Air Handlers: An Appliance of Airtight Defiance?

Charles R. Withers and James B. Cummings, Florida Solar Energy Center

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Abstract

Many studies have been performed around the U.S. that quantify the air tightness of entire air distribution systems; however, there is very little published data on air handler tightness. Even relatively small holes in or near the air handler can be critical since the static pressures during system operation are greater in this area than in any other part of the air distribution system. A study of air handler tightness was conducted using 69 heat pump, gas heat, and hydronic heat systems. Air handlers in Florida single-family homes built after January 2001 were tested during the period from June 2001 through June 2002. Testing occurred in 23 units in attics, 23 units in garages, and 23 units indoors. The study finds that leakage in the air handler cabinet alone is sufficient to disqualify some air distribution systems from receiving “leak free” credit in energy codes. On average the air handler and duct connections to it have a Q25, total that is 2% of total system rated airflow, allowing only 3% more to be spread over the hundreds of square feet of the entire duct system under 1998 IECC standards. This paper will discuss the tested air leakage rate, Q25, total, of air handlers and the significance of operational leakage rate, Q, at the positive and negative pressure regions. Characterization of air handler leaks will explain why the air handler and duct connections to it have an operational leakage that is about 5% of the total rated air flow on average, and what can be done about it.