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Better Breathing Thanks to Good Science

About 15 years ago researchers started applying scientific methods and instruments to the study of buildings.

June 16, 2001

As a result, over the last decade and a half, many cherished beliefs about buildings and construction have been turned upside down. One belief that I, as a product of the South, had a hard time surrendering was the notion that there was no such thing as an air conditioner that was too big.

Bigger is better is an American mantra and, when it came to air conditioners, it was unquestioned wisdom. If a three-ton air conditioner was good, a five-ton unit was better. Not that we really knew what a ton of air conditioning was, but we knew that we wanted as much of it as we could get.

Building science research has not only turned this assumption upside down but also has revealed that following this mantra can lead to serious health problems. The right size for an air conditioner is the size that will cool your home on a hot summer afternoon with only about 15% to spare. Why? We found that when we install more air conditioning than this, it will do what we call short cycling. When the air conditioning (A/C) unit is oversized, it can cool the house like it's doing a part time job. It only runs for a few minutes--maybe five or ten at a time--and then shuts down for a few minutes, before starting up again. In these short cycles it cools the house, but it does a very poor job of removing humidity.

Usually as an A/C cools the hot air in a home, moisture condenses out of the air and gets discharged into the condensate line, because cooler air can not hold as much moisture as warmer air does. But an A/C doesnˆt get cold enough to remove water until it has run for three to five minutes. It also canˆt pull enough indoor air through the unit to wring out the water during such short run times. So homeowners get the cave effect in which a home is cool, but damp feeling. The excess moisture that builds up in the house encourages mold and dust mites to reproduce like madònot a good situation for the occupants.

In my work as a home performance specialist, I see a fair number of people who get sick unknowingly from just such a problem. One day I received a call from a woman who complained that her home smelled like a boys' locker room on Friday afternoon. She also told me that their grandson got sick every time he stayed with them and left coughing with a runny nose. She too had begun to have allergies all the time. I asked about the air conditioning and was told that it was fine. They had just replaced it during the previous winter with a new high efficiency one.

I got to the house and found that her nose was quite accurate. The house had a pungent odor that was very reminiscent of old sneakers. The humidity in the home was around 68% and they were keeping the house at a cool 71 degrees. I still found the house uncomfortable as I am not one who enjoys experiencing relative humidity levels of more than 55%.

I checked the home for obvious sources of moistureòaquariums, lots of plants, or a steam roomòbut found none. I also looked for filter marks in the doorways, any big exhaust fans, or other indications that the house experienced pressure swings and found nothing. My pressure testing told me that the house was operating at a negative pressure of 3.9 Pascals, a sure sign that the air conditioning ducts were leaking pretty badly.

Even with the bad duct leaks, the owners told me that the new A/C unit only ran for five or ten minutes each time it came on during the hottest part of the afternoon. I then went out to see what the ground looked like where the A/C unit's condensate line exited the home. The soil was a little damp, but there was nothing like the amount of water coming out of that line that one should see in a home like this. These signs began to add up to a familiar picture.

At the urging of all the A/C contractors they had talked with, these homeowners had replaced their old three-ton unit with a new four-ton A/C system. They had gotten a bigger unit because when they told the A/C contractors that the old unit sometimes didnˆt keep the house comfortable, the solution they all immediately offered was more tonnage. Because bigger is betteris the standard response to any complaint, I was not surprised to hear this. I knew from experience that a home of only 1,540 square feet that was built in the early ˆ90s didnˆt need a four-ton unit. I also knew that the ducts were leaking very badly and that if they had been fixed, the old system would have been able to cool the home just fine.

When the new unit, which was oversized by 1.5 to 2.0 tons, ran in its very short cycles through the leaky ducts, it did not stand a chance of properly dehumidifying the house. The resulting high indoor humidity levels created a perfect incubator for molds. The house had a good crop behind the vinyl wallpaper in the kitchen, in both bathrooms, and under the paneling in the living room.

To fix their problems, I got the leaky ducts sealed up, downsized the system to a 2.5-ton system, and remediated the mold. (I told them a 2.0-ton unit would do the job, but that was too much change too fast for them.) All was well, and both the old and young folks stopped their constant coughing and sneezing. Like many home performance problems, we needed to address the system, not just one item, to get the desired outcome.

So how do you assure that your air conditioner is properly sized? Sizing an air conditioner should be done according to the Air Conditioning Contractors of America (ACCA) Manual J, Sizing Calculation Methodology. If you ask a contractor to install an air conditioning unit sized to ACCA Manual J and you get a blank stare, move on to another contractor. The new unit will probably be smaller, but will do a better job of cooling and dehumidifying your home at less cost. ACCA, the contractors own association, admits that the average air conditioner in this country is from 150% to 200% as big as it really needs to be.

by Doug Garrett is principal of Building Performance and Comfort, which is based in Leander, Texas.

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