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Reference Publication: Cummings, J. B. and C.R. Withers, "Identifying Air Flow Failure Modes in Small Commercial Buildings: Tools and Methodologies for Building Commissioning Diagnostics." Fifth National Conference on Building Commissioning Proceedings, Huntington Beach, CA. April 1997. Disclaimer: The views and opinions expressed in this article are solely those of the authors and are not intended to represent the views and opinions of the Florida Solar Energy Center.

Identifying Air Flow Failure Modes in Small Commercial Buildings:
Tools and Methodologies for Building Commissioning Diagnostics

James B. Cummings and Charles R. Withers
Florida Solar Energy Center (FSEC)

FSEC-CR-1666-97

Synopsis

A recent study of small commercial buildings in central Florida found that uncontrolled air flow -- including duct leakage, return air imbalance, and exhaust air/make-up air imbalance -- is widespread. Of 70 buildings studied, only 1 was identified as having no significant uncontrolled air flow. The causes of uncontrolled air flow include failure of design, poor workmanship, O&M problems, HVAC commissioning failures, materials degradation, and building retrofits. This study also found that the consequences of uncontrolled air flow are often quite severe and varied -- including high utility bills, occupant thermal discomfort, high humidity, mold and mildew growth, moisture damage to building materials, transport of pollutants to the occupied space, and backdrafting of combustion equipment. The characteristics and causes of uncontrolled air flow have been largely unknown or misunderstood until recently, and diagnostic tools and methodologies for uncovering uncontrolled air flow have been largely unavailable. Standard methods of ensuring balanced air flows often fail because of measurement errors and flawed assumptions of test methodology. This paper introduces the reader to the nature and magnitude of UAF problems in commercial buildings, presents air flow management standards, and presents diagnostic tools and methods.

About the Authors

James B. Cummings (M.S. Trinity University) is a Senior Research Analyst at the Florida Solar Energy Center (since 1985) where he has done research on duct leakage in residences, uncontrolled air flow in commercial buildings, and indoor air quality impacts of building failure modes. He has extensive hands-on field experience in testing over 200 residences and 70 commercial buildings, including blower door, tracer gas, air flow, infiltration, pressure differential, and IAQ measurements. He has worked extensively in the development of building air flow and pressure diagnostic methods and tools. He is author to duct repair, weatherization, and HERS training manuals, as well as over thirty reports and published papers.

Charles R. Withers, Jr., Research Analyst at the Florida Solar Energy Center, has done extensive field testing and monitoring in residences and commercial buildings. He has many years of hands-on experience testing airtightness, air flows, air pressure, combustion safety, and air contaminants in buildings, and has been a contributor to a number of important diagnostic testing methodologies. He has published a number of reports and papers in the areas of duct leakage, diagnostic methodologies, and indoor air quality.

Introduction

One of the most important causes of building failures is uncontrolled air flow (UAF). UAF is defined as air moving across the building envelope or between zones or compartments of a building, where the pathways of flow, the direction of flow, and the origin of the air are unknown, unspecified, or unintended. UAF has a wide range of consequences which include high energy use, comfort problems, excessive ventilation, pressure imbalances, HVAC sizing problems, high RH, building moisture damage, mold and mildew (M/M) growth, combustion safety concerns, and indoor air quality problems. Until recently, the extent of the problems associated with UAF was not well understood. This paper introduces the reader to the nature and magnitude of UAF problems in commercial buildings and presents diagnostic tools and methods. The authors feel this information is important to building commissioning because many building failures result from UAF and the diagnostic methods and tools presented in this paper should be incorporated into standard commissioning practice for new and existing buildings.

Causes and Consequences of Uncontrolled Air Flow

UAF testing was done in 70 small commercial buildings, ranging from 880 to 22,500 square feet (sf) (Cummings et al., 1996a). Buildings 0 - 25,000 sf size represent 90% of all commercial buildings in the United States and 38% of all commercial building floor space (DOE/EIA, 1989).

Four Types of Uncontrolled Air Flow

There are four primary types of UAF: 1) duct leakage, 2) restricted return air (RA), 3) imbalance between exhaust and intake air flows, and 4) leaky buildings or excessively vented buffer zones.

Duct Leakage

Duct leakage was measured in 46 of the 70 buildings; it averaged 341 CFM25 per 1000 sf of floor space (CFM25 is air flow through leaks when the ductwork is at -25 pascals). This is about three times that in Florida residences and 70 times as leaky as the SMACNA duct leakage standard (SMACNA, 1985). The magnitude of duct leak impacts on energy use, RH, infiltration, and building pressures depends largely upon duct location in relation to the building air and thermal barriers. Ducts are found in three general locations (notes in parentheses apply to hot/humid climate summer; additional discussion in Cummings et al., 1996a); 1) within both the air barrier and thermal barrier of the building (cool and dry), 2) within the air barrier but outside the thermal barrier -- roof is the air barrier but insulation is located on the ceiling (hot and dry), 3) outside of both the air and thermal barriers of the building (hot and humid). If duct leaks occur inside the building air and thermal barriers, they cause neither significant energy waste nor increased infiltration. If the duct leaks occur inside the air barrier but outside the thermal barrier, they cause energy waste but no increase in infiltration. If the duct leaks are located outside both air and thermal barriers, then significant increases in energy use and infiltration result.

Restricted Return Air Pathways

In many small Florida commercial buildings, supply air (SA) is provided to each room but returns are located only in the central zone. Therefore, when interior doors are closed, the closed rooms are pressurized and the central zones are depressurized. High pressure in the closed rooms pushes air from the room to outdoors and to the ceiling space above. Depressurization in the central zone pulls air from outdoors and the ceiling space above. Suspended t-bar ceilings are very leaky, so most of the UAF occurs across the ceiling. The energy and infiltration impacts of restricted RA depend, therefore, in large part upon where the air and thermal barriers are located. If both are at the roof (ceiling is cool and dry), then the energy and infiltration impacts will be minimal. If the air barrier is at the roof but the thermal barrier is at the ceiling (ceiling is hot and dry), then the energy impacts will be large but the infiltration impacts will be minimal. If both are at the ceiling (ceiling is hot and humid), then the energy and infiltration impacts will be large if the ceiling is t-bar construction, or small if the ceiling is tight drywall construction.

Another form of restricted RA occurs when the ceiling space is used as a return plenum and fire walls in the ceiling space restrict air returning to the air handler (AH). "Crossover return windows" are generally, but not always, provided (with fire damper) to allow RA flow, but these are often undersized and sometimes the fire damper fails in a closed position. This creates higher pressure in zones further from the AH and lower pressure in zones closer to the AH. The pressure imbalance can create significant infiltration by pushing air out of the building (high pressure zone) and pulling air into the building (low pressure zone).

Unbalanced Exhaust and Intake Air Flows

In some commercial buildings -- especially restaurants, recreation facilities, and hotels -- large exhaust fans (EA) draw air from the building. Make-up air (MA) and outdoor air (OA) provide incoming air to balance the EA. However, combined MA and OA are typically less than total EA. In 3 of 8 tested restaurants, no MA or OA was provided. In the remaining 5 restaurants, MA and OA totalled only 65% of EA flow (Table 1.). Consequently these restaurants operate at substantial negative pressure. The UAF and resulting space depressurization produced high indoor RH, wall moisture accumulation, wall M/M growth, drywall moisture damage, loose wallpaper, uncomfortable room temperature, extinguished pilot lights, backdrafting of combustion appliances, flame roll-out from gas water heaters, and gases being pulled from sewer lines. In one restaurant, signs on the exterior doors stated "Please Pull Hard" because space depressurization was so great that people thought the doors were locked (see Cummings et al., 1996b for additional discussion). Examples follow: Example 1. In an 8-month-old restaurant, depressurization ranged from -3 to -17 pascals as air handlers (each with OA) cycled on and off with load. Depressurization caused M/M on walls, loose wallpaper, and backdrafting and flame roll-out from the water heater. Example 2. In a 10-month-old golf club house, kitchen and locker room EA produced -6 pascals. This drew humid attic down walls where it collected in cool wall materials, causing 40% of the wallpaper to be removed.

Table 1.
EA, MA, and OA (cfm) imbalance causes depressurization (pascals) in five restaurants.

	EA	MA	OA	NET	dP
golf club house	-3038	1107	0	-1931	-6.1
hotel	-12907	5140	815	-6952	-3
sub restaurant	-5603	3220	1450	-933	-25
chicken restaurant	-9222	5110	1250	-2862	-8
chicken restaurant	-10616	6157	2253	-2206	-43
Average	-8277	4147	1154	-2977	-17.0

Leaky Buildings or Excessively Vented Buffer Zones

In some commercial buildings, the building shell is very leaky, leading to excess ventilation, RH control problems, and energy waste. If building buffer zones are excessively ventilated, then condensation may occur on ductwork and pipes, causing wet ceilings and M/M growth.

Six Factors Which Result in Uncontrolled Air Flow

Six factors have been identified as major contributors to the occurrence of UAF in buildings; 1. failure of design, 2. poor workmanship, 3. HVAC commissioning failures, 4. O&M problems, 5. materials degradation, and 6. building retrofits.

Failure of Design

UAF may occur from design failure. In some small buildings, no HVAC system design occurs. HVAC system sizing, layout, and fabrication decisions are left to the contractor. In other cases, HVAC design is done but does not adequately provide for balanced RA or EA flows.

Failure of Workmanship

Poor workmanship often causes UAF. Failure to seal ducts, leaving the end of ducts wide open, flex ducts that fall off, missing or closed cross-over windows in fire walls, using building cavities as ducts (AH support platforms, wall cavities, chases, shafts, and mechanical closets are almost always leaky), and locating returns in zones served by another system.

Failure of Current HVAC Commissioning Practice

Current HVAC commissioning practice (Test and Balance; TAB) does not adequately deal with the four forms of UAF problems (section 2.1). TAB does not typically measure duct airtightness or air leakage from ductwork. Also, it often does not take into account duct leakage when determining building and zone air flow balances, often assuming that air flows at grills and registers represent total system flows. TAB generally measures air flows at grills/registers by flow hood or in ducts by means of pitot tube traverses. These measurements are generally not at the building air boundary, and therefore duct leakage which exists in those duct systems is not accounted for in the TAB test method. Figure 1 illustrates duct leakage to and from various locations -- to and from indoors, to and from buffer zones, and to and from outdoors.

Figure 1. Duct leaks occur in EA, MA, OA, RA, and SA -- to and from indoors, buffer zones, and outdoors. Measurement location is important for determining building air flow balance.

Consider an example related to duct leakage. EA is specified as 40 cubic feet per minute (cfm) for each of 200 guest rooms; total equals 8000 cfm. Exhaust fans, located at the roof, actually draw 8000 cfm from building shafts which act as exhaust plenums. Ducts, in turn, run from the exhaust plenums to the guest rooms. TAB measurements at the grills find an average of 27 cfm per room. The remaining 13 cfm per room leaks into the plenum mostly from indoors and some from outdoors. Concluding that EA was insufficient, TAB personnel increase EA to achieve 40 cfm per grill. As a consequence, the EA now pulls about 11,000 cfm from the building. While hotel pressure was +1 pascal before TAB, it is now -6 pascals. Best practice commissioning would measure air flow at the grills and at the EA discharge, require duct leaks be fixed, and check that the building operates at positive pressure (by measuring pressure!)

Consider another example related to duct leakage. A five-story government office building has OA for each of its 11 AHs. TAB measured OA by means of pitot tube traverse, but actual OA was less than measured. On the first floor, one OA duct ran from the air AH to a grill in the exterior wall of the building. It stopped, however, 1 inch short of the grill; 70% of the "OA" was coming from the mechanical room. On the second floor, the OA ducts for the two units ran from the AHs to the exterior walls, through plywood panels, and presumably to grills outdoors. It turned out, however, there were no grills (only solid brick) on the other side of the plywood, and the air flow that the TAB had measured by means of pitot tube traverse was actually air leaking from indoors. As a consequence of these OA failures, the building experienced less ventilation and less positive pressure than expected.

Consider an example related to restricted RA. In the same five-story building, fire walls restricted RA in a ceiling space used as a return plenum. "Cross-over return windows" were located in the fire walls to allow RA flow between zones. However, these were undersized by a factor of three, causing zones pressures ranging from -8 to +17 pascals with respect to (wrt) outdoors. These pressure imbalances were pushing air out of and drawing air into the building. TAB did not detect or report these RA imbalance problems.

Operation and Maintenance Failure

Failure to maintain HVAC systems can cause UAF. Condensate drains often have disconnected, broken, or leaking traps. These dysfunctional traps allow air to enter the drain pipe at high velocity, leading to ponding in the drain pan, overflow into the AH, and splattering of ponding water into the AH. This can lead to poor AC dehumidification performance and wetting of AH and duct surfaces, creating a fertile environment for M/M growth (Trent, 1994).

Dirty filters can create UAF. Upon completion of a restaurant, a HVAC contractor demonstrated to the owner that the building was at positive pressure using a smoke pencil. Some 9 months later, the building was sufficiently depressurized to backdraft the instantaneous water heater, extinguish its pilot light, and pull odorous gases from an improperly seated toilet. Inspection found that the filters for the AHs, OA, and MA had never been cleaned. After 9 months, these filters were all "caked" While EA was at full flow, OA and MA were greatly reduced. Cleaning or replacing the filters increased building pressure from -25 to -2 pascals.

Materials Failure

The most common material-related failures are associated with duct tape. Tape adhesives, when exposed to high temperatures environments, often loose their adhesion resulting in duct leakage.

Building Retrofits

When new tenants move into a commercial space, it is common to reconfigure the interior layout by moving interior walls. This can lead to restricted and unbalanced air flows. Pathways for RA are often blocked. In other cases, return grills are located in zones served by other AHs. Some of the most extreme cases of UAF in the 70-building study were caused by retrofits. This points to the fact that commissioning is necessary for retrofit as well as new construction.

Air Flow Standards and Diagnostics

Commissioning, as understood by the authors, is the process by which design, construction, building start-up, and O&M procedures are optimized, by means of a commissioning plan and agent, so that the building performs in accordance with the needs of the occupants. In order to achieve successful commissioning of building air flows, two elements are necessary:

1. airtightness, air flow, and air pressure standards
2. test methods and tools which can verify whether these standards are met.

Airtightness, Air Flow, and Air Pressure Standards

In order to control temperature, humidity, and indoor air quality, it is essential to control building air flows and pressure differentials. In order to work toward this goal, the building production team (including the commissioning agents) should work toward the following standards:

1. Fairly tight building shell. Aim for ACH50 between 4 and 8 (ACH50 is the air flow rate across the building envelope when the building is at -50 pascals wrt outdoors). If much tighter, unbalanced air flows can easily create extreme pressure imbalances. If much looser, it will be difficult to control the direction of flow across the building envelope and natural infiltration may become excessive. To achieve a tight shell, do not rely on the suspended t-bar ceiling as an air barrier; it is very leaky. Best choices: 1) make the roof deck the air barrier and 2) locate the thermal barrier (insulation) at the air barrier.
2. Tight duct systems. Ducts should be fabricated and sealed with mastic and fabglass so they will be airtight and durable for the life of the building. Tight ducts will ensure even distribution of heating and cooling and proper flow rates for EA and OA.
3. Balanced return air system. Provide sufficiently large and unrestricted pathways for RA. If a ducted return system, locate the return grills in the correct zones and size for the space they serve. If a ceiling return plenum, provide adequately sized "crossover windows" in firewalls. If central returns, provide properly sized return transfers for when interior doors are closed. In general, aim for pressure differentials across closed doors of 2 pascals or less.
4. Building at positive pressure wrt outdoors. Positive pressure keeps exterior walls closer to room temperature, and temperature and humidity will be more uniform in the space. It reduces energy use since infiltration from attics will be reduced. It moves dry interior air through exterior wall cavities thus avoiding many moisture-related problems. It minimizes "mining" of pollutants from soil, sewer lines, and combustion equipment. It achieves better space dehumidification, since the OA can be dehumidified at its point of entry more effectively than removing the moisture after it has mixed throughout the building.

Diagnostic Methods and Tools

For new buildings, diagnostic testing can be (should be) done to verify that airtightness, air flow, and air pressure standards are met. For existing buildings, diagnostic testing can be done to identify failure modes and indicate retrofit solutions. Diagnostic approaches to identifying failure modes and solutions include the following; characterization of building airtightness, air flows, mechanically induced pressure differentials, and infiltration/ventilation rates.

Building Airtightness

Building airtightness, or the cumulative hole size of the building envelope, cannot be measured directly by use of a ruler or measuring tape, because the cracks and penetrations in the building envelope are widely distributed. It can be measured only indirectly by characterizing the air flow rate through the building envelope when the building is depressurized by a specified amount (50 pascals), typically by means of a blower door; this is called CFM50.

Building airtightness can also be characterized by ACH50, which is air changes per hour when the building is at -50 pascals. ACH50 is calculated from CFM50.

ACH50 = CFM50 * 60 / building volume

Consider an example: a building has 6400 sf of floor area with 9 foot ceilings, and CFM50 is 4900. ACH50 = 4900 cfm * 60 / (6400 sf * 9 ft) = 5.1 ACH50. This building, therefore, falls within the recommended airtightness range of 4 to 8 ACH50.

Building airtightness can be measured using HVAC equipment, especially EA. Various building fans are turned on to produce negative or positive pressure. Consider a restaurant that has EA, MA, and OA. You can turn off the MA and AHs, and mask off the MA and OA intakes. Then you can measure EA using tracer gas injection or a capture tent (see section 3.2.2). Let's say total EA was 1850 cfm and building pressure goes to -15 pascals. You can refer to a chart such as Figure 2 to determine that approximate building airtightness is 4000 CFM50. (Caution: turning off the MA and OA could produce -100 pascals or more, if the EA flow is large or the building tight, which could burst windows. Therefore, hold an exterior door open when turning these fans off, and then observing building pressure as the door is closed.)

Figure 2. Air flow versus airtightness pressure chart. This chart allows prediction of pressures, air flows, or airtightness, when 2 of 3 variables are known. Diagonal lines are pressure (pascals). Assumes airtightness curve exponent n = 0.65. (Adapted from a chart developed by Natural Florida Retrofit).

Building Air Flows

Building air flows can be differentiated as 1) those that recirculate within the building and 2) those that pass across the building envelope. Those that recirculate are primarily the RA and SA of heating/cooling systems. In the absence of duct leakage or restricted RA, these recirculation air flows do not affect building pressure and infiltration rates. Those that pass across the building envelope include EA, MA, OA, return leaks from outside, supply leaks to outside, and air flows induced by RA imbalance. Characterization of air flow across the building envelope is important because these air flows impacts energy use, indoor RH, ventilation, and pressure differentials.

There are many methods and tools of air flow measurement, including air flow hood, pitot tube traverse, capture tent, building as a capture tent, calibrated blower attached to an HVAC system, and tracer gas injection. (More detail is found in Cummings et al., 1996a).

Air flow hood. This instrument is placed over registers/grills and provides air flow readings.

Pitot tube traverse. This instrument is inserted into holes drilled in the ductwork to obtain velocity measurements in a cross-sectional matrix. Average velocity through the duct section is determined; velocity is multiplied by the cross-sectional area to obtain volumetric flow. To obtain accurate readings, the traverse must be made at a location at least five duct diameters downstream from elbows or constrictions in the ductwork.

Capture tent. This tent-like enclosure is placed over an air flow orifice. A calibrated blower is installed into the side of the tent. Air flow into or out of the tent at the orifice is matched by the calibrated fan to keep the tent at neutral pressure. Air flow through the calibrated fan is then equal to the air flow into or out of the orifice. Consider an example. A tent is placed over an EA "mushroom" on a rooftop. A calibrated fan is installed into the side of the tent. With the calibrated fan off, the EA discharging into the tent creates positive pressure in the tent. Then the calibrated fan is turned on and its speed adjusted until the tent is brought to neutral pressure wrt the surrounding environment. The EA flow rate equals the flow rate of the calibrated fan.

Building as a capture tent. The building itself can be used as a capture tent. This technique works best in buildings which are either fairly airtight or have large air moving equipment. In this application, a blower door or other calibrated fan is installed in an exterior doorway of the building, to pull air out or push air into the building to match the HVAC air flow rates.

The HVAC systems are turned on in their normal operating mode. Building normal operating pressure (NOP) is measured. As an example, let's say that a building has EA, MA, and OA, and NOP is -10 pascals. Each of these air moving systems can be individually turned off, and the blower door is then operated in its place to maintain the building at NOP. Let's say masking off the OA (AH still operating) causes building pressure to go to -19 pascals, but pushing 1850 cfm into the building using the blower door returns the building to NOP (that is, -10 pascals). This means OA flow is 1850 cfm. Then MA can be turned off (and masked off) and the blower door speed increased to again achieve NOP. Let's say the blower door is now moving 3850 cfm into the building. MA is then the blower door flow rate less OA; MA = 2000 cfm. Finally turn on MA, unseal OA, and turn off EA. Use the blower door to pull air out of the building until NOP is achieved. The air flow rate through the blower door will be equal to EA.

Calibrated fan attached to HVAC system. In this application, a calibrated fan can be mounted directly onto an air moving appliance. Consider an example of measuring an AH flow rate. First measure the NOP in the supply plenum with the AH operating. Then turn off the AH, block the opening between the AH and the return plenum, remove a panel in the AH cabinet, and mount the calibrated fan into the cabinet opening, masking off the remainder of the opening. Then turn on the AH; with the return blocked the AH must draw its air through the calibrated fan. Turn on the calibrated fan to blow air into the AH and increase fan speed until the pressure in the supply plenum is at NOP. AH flow rate then equals the flow rate through the calibrated fan.

Tracer gas injection. In this application, tracer gas is metered into an air stream, such as into a duct, plenum, or AH, and the concentration of tracer gas is measured upstream and downstream of the injection point using a gas analyzer. The increase in mixed tracer gas concentration from before to after the injection point indicates the air flow rate in that duct section. Air flow is calculated by means of this equation (Grieve, 1991):

q = dose/(Cs - Cb)

where q is the air flow rate (cfm), dose is the tracer gas injection rate (cfm), Cs is the tracer concentration downstream at the sample point, and Cb is the tracer concentration upstream of the injection location. This test method has the advantage of working for a wide range of air flow rates and air flow configurations. It can be used to measure air flows in EA, MA, OA ducts, return and supply duct sections, and AHs.

We can view these air flow measurement options as a bag of tools. But which tool should be used and why would we want to use one particular tool over another? The answer depends upon a number of variables, including:

1. Where the air flow measurement is taken.
   • If you want to know EA flow rate from the building, the measurement should be taken close to the building air boundary (often at the roof level). Therefore, measurement of EA at the grills will not tell you what EA is leaving the building if there are EA duct leaks.
   • If you want to measure AH flow, measurement at the returns or supplies will not provide the correct answer if there are duct leaks, and there usually are duct leaks.
   • If you want to know OA flow into the building, it should be measured at the location where it enters the building envelope. A pitot tube traverse may not give an accurate answer if there are duct leaks.
2. Not all tools will work at all locations. Air flow measurement tool selection depends upon several variables: the air flow rate; whether the flow is in a duct section, entering a grill, or discharging from a exhaust fan housing; the shape of the air discharge; and the ductwork geometry. Air flow hoods typically cannot measure air flows of greater than 2500 cfm or EA discharge from most exhaust "mushrooms" the standard hood is too small. A pitot tube traverse cannot be used in duct sections with tightly spaced turns, because the straight section requirement cannot be met.
3. Speed and ease of use. Tool selection depends upon the situation. In general, if an air flow hood will work, it is the fastest and easiest measurement tool. However, in the numerous other situations in which a flow hood is not appropriate, the next fastest option is often the best choice. The building as a capture tent is often the fastest choice for determining the air flow rates of EA, MA, and OA. Tracer gas injection is often faster than using a capture tent. The capture tent is often faster than a calibrated fan attached to an HVAC appliance.
4. Measurement accuracy. Different tools have different accuracies depending upon their application and how the tool is used. Air flow hoods can be accurate within + 5% in many applications, but can overstate air flow rates from supply registers that "jet" to one side of the hood by as much as 80%. Pitot tube traverses can be quite accurate, but accuracy depends upon the skill and diligence of the technician and the distance from duct turns. Use of the building as a capture tent depends in large part on the relative size of the air flows being measured, the airtightness of the building, and background pressure variability due to wind. Ultimately, the decision will come down to how accurate does the measurement need to be.
5. Test disruptiveness. You may choose a test method which is less disruptive to the occupants such as measurements which can be done from the mechanical room or the rooftop instead of those that would require accessing ducts through ceiling tiles on ladders in occupied zones.

As users become familiar with the various air flow measurement options, they will gain experience in determining where the measurement should be taken, what tool will work in which locations, which tool is most time efficient, and you will learn to weigh these variables against "how accurate must the measurement be?" and "what test method will be the least disruptive?"

Building Pressure Differentials

Pressure mapping is an essential tool for determining the operational health of the building and air flow systems. Pressure mapping tells you whether the building is at positive or negative pressure, whether combustion equipment will backdraft, the direction of air flow, where air is coming from, and whether moisture problems are likely to occur. It also tells you which type of duct leakage is dominant, whether RA restrictions exist, and whether the building meets the pressure differential design specifications. Pressure mapping involves identifying the pattern of pressure differentials which exists in the building zones with the AH off, with the AH on, with interior doors closed, and with exhaust fans operating. It can be performed as one-time measurements using portable, hand-held manometers, preferably with auto-zero and time averaging capabilities. It can also be done with computer interfaced, multi-channel manometers over periods of time to see the time variations which occur during actual building operation.

Infiltration/ventilation Rates

The tracer gas decay (TGD) test is an effective tool for verifying building air exchange rates. TGD tells the building infiltration/ventilation rate as the building actually operates, under the specific set of weather and HVAC conditions which exist for that specific test period. This test also provides confirmation of the maximum air flow across the building envelope, since the ventilation rate of the building is often equal to the maximum air flow rate across the building envelope. (Example: a building has EA = 2000 cfm and OA = 3000 cfm. Dominant OA of 3000 cfm will produce a building ventilation rate of approximately 3000 cfm.) There are two exceptions: 1) When restricted RA causes some zones to be at positive pressure and other zones to be at negative pressure, pushing air out of and pulling air into the building. Resulting infiltration depends upon the relative airtightness of the partitions causing the RA restriction versus the building exterior envelope. 2) When building pressure is near neutral, especially in leaky buildings, natural infiltration occurs and this adds to the mechanically induced ventilation. This test is performed by injecting a tracer gas into the building and mixing it with fans. It often works well to inject the gas into the RA and let the AH mix the gas for 15 to 20 minutes. The decay rate of the tracer gas concentration indicates the exchange rate with outdoors by the formula ach = (60 / n) ln(Ci / Cf) where n is the number of minutes of the test, Ci is initial concentration in parts per million (ppm), and Cf is the final concentration (ppm).

References:

Cummings, J.B., C.R. Withers, N. Moyer, P. Fairey, and B. McKendry, "Uncontrolled Air Flow in Non-Residential Buildings; Final Report" FSEC-CR-878-96, Florida Solar Energy Center, Cocoa, FL, April 1996a.

Cummings, J.B., C.R. Withers, N. Moyer, P. Fairey, and B. McKendry. "Field Measurement of Uncontrolled Air Flow and Depressurization in Restaurants". ASHRAE Transactions, Vol.102, Part 1, p.859, 1996b.

Grieve, P.W. 1991. Measuring ventilation using tracer-gases. Bruel & Kjaer, BR 0608-12.

DOE/EIA-0246(89), "Commercial Buildings Characteristics 1989", Energy Information Administration, Commercial Buildings Energy Consumption Survey, June, 1991.

SMACNA, "HVAC Air Duct Leakage Test Manual", 1st Edition, Sheet Metal and Air Conditioning Contractors National Association, 1985

Trent, W. and C.Trent. 1994. Indoor air quality and the condensate trap. Presented at the Ninth Symposium on Improving Building Systems in Hot and Humid Climates. Arlington, TX.

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