Last week I discussed humidity and the importance of a properly sized and maintained air conditioning (HVAC) system.  When most of us think about HVAC, we think about that gaudy dog house looking thing that sits outside, which most of you try to cover up with some type of bush.      May I send a special thank you, to those of you who plant those nice holly bushes?  Yet hidden away in the dark crawlspace or the hot, humid attic, lies an unappreciated and under examined, duct system.  This unassuming fellow comes in many shapes and sizes, and too often, sad to say, this vital part of the HVAC system brings the energy performance of the home to surprising lows.

    In one study, for example, researchers with the Florida Solar Energy Center (FSEC) replaced several oversized air conditioning units with “correctly” sized machines that matched the house cooling loads.  In theory, performance should have improved — but instead, the new systems saved little if any energy, while the homes’ indoor humidity actually went up. 

    Why the disappointing results?  Ductwork. When the researchers installed the new equipment, they left the old ductwork in place.   

    Too often this is the case.  A homeowner takes out their old 7 or 8 SEER unit and puts in a new 14 or 16 SEER machine, thinking their power bills are going to go down 50 percent.  Then reality sets in and their bills only changes, 5 percent.  This is when the angry homeowner calls and screams, why?  The short answer is there’s probably a mismatch between their duct system and the equipment’s required airflow.

    A 1994 Department of Energy study estimated that nationwide, heating and cooling systems were operating at 60 percent of the design airflow because of undersized ductwork.

In Mississippi and Alabama’s hot, humid climate, improperly sized ductwork can also cause moisture problems.  A large number of HVAC contractors throughout the United States use flexible duct in attics and crawlspaces.  There is not one thing wrong with this practice if the duct is sized and installed correctly to match the unit installed.  Unfortunately, careless installation practices often result in ducts being intertwined and tangled around each other, and in most cases, this type of system is very difficult to seal tight, resulting in numerous leaks throughout your home’s system.  Unfortunately, a “typical” duct system will lose 20 percent to 30 percent of the airflow to the attic or crawlspace.  Even worse, leaks in an HVAC return will suck hot, humid, unconditioned air into the HVAC system, increasing the cooling and humidity needs.

    What can you do?  Start with assuring that the ductwork is the proper size for the equipment you are having installed.  If it’s not, replace it.  You can take what your salesman will tell you is the best piece of equipment on the market and absolutely ruin it by putting it on an existing, BAD duct system.  If it is the proper size, assure that it is sealed thoroughly.  Codes allow either duct-sealing mastic, or specialty tapes to meet their duct sealing requirements.  Ironically, our beloved “duct tape” works on everything except ductwork.

    Chad E. Vaughn is vice president of The Service Company of Mississippi, LLC. E-mail him at Chad.Vaughn@the-service-company.com or visit the company’s website at www.the-service-company.com.