July 2006

New FSEC Study Offers Strategies for Improving
Air-Conditioner Dehumidification Performance

Making purchases of almost any type of home product or appliance, from a new refrigerator to living room furniture, can be difficult because of the many choices, varied options and wide range of prices.

The problem of selecting the right product, however, can be especially difficult when choosing a new home air conditioner, particularly for people living in hot, humid climates where the unit is used so often during much of the year and rising energy costs can make the wrong choice a major cash guzzler.

photo of man unscrewing panel from aire conditioning unit

FSEC researcher Don Shirey prepares
air handler for laboratory testing.
Photo: Nicholas Waters

A recent research project by Don Shirey and Richard Raustad of FSEC, assisted by former FSEC staff member Hugh Henderson, provides insight into understanding and quantifying the moisture removal performance of cooling coils at part-load conditions (e.g., when the compressor of a residential air conditioner cycles on and off), which under certain circumstances can dramatically reduce the system’s dehumidification ability and possibly even cause indoor air quality problems.

The researchers conducted detailed performance measurements on eight cooling coils, collected performance data at seven field test sites, used an engineering model to look at dehumidification performance degradation, and reviewed the existing literature on the subject to come up with some “best practices” that will help homeowners choose and operate their air conditioners.

One major finding clearly supported the importance of proper equipment sizing. Single-stage cooling units that operated at less than half of their full load with the supply air fan running continuously (fan “ON”) provided very poor dehumidification. Homeowners need to be sure their air conditioners are not oversized as compared to the expected cooling loads, or the resulting time operating at part load can result in higher humidity levels in the home.

They found that having two stages of cooling capacity also greatly reduced the degradation of dehumidification performance at part load. And the study also confirmed that operating a system in the “AUTO” fan mode, with the supply air fan cycling on and off with the compressor, is the best strategy for improving the system’s dehumidification performance.

photo of computer screens and data logging equipment photo of air conditioning unit in lab
The laboratory facility was comprised of a control room (left) and two chambers to control various air conditions. The chamber used to control typical outdoor temperatures is shown.
Photos: Richard Raustad

Shirey noted that the project, which the researchers worked on for more than three years, came up with some hard data to support a number of strategies that would help improve an air conditioner’s performance and maximize its energy efficiency. “We really wanted to conduct a study like this for years since many ideas about part-load dehumidification performance and techniques for improvement have been suggested, but we needed data measurements under controlled conditions to prove or disprove them,” he explained. “For example, sizing can clearly make a huge difference, especially in terms of moisture removal. Continuous supply fan operation, while the compressor cycles on/off to satisfy the thermostat, is going to reduce the dehumidification ability of the system.”

Another suggestion for homeowners is to pay attention to how the supply air fan is controlled after the compressor turns off. “Many systems have the supply air fan continue running for a certain period after the compressor is shut off (e.g., 1 to 3 minutes),” he added. “This is fine when humidity levels are not a problem and extra cooling left in the system from the cold coil can be used. But in humid climates like Florida, this feature needs to be disabled or the air flowing over the wet coil is going to bring this moisture back indoors.”

A problem in doing this is that while many air-conditioning technicians know their brand of systems well, they won’t always know how to disable this feature on other units, and it’s not always obvious what needs to be done. Florida homeowners should insist that this “fan overrun” strategy be disabled.

In addition to coming up with suggestions for homeowners, the researchers noted some strategies for the manufacturers to consider for future units. Manufacturers need to slow down the indoor fan proportionally when the system operates at lower capacity. “Some manufacturers understand the importance of this in terms of dehumidification performance,” he pointed out, “but not all manufacturers have gotten the point yet. I suggest that consumers do their homework and learn about the performance of systems they are considering before choosing their next home air conditioner.”

One way to do that homework is to read through the FSEC contract report of their study at http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1537-05.pdf (13Mbyte). The full 613-page document is a lot more than most people will need to get the key recommendations. But if you’re concerned about operating your air conditioner efficiently and letting it keep the house at a comfortable dehumidification level, you ought to look through at least the report’s abstract and conclusions: http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1537-05-es.pdf. It makes it clear that sizing a system for peak performance doesn’t always result in the performance you want when the system operates at less than full load.

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