Acknowledgements

Funding for this project was provided by the Florida Energy Office, Department of Community Affairs. We would like to thank Ed Cobham and the other members of FEO staff for their continued support of research into uncontrolled air flows in buildings.

We would also like to thank John Toth of FSEC who contributed considerable time, energy, and talent to doing field testing, organizing our files, and organizing our slides; and to Tom Hansen of Future Controls, our Test and Balance expert, for his considerable help in deciphering complex control systems and assisting us with various aspects of Test and Balance; and to John Tooley of Natural Florida Retrofit for help in field testing several buildings. Thanks to members of the advisory committee for giving of their time and energy to participate in this project. Thanks to the three paid consultant members of the advisory committee, Joe Lstiburek, Terry Brennen, and Bill Turner, for contributing their expertise and encouragement to our effort. Thanks to the following volunteer members of the advisory committee for their considerable contributions to this effort.

1. Executive Summary

The objective of this study was to develop the capability to substantially reduce energy use, building degradation and indoor air pollution caused by uncontrolled air flow in nonresidential buildings. For purposes of the study, “uncontrolled air flow” was defined as air moving across the building envelope or between zones or components of a building, where the pathways of flow, the direction of flow, and the origin of the air are unknown, unspecified, or unintended.

The study comes to three major conclusions as follows:

A major goal of this study has been to characterize the nature and extent of uncontrolled air flow through testing, measurement and monitoring in 70 small commercial buildings. Results showed that the nature of uncontrolled air flows in these buildings varies widely, and is strongly dependent on a large variety of complex building system interactions. As to extent, the study found that uncontrolled air flow is ubiquitous. Out of the 70 buildings studied, only one was deemed to be a “good” building. Repairs were made on 20 of these buildings. Before and after monitoring showed average energy savings of 15%.

Perhaps the most profound and compelling finding of the study is that, given the present state of practice, whether a building will avoid serious, or even catastrophic problems due to uncontrolled air flow, is primarily a matter of ijk. Building practitioners do not have access to information about uncontrolled air flow and its potential consequences in buildings. Even the research community has only recently “discovered” uncontrolled air flow, as evidenced by the fact that this study is the first major research effort of its kind. This study has found uncontrolled air flow to be surprisingly complex, and the impacts and consequences to be quite well camouflaged. Under these circumstances, it is not surprising that a good building depends almost entirely on chance.

The salient characteristic of uncontrolled air flow that appears most elusive is the fact that our buildings are functioning as pressure vessels. Mechanical air distribution systems force air through not only “leaky” ducts, but also through the various zones of this complex pressure vessel. Because we cannot see the uncontrolled air flow that results, we normally are not aware of it. As a result, forced air systems are typically treated as discrete, separate systems that begin and end with the fans, blowers, and duct systems. In actuality, the building is a complex series of pressure vessels that connect together the two ends of the air handler and duct systems, and are thus an integral part of them.

It appears that this lack of information on building air flows has forced practitioners to accept things on faith. They are condemned to the smart air syndrome -- the belief that the air is “smart’ enough to flow where it is supposed to flow, which is only within the designated ducts of the mechanical systems.

There is scant evidence from this study that the impacts of uncontrolled air flows in buildings are understood or appreciated. Buildings are designed and tested and balanced considering only the air flows at designated air inlets and outlets of the duct systems. Even though duct systems have been shown to be quite leaky, they are practically never tested for tightness. The field evidence points to two ill-fated assumptions: 1) the ducts will not leak, and 2) forced air flow begins and ends at the designated termination points of the air handler’s duct systems. Numerous examples that show the widespread existence of the smart air syndrome are contained in the body of this report.

This study conclusively shows that duct systems normally leak, often extensively. Given this fact, and the fact that the building acts as a pressure vessel, it is clear that building pressure measurements are the only accurate means of determining air flow balance in buildings. The study concludes that building pressures must be measured under a variety of building and mechanical system operating conditions in order to accurately evaluate and understand the impacts of the forced air flow in buildings. Air flow measurements alone are simply insufficient.

The extensive test and measurement data from the 70 small commercial buildings studied here show that these buildings are considerably more “leaky” than residences. The data also show that uncontrolled air flow is more pervasive and more complex in these buildings than in residences. The data also disclose a labyrinth of complex building system interactions that make diagnostic generalizations virtually impossible. Almost any given diagnostic result (for instance, the degree of supply duct leakage) can have an enormous variety of impacts, depending quite literally on everything else in the building. Simple generalizations and rules-of-thumb inevitably have proven insufficient. Nonetheless, this study provides answers to many previous questions. It also provides much needed insight, and discloses an extensive array of additional gaps in our knowledge base. (See especially Sections 4, 7 and 8 of this report.)

It has become clear from the study that more research and extensive education and training are required before practitioners will be able to successfully evaluate the impacts of these complex building system interactions. One recommendation of this study is to transfer this “new” knowledge to the building industry -- its researchers, its practitioners, and its regulators. Section 7 of this report details the specific needs in this area and recommends implementation strategies for proceeding with this important task.

It has also become clear that, given the proper attention, most uncontrolled airfiows and the problems they engender can be avoided. Section 6 of this report provides extensive discussion on the characteristics of a good building. It is now clear that a much improved set of “best practices” for building design, construction, and commissioning can be achieved. Extensive effort on the part of many individuals and organizations will be necessary to widely effect such a standard in our diverse building industry.

This study is important because uncontrolled air flows in buildings often have serious and sometimes catastrophic consequences. Increases in energy and demand costs may be the least critical of these. Often more critical and costly are the decreases in health and safety, building material durability, indoor air quality, indoor comfort, building moisture control, worker productivity, and business revenues that are experienced in problem buildings. Much too often, uncontrolled air flows cause massive building failures that result in dramatic remediation and litigation costs. In some cases, these costs have exceeded the original cost of constructing the building. Indoor air quality problems alone are estimated to result in tens of billions of dollars in unwanted costs annually in U.S. buildings.

The study presents strong evidence that many serious indoor air quality problems either arise directly from, or are exacerbated by, uncontrolled air flows in buildings. One of the important overall conclusions of this study can be concisely summarized as follows:

It is highly unlikely that indoor air quality can be ensured in any building
unless the potential for uncontrolled air flow in that buildings is eliminated.

In other words, if we can’t control the air flow, how can we expect to control the air quality?

In summary, virtually all uncontrolled air flows in buildings stem from one or more of the following three straightforward, building-system characteristics:

The results of this study show that buildings do not have to be plagued by these unwanted air flow problems. Good buildings can be achieved.

2. Project Description

2.1 Introduction

“The overall objective of this project is to develop the capability to substantially reduce energy use, building degradation, and indoor air pollution caused by uncontrolled air flow in non-residential buildings.” (Workplan: Uncontrolled Air Flow in Non-Residential Buildings, February 4, 1993). The workplan goes on to say, “This involves gaining knowledge of air flow and pressure differentials in non-residential buildings, identifving tools and testing techniques for research and diagnostic work, and developing recommendations for dealing with uncontrolled air flow in non-residential buildings.”

Before proceeding, a definition of uncontrolled air flow is helpful. Our working definition has been:

“Uncontrolled air flow - air moving across a building envelope or between
zones or components of a building, where the pathways of flow, the direction
of flow, and the origin of the air is unknown, unspecified, or unintended.”

Seventy small commercial buildings in central Florida were tested for various types of uncontrolled air flow, including duct leakage, return air imbalance problems, and exhaust air/intake air imbalance. These buildings ranged in size from 704 square feet to 22,461 square feet, and averaged 5030 square feet. Typical tests included building airtightness, duct system airtightness, infiltration rates, and pressure differentials; and air flow rates of supply, return, air handlers, exhaust fans, outdoor air, and make-up air. This study concludes that uncontrolled air flow is a serious and wide-spread problem that contributes to energy waste, elevated peak electrical demand, high relative humidity, building materials moisture degradation problems, mold and mildew growth, combustion safety concerns, and indoor air quality complaints.

Uncontrolled air flow is a function of the intensity of drivers (primarily mechanical air moving systems) and building complexity and tightness (see section 2.2 for discussion of uncontrolled air flow potential). Compared to residences, small commercial buildings have larger air flow drivers because they have larger HVAC systems which run a greater proportion of the time. Cooling systems are considerably larger, averaging 3.38 tons per 1000 square feet in these 70 buildings compared to 1.8 tons per 1000 square feet in typical residences, and in some small commercial buildings the air handlers run continuously. Exhaust fans are larger and operate longer periods of time in commercial buildings. Additionally, commercial buildings have outdoor air and make-up air which do not normally exist in Florida residences.

Duct leakage, as measured by duct depressurization test, is three times greater in commercial buildings than in Florida residences (area normalized). Even considering that the cooling systems of commercial buildings are nearly twice as large (in terms of tons of cooling capacity per 1000 square feet), this duct leakage is considerably in excess of duct leakage in Florida residences. According to SMACNA, these commercial duct systems are approximately 70 times more leaky than the SMACNA duct tightness standard. In Florida residences, duct leaks are almost always to and from unconditioned spaces (attics, garages, reducing exhaust air, and airtightening of leaky t-bar ceilings).

Cooling energy use was monitored for four to six summer months in each building. Repair of uncontrolled air flow was done in the middle of the summer. On average, cooling energy consumption decreased from 87.4 kWh/day to 75.1 kWh/day, or 12.4 kWh/day. On average, cooling energy use declined by 14.7% from repair of uncontrolled air flow. Based on the assumed $0.075/kWh electricity cost, projected annual cooling energy savings are $182. Given that the average projected retrofit cost is $454, simple payback is 2.5 years. This indicates that UAF repairs can be very cost-effective retrofit measures.

This report concludes that uncontrolled air flow is widespread in commercial buildings. Energy waste is one important consequence. However, other consequences of occupant discomfort, cooling/heating equipment being improperly sized, high humidity, moisture problems, indoor air quality complaints, and sick buildings are of equal or greater importance. Since uncontrolled air flow problems are widespread and often severe, there is a strong need to bring about changes to the way buildings are created and used.

A plan was developed to implement the findings of this project (chapter 7). The plan suggests three areas of further work. 1) More research needs to be done to understand the extent, consequences, and solutions to uncontrolled air flow in commercial buildings. 2) Based on findings from this research, various standards relating to building design, construction, commissioning, and maintenance need to be modified. 3) To bring about resolution of uncontrolled air flow problems, training programs need to be established to bring improved design, construction, commissioning, and maintenance skills to those responsible for buildings.

Uncontrolled air flow may be caused by duct leaks, return design problems, and exhaust/intake imbalance. Another and more inclusive way of stating this is that uncontrolled air flow occurs:

In order to achieve project objectives, we have tested 70 commercial buildings and made repairs of uncontrolled air flow (UAF) in 20 commercial buildings. (Note: by commercial buildings, we mean all non-residential buildings except industrial.)

2.2 Building Categorization

Commercial buildings fall into a wide variety of building uses and types. By comparison, there is much less diversity in single family homes. Most homes have two, three, or four bedrooms, a living room, a dining room, a kitchen, one or two bathrooms, and a garage. There are, of course, regional variations regarding foundation style (slab on grade, crawlspace, or basement), number of stories, and attic configurations. In terms of air flow, etc. In small commercial buildings, however, duct leaks commonly exist in four different ambient environments. 1) In some cases, the duct leakage occurs inside both the building air barrier and thermal barrier (insulation). In these cases there is not much air exchange or heat exchange with outdoors. 2) In other cases, the duct leaks occur inside the building air barrier but outside the thermal barrier (such as when the ducts are in the space between the ceiling and the roof deck, but the insulation is on top of the ceiling tiles) and therefore have significant energy penalties. 3) In yet other instances, the duct leaks exist outside both air and thermal barriers (such as in vented ceiling spaces or vented attic spaces), and the energy penalties of duct leakage can be severe. 4) Ducts may also be located on the roof. In many cases, a small portion of the ductwork is located on top of the roof. In a small number of cases, the entire duct system is on top of the roof. The energy penalties of duct leakage are generally worst in case 3, but are also substantial in cases 2 and 4.

Small commercial buildings are more leaky and often have greater complexity than residences. Testing in 70 small commercial buildings found them to be 30% more leaky than residences, primarily because suspended t-bar ceilings are quite leaky. ACH5O, the air exchange rate of the building when depressurized to -50 pascals by a blower door, averages 16.7 compared to 12.7 in Florida residences. The difference is even more pronounced in new construction. New commercial buildings are more than twice as leaky as new Florida residences.

Small commercial buildings are very leaky primarily because suspended t-bar ceilings are on the order of 10 times more leaky than gypsum board ceilings. Because the ceilings of most commercial buildings are so leaky, overall building leakiness depends primarily upon whether the ceiling space or attic space above the ceiling is well ventilated to outdoors. In those which have tight ceiling spaces, building airtightness may be 5 ACH5O or less. Several of the 70 buildings were considerably tighter than any Florida residence we have tested. In the majority of cases, where the ceiling space or attic space is vented, building airtightness is 15 ACH5O or greater. Also of note is that attached units, such as those in strip malls, are more than twice as leaky as stand-alone commercial buildings. Because much of the leakage of these attached units is to adjacent units, the energy impacts of this excessive building leakage may be relatively less severe.

Commercial buildings are often more complex than residences -- that is, they have a greater number of partitions and compartments. A building can be thought of as a matrix of barriers to air flow and pathways for air flow. Interior walls, closed interior doors, firewalls, and multiple stories create potential barriers to air flow which can interrupt the flow of air throughout the building. This disruption of air flow can create pressure imbalances which can cause elevated infiltration rates, accumulation of moisture in building cavities, and backdrafting of combustion equipment. Filters and coils, as they become dirty, become barriers to air flow and can create pressure imbalance in the air distribution system.

Repair of uncontrolled air flows was done on 20 of the 70 buildings. Repair candidates were selected based on the perceived potential for energy savings and whether the repair was financially feasible within the project budget. Repairs included sealing of duct leakage, provision of return air pathways, reducing outdoor air flow, airtightening the building envelope, and turning off attic exhaust fans. The majority of repairs were duct repair. Three major types of repairs that were not attempted were provision of make-up air, homes often have air distribution systems, but no make-up air, no outdoor air, and small and infrequently used exhaust fans. Variations in homes is much smaller than in commercial buildings.

Commercial buildings have a much wider range of sizes, uses, construction styles, height, and mechanical systems. A conceptual framework was developed for thinking about the diversity of building types and configurations found in commercial buildings based on the potential for and consequences of uncontrolled air flow.

2.2.1 Uncontrolled air flow potential

A two-dimensional matrix has been developed that attempts to describe the potential for uncontrolled air flow in a building. The two axis are “drivers” and “building” (Figure 2.1).

“Drivers” are the forces which move air within a building and across the building envelope. They include air handlers, exhaust fans, outdoor air, make-up air fans, and operation of combustion equipment (Figure 2.2). Drivers can also include wind driven air flow and pressure differentials, and stack effect pressures (especially in tall buildings in cold weather). Drivers range from mild to intense. The more intense or larger the drivers, the greater the potential for uncontrolled air flow to occur.

“Building” refers to building size and complexity, but complexity is most important. A building is an interwoven fabric of barriers and pathways. A wide-open retail space, for example, has few compartments, and is therefore “simple”. A building which has many partitions and subdivided spaces is complex. Partitions provide the potential to restrict air flow and therefore have the potential to create substantial pressure imbalances and UAF (Figure 2.3).

Simple buildings with mild drivers fall into the “low potential” portion of the matrix and have less potential for UAF and large pressure differentials. Examples include restaurants (large exhaust, make-up air, and outdoor air) and some recreation and light industrial/warehouse facilities. Complex buildings with mild drivers (only air handlers) also fall into the "moderate potential" portion of the matrix and have substantial potential for UAF because of closed doors when returns are centrally located.

Complex buildings with intense drivers fall into the "high potential" portion of the matrix and have the greatest potential for UAF; these would include hotels, hospitals, larger restaurants, and some sports facilities.

2.2.2 Uncontrolled air flow consequences

While uncontrolled air flow potential can be defined by a two-dimensional matrix consisting of drivers and building complexity. The potential for uncontrolled air flow can be thought of as the area defined by the two matrices (Figure 2.4). Uncontrolled air flow consequences can be conceived as a three-dimensional matrix defined by drivers, building complexity, and sources, and the extent of consequences can be thought of a volume defined by the three parameters of drivers, building complexity, and sources (Figure 2.5). Sources include heat, cold, moisture, and air contaminants. When transported by UAF, these sources often have negative consequences. Sources fall into three primary categories; 1) cold/heat, 2) moisture, and 3) pollutants.

2.2.2.1 Cold and Heat

When drawn into the building by UAF, cold and heat can cause the following consequences.

2.2.2.2 Moisture or dryness

When drawn into the building by UAF, moisture or dryness (air with low moisture content can cause the following consequences.

2.2.2.3 Pollutants

Pollutants can be drawn itno buildings or not properly diluted (diminished ventilation) as a result of uncontrolled air flow. UAF can cause indoor air quality problems by four mechanisms; 1) mining of pollutants, 2) transporting of pollutants, 3) generation of pollutants, especially microbial growth, and 4) diminishing of ventilation.

Mining of pollutants occurs when depressurization caused by UAF draws pollutants from the soil (radon, methane), sewer pipes (methane, sulfur dioxide), or combustion equipment (carbon monoxide, moisture, particulates, and nitrous oxides). Space depressurization draws pollutants through cracks or penetrations in the slab (soil gases such as radon or methane), from sewer lines when plumbing fixtures are not seated properly or when traps are dry, and from combustion appliances when backdrafting or spillage is induced.

Transport of pollutants occurs when UAF carries pollutants from zones that have air contaminants. Following are examples. Return leaks in ducts or air handlers located in crawl spaces, garages, attics, etc. may transport radon, volatile organic compounds (oil and gasoline in garage), and insulation particles from these zones to the occupied space. In at least two reported incidents in Duval and Brevard counties in Florida, a total of seven persons died in their homes from carbon monoxide poisoning when cars were left running in closed garages. Leaks in the air handlers and ducts in the garage transport carbon monoxide-laden air into the house, resulting in lethal carbon monoxide poisoning. Return leaks in a roof-top package air conditioner may be drawing air from contaminated sources such as cooling towers, plumbing stacks, or exhaust fan discharges.

Generation of pollutants occurs when UAF actually creates or increases the pollution source. Four examples are presented.

Diminished ventilation may occur if outdoor air, exhaust air, or. make-up air ducts have leaks. Consider some examples. Outdoor air ductwork is located in a ceiling space. If the ductwork leaks, then building air is drawn into the outdoor air ducts and thus reduces building ventilation air. If exhaust fan ductwork leaks (blower at or near the grill), so that air is discharging from the ducts into the building, then polluted air may be re-entrained into the building and the overall ventilation rate may be diminished.

2.2.3 Types of air quality consequences

Uncontrolled air flow generates very substantial energy consequences. • However, these consequences are often’ dwarfed by the indoor air quality and materials damage consequences. Air quality problems produced by the four UAF mechanisms listed in the preceding section can result in various types of consequences.

1. Odor problems
2. Health consequences
   a) chronic
   b) acute
3. Reduced worker productivity
4. Sick buildings
   a) headaches
   b) respiratory stress
   c) increased illness
   d) anxiety. developed among workers
5. Evacuation of buildings, temporarily or long-term
6. Law suits

UAF may lead to increased sick leave and reduced worker productivity. Add to this the possibilities of having to evacuate the building, renovation costs, and law suits, the air quality:nationwide consequences of UAF may run into the billions of dollars each year.

2.2.4 Consequences are somewhat random

Just because there are intense drivers, a complex building, or both, does not ensure that uncontrolled air flow will result. These factors simply increase the potential for uncontrolled air flows.

Just because there are uncontrolled air flows does not mean there will be consequences. The presence of uncontrolled air flow simply increases the potential for significant consequences. Whether there are consequences or not depends in large part upon luck (often the randomness of how buildings are put together) and of course whether there are sources.

Just a matter of luck. Whether uncontrolled air flow leads to serious consequences is often just a matter of luck. Consider the case of one manufactured office building (#16 on Master Table in Appendix B) tested in this project. Seven package air handlers are mounted on the exterior wall of the building, through-the-wall returns pull air from office, return transfer grills in the office doors were greatly undersized (allowing only about 5% of the needed air flow), and office doors are closed most of the time (Figure 2.6 -- note the numbers are pressure differentials and the arrows indicate the direction of pressure gradient and air flow). The closed doors then acted as barriers to air flow, creating strong depressurization in the closed offices, and thus pulling a majority of the return air from the space above the ceiling, which had 34 passive roof vents to outdoors (Figure 2.7).

Two fortunate circumstances prevented this major form of uncontrolled air flow from being a serious problem. First, the roof vents had dampers and these dampers were quite tight. Second, the ceiling insulation (batts) was attached to the. bottom of the roof deck. Therefore, this ceiling space, which was acting like a return plenum, was located inside both the air barrier and thermal barrier of the building. If the insulation had been located on top of the ceiling tiles or the roof vents actually ventilated the ceiling space, then there could have been substantial energy and humidity consequences.

Importance of sources. The importance of sources can be illustrated by considering the following example. If we have a building with considerable UAF, and it is drawing considerable air from outdoors, but the air being drawn into the building is virtually identical to the desired indoor conditions (75F and 50% RH) and has no pollutants, then there may be no energy, comfort, humidity, building material damage, or indoor air quality consequences from UAF. This could occur, for example, in a city like San Diego, California when cool and clean breezes blow from the ocean much of the time. In real life, however, the air brought into buildings by UAF is generally too cold, too hot, too humid, too dry, or polluted. Even in San Diego on a perfect day, air can be drawn from an attic space (which may be hot) or a loading dock area (which may be polluted), and there may be negative consequences.

3. Diagnostic and Testing Protocols

One of the primary objectives of this project was to develop diagnostic and testing methodologies which can be used to diagnose UAF problems in buildings. Since this project is research, the testing actually done on these buildings was more comprehensive than we expect will be used in typical “real world” diagnosis. Nevertheless, many of the research procedures and testing methodologies developed in this project will be incorporated into real world diagnostics. A number of new measurement techniques were developed and refined in this project, and they allow mçasurement of air flows and pressure differentials in buildings more accurately and quickly than is commonly available in the industry.

Following is a discussion of the various diagnostic and testing protocols used to characterize airtightness, pressure differentials, and air flow rates in this project.

3.1. Not All Tests Were Done In All 70 Buildings

Not all tests were done in all 70 buildings. In terms of building airtightness, one business owner would not allow us to do a blower door test because he felt it would disrupt his business activities. Building airtightness tests exist for the other 69 buildings.

In 24 of the 70 buildings, duct system airtightness was not measured. Three factors determined whether the duct airtightness test was done.

In 13 of the 70 buildings, the building infiltration rate with the air handlers (and normally operating exhaust fans) turned off was not measured. This occurred almost exclusively in cases where the occupants were very reluctant to allow us to turn off the air conditioning systems for a one-hour-plus period. Business owners are generally very sensitive to things that affect worker productivity or customer comfort. In many buildings, turning off the cooling system on summer days results in rapidly rising temperature and humidity. In restaurants, it is virtually impossible to turn off the exhaust fans because the cooking appliances require continuous exhaust.

In one small real estate office building, for example, the air handler-off infiltration test was done over the lunch hour when most of the employees were out to lunch. During the 70- minute test, the temperature in the space rose by about 5F. Because the air conditioner was undersized (this was a thermally inefficient building because of a very leaky building shell and poorly located insulation), the temperature in the space never recovered and the occupants were hot (and not particularly happy with us) throughout the remainder of the work day.

Typical protocol. A typical protocol includes visual inspection, building and duct airtightness testing, pressure differential measurement, infiltration/ventilation tests, air flow measurement, and visual inspection of building mechanical components which may contribute to uncontrolled air flow and pressure differentials. The objective of the testing is to characterize air flows and pressure differentials within the building, characterize the air flow balance across the building envelope, identify the cause of air flow and pressure imbalances, and understand the interacting relationships between building airtightness, air flows, pressure differentials, the operation of building equipment, indoor air quality, ventilation, and energy consumption.

3.2 Visual Inspection

The first step in the diagnostic process is visual inspection and obtaining information from persons familiar with the building. Following are steps typically taken.

3.3 Important Definitions

Following are definitions that are important to understanding some of the following discussions.

Attic space is a space above the ceiling that has trusses or joists which can be walked on and often has sloped roof, wood decking, intentional ventilation, and insulation at the attic floor level.

Backdrafting is reversal of flow of gases down the chimney or vent of a combustion appliance.

Ceiling space is the space above a ceiling that has no structural members that could support a person’s weight, may have insulation at the ceiling or roof deck level, and generally is not intentionally vented.

Pressure pan is a pan that can be placed over supply and return registers to measure pressure difference. It is made of a cake pan (or other type of pan). Gasketing is put on the pan rim to facilitate an airtight fit and a tap penetrates the pan to allow measurement of pressure in the ductwork.

Primary air barrier is that portion of the building envelope which provides the greatest resistance to air flow and the greatest pressure drop when the building is exposed to a significant pressure differential compared to outdoors. In the ceiling/roof plane, the roof deck may be the primary air barrier or the ceiling may be the primary air barrier. Note that this means only that the primary air barrier is only relatively tight; that is, it is tight only by comparison to any other air barriers in series with the primary air barrier. A suspended t-bar ceiling may be the primary air barrier, but it nevertheless may be very leaky.

Primary thermal barrier is that portion of the building envelope which provides the greatest resistance to heat flow. In the ceiling/roof plane of the building, insulation may be located at the ceiling, at in the roof deck, or suspended in between. Often the primary insulation barrier is located on top of the ceiling tiles, but the (flat) roof assembly may also have some significant R-value, so the ceiling space is actually sandwiched between two thermal barriers.

Spillage occurs when only a portion of the combustion gases leave the building through the vent pipe. The remainder spill into the space. Spillage often occurs during start-up of a combustion appliance when the chimney or vent is cold. Ad the flue gases warm, draft strength increases and complete drafting of combustion gases results. Continuation of spillage beyond one minute indicates a draft problem.

3.4 Building Airtightness Testing

Typically, the second step in the diagnostic process is building airtightness testing. The building is prepared by turning off vented combustion equipment and air moving equipment including air handlers, exhaust fans, make-up air fans, and clothes dryers. Outdoor air, exhaust fans, and make-up air openings are sealed off, since these holes in the building do not respond passively to building air flow and pressure dynamics.

A multi-point airtightness test is performed (we follow the ASTM E 779-87, “Standard Test Method for Determining Air Leakage Rate by Fan Pressurization”, except we only depressurize the building), using from one to six calibrated fans (blower doors), depending upon the building airtightness and size, and generally obtaining air flow at five to eight building pressures in the range from -10 pascals to -60 pascals depressurization. (Note: all pressures expressed in this paper are “with respect to outdoors” unless otherwise indicated). Knowledge of building airtightness assists in interpretation of other field testing, especially pressure differential measurements, and in developing recommendations for air flow and pressure balancing.

3.5 Identification of Building Air Barriers

With the building depressurized to -50 pascals by the calibrated fan(s), pressures in various zones of the building are measured in ordet to identify the primary building air barrier; in other words to determine which portions of the building are “indoors” and which are “outdoors”. Pressure may be measured in the ceiling space, attic space, wall cavities, chases, soffits, mechanical rooms, ducts, space between floors, etc. Consider an example; if the ceiling space of an office buildings is at -5 pascals when the occupied space is at -50 pascals, this indicates that the ceiling is the primary air barrier. It may also indicate that the ceiling space is reasonably well ventilated to outdoors and that ducts located in the ceiling space are in a zone that is effectively “outdoors”. If the ducts are “outdoors”, then it will be more important to test for duct leakage.

If, on the other hand, the ceiling space is at -49 pascals when the occupied space is at -50 pascals, then the ceiling space and the ducts are located “indoors”. (Note that being inside the building air barrier does not ensure, however, that the ceiling space and the ducts are inside the thermal barrier; for example, the insulation may be on top of the ceiling tiles while the roof deck is the air barrier.)

Consider another example; a mechanical room containing a gas water heater is at zero pressure when the occupied zones are at -50 pascals. This indicates that the mechanical room is well connected to outdoors arid poorly connected to indoors, and that the combustion equipment located in that room will not be significantly affected by any pressures which may be created in the occupied zones. On the other hand, if the mechanical room is well ventilated to the occupied space and is at -48 pascals when the occupied space is at -50 pascals, then the combustion equipment located in that room could be significantly affected by any pressures which may be created in the occupied zones. This could lead to spillage or backdrafting of combustion equipment.

3.6 Duct System Airtightness Testing

Airtightness of the duct system can be measured by means of a depressurization test. Airtightness of the duct system can also be indicated by means of a pressure pan test.

3.6.1 Duct system depressurization test

Duct system airtightness may be measured using calibrated fans (duct test rigs or duct testers). All registers except one supply and one return (in proximity to the air handler) are masked off. Outdoor air inlets, if any, are masked off. Calibrated fans are attached to the open registers. An air flow barrier is placed in the air handler (at the filter, coil, or blower) to divide the system into supply and return. Air is drawn from the duct system by the calibrated fans and a multi-point (multiple pressures) airtightness test is done, with each side of the system at the same pressure (duct pressure is measured near the air handler and referenced to the zone in which the ducts are located). CFM25 (air flow through leaks in the duct system when the ducts are at -25 pascals) is determined for both the supply and the return side of the system. The combined CFM25 (add supply and return sides together) represents the combined leakage to outdoors, unconditioned building space, and conditioned building space, and can be expressed as CFM25total.

The duct system airtightness test can be repeated to determine what portion of the duct leakage is to outdoors (or to buffer zones which are well ventilated to outdoors). Using a calibrated fan, the building is depressurized to the same pressure as the duct system, usually at just one pressure of -25 pascals, and the duct system airtightness test is repeated. Since the occupied zone and the ducts are at the same pressure, the duct test rig is measuring only duct leaks to outdoors. The resulting CFM25 can be expressed as CFM25out.

If the ducts, plenums, and air handler are within the air barrier of the building, then the second test (with the building depressurized) is not needed. Note that in some cases, the ducts may be inside the building air barrier but there are leak pathways between the “designated” duct system and interstitial cavities that lead to outdoors.

In other cases, the ducts are located within the air barrier of the building, but outside the thermal barrier, such as when the roof deck is the building air barrier, the ducts are located in a ceiling space, and the insulation is on top of a t-bar ceiling. (T-bar ceilings are used in the vast majority of all commercial buildings. They are composed of t-shaped metal framework suspended from above with ceiling tiles supported within the metal framework). Duct leakage to and from these ducts causes considerable heat gain from the ceiling space during the cooling season and heat loss during the heating season. Even though this leakage occurs within the air barrier of the building, it occurs outside the thermal barrier of the building and consequently causes significant energy penalties (the extent of the energy penalties largely depends upon the thermal resistance inherent in the roof construction, the color of the roof surface, whether the roof surface is covered by gravel, and the extent of roof shading).

3.6.2 Pressure pan test

Duct system airtightness can be indicated (though not measured) by means of a pressure pan test. In this test, the building is depressurized to -50 pascals by the blower door. The air handler is turned off. A pan, similar to a cake pan, is attached to a pole and placed over each supply and return register, one register at a time. A micromanometer is attached to a pressure port on the pan so that the pressure in the ductwork (with respect to the room) can be measured. If the duct pressure is the same as the room pressure, then there is no duct leakage (to outside the building envelope). If the duct pressure is considerably different from room pressure, this indicates substantial duct leakage near that register. The following table provides a means of interpreting the pressure pan results.

Table 3.1
Interpretation of pressure pan results when house is depressurized
to -50 pascals. (Cummings, Tooley, and Moyer, 1993.

The interpretation of pressure pan test results shown in Table 3.1 assumes that the duct system is located in a zone which is well ventilated to outside, so that when the occupied space is at -50 pascals, the duct zone is near neutral with respect to outdoors. In single family residences, this assumption is most often correct. In commercial buildings, however, the zone containing the ductwork is frequently located inside the primary air barrier and may experience pressure much closer to indoors than outdoors.

Consider the following example. In Realty 2 (building #37), the attic space is at -43 pascals with respect to outdoors when the occupied space was at -50 pascals with respect to outdoors (the attic is at +7 pascals with respect to the occupied space). Pressure pan tests were done on the 10 supplies and 1 return of the duct system. The indicated pressure ranged from 0.7 pascals to 5.0 pascals and the average was 2.0. Since the maximum pressure that could occur was 7 pascals (50 pascals - 43 pascals), these pressures indicate large duct leakage.

In order to convert these number to values which can be interpreted on Table 3.1, multiply the pressure pan pressures by 50 (the assumed pressure difference between the occupied zone and the duct zone) and divide by the actual pressure difference between the occupied zone and the duct zone. In this example, multiply by 50 and divide by 7. The converted pressure pan pressures would range from 5 to 35.7 pascals. In many commercial buildings, pressure in the duct zone is the same or virtually the same as in the occupied space (blower door operating); consequently the pressure pan test will not work in these buildings. In conclusion, while the pressure pan test is widely useful in residences, it is often not an effective diagnostic tool in commercial buildings.

3.7 Building Pressure Differentials

Pressure differentials were measured in the building with the building and HVAC systems in various modes of operation. Pressure in the building is measured with respect to (wrt) outdoors once with the air handlers turned to continuous “on” and the exhaust fans operated in normal operation. Pressure in the building is measured a second time with all mechanical systems turned off. Pressure in various rooms and zones of the building are measured with doors open and closed and various HVAC equipment turned on and off in order to characterize pressure differentials between various zones of the building and between those zones and outdoors. A primary objective is to characterize the effect of the air moving equipment on building and zone pressures, especially negative pressure.

Pressure differential measurements are made with 2-channel and 8-channel digital micromanometers with resolution to 0.1 pascals. The hand held units have time-averaging capabilities which allow discriminating small pressure differentials even when significant fluctuations exist because of wind. The 8-channel micromanometer with interface to computer display and memory was used to sample at up to eight locations simultaneously throughout the building and mechanical systems, plot continuously on a computer screen, and store data for later analysis.

3.8 Infiltration/Ventilation Rates

Using tracer gas decay methodology (ASTM E 741, “Standard Test Method for Determining Air Leakage Rate by Tracer Dilution”), the building infiltration/ventilation rate was measured, once with the HVAC equipment operating and then again with the HVAC equipment turned off (if possible or practical). In 13 of the 70 commercial buildings, the “equipment off ‘ test was not done because it was difficult to find a period when the occupants would allow turning off the mechanical systems.

Foxboro Miran 101 Specific Gas Analyzers and a Bruel and Kjaer 1302 multi-gas analyzer were used in various types of infiltration testing. Sulfur hexafluoride and nitrous oxide were the two gases that were commonly used as tracers. The typical test method using the Miran 101 was as follows:

3.9 Air Flow Rates

In virtually all buildings, HVAC system air flow rates were measured. A number of different measurement techniques were used.

Various methods were used because 1) some methods work better than others because of intake or discharge configuration, 2) some methods take more time, and 3) some methods take duct leakage into account and others do not. Air flow hoods are useful for getting a “quick and dirty” picture of overall flows of the HVAC systems. They are not the best choice, in many cases, for measuring exhaust air flows, make-up air flows, or outdoor air flows. They also may overestimate air flows, especially at discharge grills/registers (more discussion in 3.8.1).

Tracer gas injection in conjunction with a gas analyzer can measure total air flow through ducts, air handlers, exhaust fans, and make-up air fans. Calibrated blowers can be used to measure air flows through air handlers, exhaust fans, make-up air fans, and even outdoor air. Both methods can deal with most intake or discharge configurations and accurately take duct leakage into account, but they are both more time consuming compared to using a flow hood.

Since an important aspect of diagnosing uncontrolled air flow is accurately measuring net air flows across the building envelope, the more time consuming methods are often called for. Following are detailed descriptions of the various air flow measurement methods.

3.9.1 Flow hood

Air flow at supply registers and return grills is measured by air flow hood. Outdoor air is typically measured with a flow hood, by placing the air flow hood over the outdoor air intake opening. Both Shortridge and Alnor hoods were used in this project.

These air flow hoods provide the fastest means for determining flow from registers and grills. They are generally quite accurate measuring flow into return grills and exhaust grills. They are not so accurate measuring air coming from some supply registers, especially those that discharge mostly to one side or have jetting” of air into the hood (these occur in small commercial and are most common in residential systems). With some register configurations and air discharge configurations, we have found measurement of air flow from supply registers often are 20% too high and can be as much as 50% to 80% too high. The larger 24”x24” grills with diffuse holes or discharging equally to all four sides can usually be measured with very little error.

3.9.2 Tracer gas

While tracer gas is most commonly used to determine the building infiltration or ventilation rate, it can also be used to determine return leak or outdoor air flow rates. Tracer gas is distributed into the building (as in the tracer gas decay infiltration test) and well mixed (about 15 minutes with the air handlers operating). Tracer gas concentration is then sampled at three locations for each air handler; A) in the room near the return grill, B) at the discharge of a supply grill, and C) at the return leak or outdoor intake location (since some tracer gas may be re-entrained into the outdoor air intake). To get return leak fraction (RLF), run the test once with the outdoor air intakes masked off. Then rnn the test a second time with the outdoor air grills open.

RLFt, the proportion of return air flow that enters the return air distribution system through all leaks, whether from inside the building shell or not, is calculated by the following equation (Cummings and Tooley, 1989).

RLFt = (A-B)/(A-C)

where

A is the tracer gas concentration of air entering the return grill(s)
B is the tracer gas concentration of air coming from a supply grill
C is the tracer gas concentration of air at the return leak site

Note that as the concentration of tracer gas at C approaches the value of that at A, the accuracy of the test diminishes rapidly.

RLFo, the proportion of return air flow that enters the return air distribution system through leaks from outside the building, is calculated by the following equation:

RLFo = (A-B)/A

Note that this test is done with outdoor air intake sealed off.

To obtain outdoor air fraction, repeat the return leak fraction test but this time with the outdoor air intake open. OAFt, the proportion of return air entering through both the outdoor air opening and return leaks from outdoors, is calculated by:

OAFt = (A-B)/(A-C)

where

C is the concentration of tracer gas enetering the outdoor air opening.

Outdoor air fraction (OAF; air entering only through the outdoor air vent) is calculated as follows:

OAF = OAFt - RLFo

Total return leak flow is calculated by:

RLFflowt = RLFt * air handler flow rate

Return leak flow from outdoors is calculated by:

RLflowo = RLFo * air handler flow rate

Outdoor air flow rate (OAflow) is calculated by multiplying OAF times total air handler flow rate.

OAflow = OAF * air handler flow rate

3.9.3 Measuring air flow rates using tracer gas injection

Flow rate of exhaust fans and make-up air fans (and air flow through most duct systems) can be measured by means of tracer gas injection and sampling with a gas analyzer downstream. Tracer gas is continuously injected into the air stream such that the gas is well distributed (tubing with holes is often used), and the gas is sampled downstream in a distributed manner (a loop of tubing attached to a sampling pump). The injection rate of the tracer gas must be accurately measured. Tracer gas concentrations before the injection point (Cs) and downstream (Cb) must be accurately measured. Flow rate is calculated by means of the following formula (Grieve, 1991):

q = dose/(Cs-Cb)

where

q is the air flow rate in cfm
dose is tracer injectin rate in cfm
Cs is the tracer concentration at the sample point
Cb is the tracer concentration upstream of the injection location

Gas flow meters.
Two sizes were purchased to allow a wide range of gas flow rates. Computer software was purchased which allows accurate determination of flow rates through these two meters for a wide range of gases.

A water-displacement procedure was developed to calibrate the flow meters. An airtight, five-gallon container of water was placed on an accurate weight measurement scale. Tracer gas then flowed into the container and displaced the water. The change in water volume was determined by means of change in weight. In order to avoid compression of the tracer gas within the five gallon container, pressure (wit the room) in the container was measured. The flow rate of water out of the tank was modulated so that pressure inside the container remained within about ±10 pascals of the room environment. The calibration results indicate that the computer software/flow meter was accurate to within 3%, and the data was extremely linear (r2 = 0.9915 fit to straight line indicates little scatter in the experimental data).

Using the calibrated flow meter and a tracer gas analyzer (either our Bruel and Kjaer 1302 or Miran 101), the air flow rate through a duct, air handler, or exhaust system could be measured quickly. On a test building, for example, we were able to measure the flow rate through six roof-top package units in less than one hour. By comparison, measuring that same rate using a duct blaster or blower door would have required considerably more time. Depending up the air handler configuration and outdoor air flow rate, an air flow hood could provide a fast and relatively accurate means of measurement. Using a capture tent with a calibrated blower might have been almost as fast, depending upon how much time is involved in assembling and moving the tent (see section 3.8.4 for further discussion of the capture tent method).

3.9.4 Calibrated fans

Calibrated fans, such as those used with blower doors and duct test rigs, can be used to measure air flows. They can be installed in capture tents, attached directly to the HVAC system, or used with the building as a capture tent.

Capture tent. Air flows, especially the discharge of exhaust fans or the intake of make-up air fans, can also be measured by means of calibrated fans and a capture tent (polyethylene sheeting on a PVC frame will do). A tent is placed over the discharge of the exhaust fan, for example, and a calibrated fan is mounted into the side of the tent (a flow conditioner may be needed to reduce turbulence at the intake side pressure sensor). A micromanometer measures the pressure in the tent wrt outdoors. The calibrated fan is turned on to draw air out of the tent and reduce the pressure in the tent (recall that the exhaust fan is blowing air into the tent) until the pressure in the tent is neutral wrt outdoors. Air flow through the calibrated fan is then equal to the flow through the exhaust system. Make-up air may be measured in an analogous manner, with the calibrated fan blowing air into the tent. Figure 3.1 illustrates use of a capture tent and calibrated fan.

Calibrated fans mounted directly to the HVAC system. The calibrated fan may be mounted directly to a portion of the HVAC system. flow of an air handler, the following steps the supply plenum as the system normally operates. Then remove a panel from the air handler, attach the calibrated blower to the opening in the air handler, seal off any remaining openings around the calibrated fan, place a barrier at the bottom of the air handler to isolate the return side of the system, turn on the air handler, then turn on the calibrated fan and increase its speed till static pressure in the supply plenum is equal to that measured previously. The air flow through the calibrated fan is equal to the air handler flow rate (Figure 3.2).

Building as a capture tent. The building can also be used as a capture tent. Look at an example of measuring exhaust fan flow in a restaurant. With the exhaust fan(s) turned on (all other air moving equipment turned off), the pressure in the building is measured (this could be -60 pascals or greater in many restaurants).

The calibrated fan is turned on to blow air into the building until the pressure in the building is neutral wrt outdoors. The air flow through the calibrated fan is equal to the exhaust fan flow rate. (Note this method works well if the building is tight or the exhaust fans are large. Restaurants may be tight and often have large exhaust fans).

Note, however, that this is the flow rate that occurs when the building is at neutral pressure. If the normal operating pressure (NOP) of the building is much different
from neutral, -10 pascals for example, then the measurement of exhaust or makeup air may be different than that which actually occurs, because building depressurization will reduce exhaust fan flow and increase make-up air flow. To correct for this, the calibrated fan is turned on to move air into the buildinguntil the NOP is reached (-10 pascals); record the flow rate through the calibrated fan (say 3600 cfm). Now calculate the air flow rate into the building due to the -10 pascals depressurization using the airtightness curve developed from the building airtightness test. (Example: say C 300, n .65, then q would be equal to 1340 cfm at -10 pascals). Exhaust fan flow is equal to flow through the calibrated fan plus the building leakage; 3600 cfm plus 1340 cfm = 4940 cfm.

The flow of make-up air can be measured in an analogous manner. Turn on only the makeup air fan and use a calibrated fan to pull air out of the building until reaching NOP. Makeup air flow is then equal to the air flow through the calibrated fan minus the air flow through the building envelope (this assumes the NOP is negative). Alternatively, the make-up air flow can be measured with the make-up air and the exhaust fans operating simultaneously (note that the make-up air flow is usually less than the exhaust air flow). If the building is at its NOP, then make-up air is equal to exhaust fan flow minus building leakage at NOP. If the building is depressurized beyond its NOP, then turn on the calibrated fan to blow air into the building until it reaches its NOP. The make-up air flow is equal to the exhaust fan flow minus building leakage minus calibrated fan flow. Note also that in this situation, the calibrated fan flow is equal to OAflowT (the sum of outdoor air flow and return leaks that draw air from outdoors).

4. Project Findings

Air flow across a building envelope is a function of hole size and pressure differential across that hole. Without a hole there is no air flow. With no pressure differential, there is (almost) no air flow. It is the combination of holes and driving forces which create most uncontrolled air flows in buildings.

Mechanically induced pressure differentials are a function of the net air flow into a space and the airtightness of that space. This applies to occupied and unoccupied building spaces, building spaces used as ducts, and ducts themselves.

In commercial buildings, we expected mechanically induced pressure differentials to be the big story. And certainly they are. However, a big concern, and a surprise to most of us, emerged -- that many small commercial buildings have big holes, especially in the ceiling plane, and that these big holes may be the source of substantial energy problems. A significant number of commercial buildings are tighter than most Florida homes (12 of 70 buildings have ACH5O less than 5). On the other hand, a large number of commercial buildings are much leakier than almost all Florida homes built in the last 20 years (26 of 70 buildings have ACH5O greater than 20). On the whole, central Florida commercial buildings are considerably more leaky than central Florida homes.

Also of considerable concern is the magnitude of problems associated with ceiling insulation systems. Many ceiling insulation systems are functioning poorly, because the insulation is missing, has been moved around, or is being by-passed by uncontrolled air flow. Following is a discussion of the major findings of this project.

4.1 General Building Information

A large portion of the project findings are contained in a single table contained in Appendix B. The following general summary can be presented for our sample of 70 central Florida small commercial buildings. Average floor area was 5030 square feet. Five buildings were manufactured (non-metal), either an office trailer or a modular office space. Four buildings were metal. Twenty buildings were frame or predominantly frame. The remaining 41 buildings were masonry or predominantly masonry (Figure 4.1).

Six buildings are on crawl spaces. The remaining 64 are slab-on-grade construction.

Five are two story. The remaining 65 are one story buildings.

Nine buildings have no attic or ceiling space. The bottom of the roof deck, in most of these cases, is the ceiling of the occupied space. Tn two cases, the space above the ceiling is a warehouse. In other words, this is an office space in an unconditioned warehouse. Fifteen buildings have attics. The remaining 44 buildings have a ceiling space above the ceiling.

There seems to be confusion among contractors about whether ceiling spaces should be vented and about construction details at the eaves. It is common, for example, to install perforated soffit facia while attempting to block off the soffit space from the ceiling space by insulation batts. The insulation batts, of course, do not stop air flow so the ceiling space is unintentionally vented. In general, it appears that small commercial buildings in Florida contain a hybrid of residential and commercial construction materials and methods which often do not make sense from the perspective of heat flow and air flow control.

4.2 General HVAC Information

Cooling capacity. Cooling capacity in commercial buildings is greater than in residential buildings. Total cooling capacity per building ranged from 2 tons to 129 tons, with an average of 16.9 tons. Number of air conditioning systems ranged from one to eight per building, with an average of 3.1 units per building. While Florida residences typically have 1.5 to 2.0 tons of air conditioning capacity per 1000 square feet floor area, the commercial buildings in our sample averaged 3.38 tons per 1000 square feet. They ranged from 1.54 tons per 1000 square feet to 11.1 tons per 1000 square feet. Figure 4.2 presents a breakdown of cooling capacity for the 66 buildings for which cooling capacity is known.

The three buildings with the highest capacity per floor area are fast-food restaurants. Cooling capacity in these three averages 9.8 tons per 1000 square feet. Cooling capacity in all eight buildings which are restaurants or contain restaurants (and for which we know cooling capacity) is 6.1 tons per 1000 square feet, or twice the capacity of the non-restaurant commercial buildings in this study. Interestingly, the fourth largest capacity per floor area is in a 1512 square foot dentist office with a 9 ton air conditioner.

Air handler location. Air handlers are located in mechanical rooms in 13 buildings and in mechanical closets in 16
buildings. We define a mechanical room as a space containing mechanical equipment (air handlers, etc.) in which persons can move around. A mechanical closet is smaller, too small to allow persons to move around or in some cases even enter. Air handlers are in attics in four buildings, in ceiling spaces in two cases, and in the occupied space in six instances. Air handlers are outdoors in five cases and on the roof in 19 cases. Air handlers were in unconditioned warehouses in three buildings (Figure 4.3). (In eight buildings air handlers are located in two locations. The numbers reported in this paragraph consider only the dominant location for each building.)

Ductwork location. Duct systems are generally located in the ceiling space or attic space. Some are located outdoors and some are located in a warehouse space. The attic space and outdoors are always unconditioned. Some ceiling spaces are conditioned and some are unconditioned. Some warehouses are conditioned and some are unconditioned. In two buildings, the ducts are primarily outdoors. In 48 buildings, ducts were located in unconditioned building space, either in an unconditioned ceiling, an attic, or an unconditioned warehouse. In 17 buildings, ducts were located in conditioned space. Conditioned space is defined as either the occupied space or another building space that is inside both the air barrier and thermal barrier of the building. When ducts are located in the conditioned space, penalties from duct leakage are greatly minimized.

Types of ducts. Three types of duct materials dominate air distribution system construction. Nine systems were entirely metal. Twenty-one systems were entirely ductboard. None were entirely flex duct. Hybrid systems were the norm. (In the following, the dominant duct type is listed first.) Nine systems were metal with ductboard. Three were metal with flex duct. Twenty-three systems were ductboard with flex. One system was flex with ductboard. Two buildings had no ductwork (Figure 4.4).

Building cavities as ducts. Building cavities are also used as portions of air distribution systems. In Florida single family residences, it is common for air handler support platforms to be used as part or all of the return air ductwork. Wall cavities, panned floor joists, mechanical closets, dropped ceiling cavities, and spaces below staircases are also used as ducts in homes. Since these building cavities are almost always very leaky, they represent a large portion of all duct leaks in existing Florida residences. In total, it is estimated that 60% of all duct leak air flow in Florida residences occurs in building cavities used as ducts.

It is also common to use building cavities as ducts in small commercial buildings. In 34 of 70 buildings, one or more types of building cavities are used as ducts (Figure 4.5). In eight buildings, the mechanical room is used as a return plenum. In six buildings, a mechanical closet is used as a return plenum. In most of these 14 cases, the ceiling of the room or closet is t-bar, and in many of those cases the space above the mechanical room is unconditioned attic or ceiling space. Since t-bar ceilings are very leaky, it is common for these mechanical rooms or mechanical closets to experience 20% to 40% return leakage (20% to 40% of the systems return air is originating from leakage through the ceiling or other leak pathsways from unconditioned space).

In five buildings, wall cavities are used as ducts or plenums. In eight buildings, the ceiling space is used as a return plenum. In 11 buildings, air handler support platforms are enclosed to form a return plenum, similar to what is found in many residences. In two buildings, chases are used as return ducts. In another building, the space between two dropped ceilings (one above the other) was enclosed to form a return plenum. In this case, one of the ceiling tiles in the upper ceiling had been removed and as a result there was an enormous return leak drawing from an unconditioned ceiling space (CFM25 for that leak site alone was about 4000). Even if all the tiles had been in place, there would still be considerable leakage from the unconditioned ceiling space because t-bar panel construction of the top portion of this plenum was very leaky. (Note that while 34 buildings use building cavities as ducts, the total of the various building cavity types comes to 41 because more than one building cavity type exists in 7 buildings.)

In general, use of building cavities as ductwork is poor practice. Building cavities virtually always leak, and in many cases the leakage can be very severe. Ceiling spaces used as return plenums can be a significant exception to this general rule. If the negative pressure in the ceiling space is fairly small (which is what we observed in most builings) and the ceiling space is reasonably tight to unconditioned spaces (which we found in some cases), then leakage associated with use of the ceiling space as a plenum can be relatively small. It is possible to design the ceiling return plenum to operate at neutral pressure with respect to outdoors. If outdoor air pressurizes the building to +2 pascals, for example, and the plenum runs at -2 pascals with respect to the occupied space, then the plenum will be at neutral pressure with respect to outdoors. Consequently, the plenum can experience next to no duct leakage. (Note that we have observed ceiling return plenums in several buildings operating at positive pressure with respect to outdoors, so in effect these return plenums are experiencing “supply leaks”.)

On the other hand, if ceiling space return plenums are rather leaky to outdoors and are significantly depressurized wrt outdoors, then use of the ceiling space as a return plenum will produce substantial air distribution system leakage. Use of the ceiling space as a return plenum can also be poor practice if the roof has little insulation R-value.

Outdoor air. Twenty-seven buildings have outdoor air. “Outdoor air” is air intentionally drawn into (or pushed into, if there is a dedicated outdoor air fan) the return side of the air distribution system and is generally designed to provide ventilation for occupants in the space. The other 43 buildings do not have outdoor air. They rely on naturally occuring infiltration, leakage in the distribution system, or operation of exhaust fans to bring in ventilation air.

Outdoor air only occurs when the air handler is on. Therefore, when the air handler is off, either because the system has been turned off, or the air handler has cycled off with the compressor, outdoor air ceases, building pressure will decrease, and the building ventilation rate may decrease.

Make-up air. Five buildings have make-up air. “Make-up” air is mechanically induced air blown into the building simultaneously with the operation of the exhaust fans. It is filtered, but in most cases (and in all five cases found in this study) it is unconditioned. Typically the same control switch activates both the make-up air and the exhaust air. Make-up air is designed to reduce the air flow imbalance on the building and thereby reduce space depressurization and space conditioning energy use. Typically make-up air is designed to be 75% to 80% of the flow rate of the exhaust fans (Gaylord Industries, Inc. Smoke Pollution Control Ventilator Model CG-AB-SPC technical bulletin).

As can be seen in Table 4.1, make-up air is substantially undersized in the five restaurants that have make-up air, averaging only 50% of exhaust air flow. Outdoor air is insufficient to make up the balance, averaging only 12% of exhaust air flow. Combined, make-up air and outdoor air represent only 62% of total exhaust air. As a consequence, these buildings operate at negative pressure.

TABLE 4.1.
Building air flow and pressure imbalance in five commercial buildings which have make-
up air and which contain restaurants. Note that while make-up air operates when the exhaust fans operate,
outdoor air may or may not occur when the exhaust fans
operate. Therefore, the net air flow balance may actually be worse than indicated.

In four other buildings containing restaurants, there is no mechanically induced make-up air. In one case, the kitchen exhaust fans are not operated at all (building #19). In another, an exterior kitchen door is left open throughout the day thereby reducing building depressurization from -26 pascals to -4.6 pascals (building #62). In the third, the building is depressurized to -1.8 pascals and passive make-up air comes primarily through the leaky t-bar ceiling from a residential-type attic (building #69). In the fourth, the exhaust fan operates 8 to 10 hours a day. After the duct leaks were repaired, the exhaust fan caused depressurization of -1.8 pascals and most of the passive make-up air comes through a leaky interior knee-wall facade above the bar (building #3 1). (See section 5.5.9 for additional discussion of building #31.)

Exhaust fans. Forty-three buildings have exhaust fans that discharge air from the occupied space to outdoors. There are a few additional buildings that have exhaust fans that discharge air into ceiling spaces that are within the air barrier of the building. Operation of these latter fans does little to affect building ventilation rates or pressure differentials. The size of exhaust fans (building total) in these 43 structures varies from as little as 44 cfm to as much as 10,606 cfm and averages 1767 cfm. In some buildings the exhaust fans operate virtually whenever the building is occupied, including restaurants, sports facilities, and some other buildings. In others, the exhaust fans are manually operated, either wired into the light switch or on a separate switch, and their operation time is then a function of user interaction.

4.3 Ducts Are Leaky

Testing of duct system airtightness was done in 46 of the 70 buildings. In a majority of these buildings, ducts were found to be very leaky. On average, CFM25 (air flow through leaks when the ducts are depressurized to -25 pascals) was 1209, or 341 CFM25 per 1000 square feet of floor area. By comparison, leakage in 100 central Florida residences (old and new together) was only 116 CFM25 per 1000 square feet, or about one-third as much (Cummings, Tooley, and Moyer, 1991). Note that the ducts measured for airtightness excluded ceiling return plenums in all but one case (building 17).

One factor that affects the amount of duct leakage is that commercial buildings have about twice the installed cooling capacity per 1000 square feet, so ducts in commercial buildings should have about 45% greater surface area. Nevertheless, even considering that difference, duct leakage in these small commercial building is disproportionately much greater than in residences. Note that SMACNA (Sheet Metal and Air Conditioning Contractors Natiohal Association) has set a leakage standard for ductwork that is much tighter than what is found in residences or commercial buildings. For Class 6 ductwork (ductboard, flex duct, and other sealed ducts), the standard expects 1.2 CFM25 per 100 square feet of duct surface area. For a typical residence, this might be 3 CFM25 per 1000 square feet of floor area. In commercial buildings, this might be 5 CFM25 per 1000 square feet of floor area. Therefore, duct systems in small commercial buildings are about 70 times more leaky than the SMACNA standard.

While the excessive leakiness of commercial duct systems is evident, the impact of duct leakage on energy consumption is not as clear. In some buildings, we expect that duct leaks have little impact on energy consumption when the ducts are fully inside the building. When they are outside the building or in an attic, the energy penalties should be quite large. However, when the ducts are partly inside the building, that is, they are inside either the primary air barrier or the primary thermal barrier, the energy impacts are less certain. We anticipate that a significant amount of energy lost from the duct leaks will be recovered in this scenario.

Energy savings from repair of UAF was examined by repairing UAF in 20 of the 70 buildings. Energy consumption in these 20 buildings decreased by an average 15%. There are still many questions to be answered about the savings that are likely to result from repair when the ducts are in. various environments. The candidates selected for energy monitoring generally had ducts that were outside the conditioned space and therefore would be likely to experiencc significant energy savings.

A number of factors dictate what, the energy costs of duct leakage will be. Not all duct leaks are created equal, even those that are the same size. The impact of duct leakage depends upon whether the ducts are completely “inside” the building, partially “inside” the building, outdoors, or -- in the worst case --‘ in a ceiling or attic space that is outside the air and thermal barrier ‘of the building. By “inside the building” we mean inside the primary air barrier, inside the primary thermal barrier, or inside both the air barrier and the thermal barrier. If the duct leaks occur in a space that is outside the building air and thermal barriers, then only a small portion of the energy lost by duct leaks may be recovered. If the ceiling space is inside the air and thermal barrier, then we expect that the majority of lost energy is covered to the occupied space.

Note that being located in an attic is even worse than being located outdoors. Attics are generally hotter than outdoors, especially during the portions of the day when air conditioning system operate the most. Therefore, any air drawn into duct leaks will be hotter (and generally have a higher dewpoint temperature) than outdoors.

Another factor to consider is the direction of mechanically driven air flow. Consider this example. Building 35; a fast food restaurant serving chicken, had a completely disconnected supply duct discharging approximately 500 cfm of air into the ceiling space. Insulation batts were located on top of the ceiling tiles. Kitchen exhaust fans were drawing about 9200 cfm of air from the restaurant’ while, on average, make-up air fans and outdoor air were discharging about 5800 cfm into the restaurant. The net air flow imbalance caused the entire restaurant to be depressurized to -43 pascals. This negative pressure was sucking air into the building from all directions, including from the ceiling space. Therefore, the approximately 500 cfm of supply air that was being dumped into the ceiling space would be almost immediately drawn into the occupied space. What portion of the lost energy was being recovered? 25%? 50%? 75%? 100%? We do not know. What would be the energy savings that would result from repairing that large duct leak? Again we do not know. Before retrofit programs can effectively determine which duct systems should be repaired, more investigation needs to. be done to understand the energy losses ‘associated with duct leakage in various types of ceilings spaces.

Consider another example, building 69. This is a convenience store with a small kitchen. A kitchen exhaust fan draws 1546 cfm from the building and depressurizes the space to -1.8 pascals. Since nearly all of the building leakage is in the ceiling, nearly all of the air drawn into the building comes from the vented, residential style attic. Since this air may be .11 OF or hotter in the summer, the’ energy impacts will be great. Consider, however, if make-up and outdoor air were added to the building so that it ran at a slight positive pressure, say +0.5 pascal. Air would then be passing from the occupied space into the attic. Under this new condition, cooling energy use would be much reduced. Savings are
greatest from make-up air, because this air does not need to be conditioned before being injected into the building and when properly designed, a large proportion of the make-up air is drawn directly into the exhaust air stream and is not distributed more generally into the occupied zones. Savings are also significant with outdoor air (compared to drawing air from the attic). Both outdoor air and air drawn in from the attic must be conditioned, but the air from outdoors is cooler and generally drier than air from the attic.

4.4 Ceilings Are Leaky

Commercial buildings, on average, are leaky, and the primary location for building leakage is the ceiling. ACH5O, the air exchange rate of the building when depressurized to -50 pascals by a blower door, averages 16.7, about 50% greater than Florida residences, and about 150% leakier than new Florida residences. Not only are these commercial buildings more leaky than residences, the range of leakiness is much greater. As can be seen in Figure 4.6, there is a bi-polar distribution of building airtightness, with peaks at the 5 to 10 ACH5O bin and at the 20 to 25 ACH5O bin.

This twin peak occurs, we believe, because of t-bar suspended ceilings which are used in the greatest majority of commercial buildings. (In our sample, 54 of 70 buildings have suspended, t-bar ceilings. Six of the remaining 16 have no ceiling other than the bottom of the roof deck. Six have gypsum board. Four have some other form of ceiling tiles but are not suspended on t-bar framework.) These suspended ceilings are very