"AC smells when it first comes on."
Column #842 4/16/2011
On The Level
Q. Our home was built in 1994 and has gas heat and separate air conditioning but uses the same ducts. The system does not have a humidifier. When we start our AC system we notice what seems to be a mildew smell coming from the ducts. After a while it disappears. This happened again last week when the weather warmed up into the mid 80s and the air-conditioning came on for the first time this year. I sprayed Lysol with bleach cleaner into the cold air return and into the area around the condensing unit with not much success. We have been in the home for four years and as far as I know the ducts have never been cleaned. I checked the condensing part of the system and there is not any standing water that I could see. I looked down the cold air returns and see there is a good accumulation of dust on the sides and bottom of the ducts. Could this be the source of the odor? Any suggestions?
A. Mildew odors in forced air heating and cooling systems are annoying and can be a real pain to locate and cure. You are on the right track looking for a wet spot as the culprit but rather than standing water I'll bet it's a condensation point either on the coils themselves or somewhere along the ductwork.
Gas heating systems with air conditioning are not immune to this sort of thing however, unlike heat pumps, seem to receive more regular cleanings as a part of their routine maintenance than heat pumps seem to get. I routinely see heat pump systems many years old that have never met a service technician. Many homeowners apply the "if it ain't broke, don't fix it" principle to air systems, mistakenly ignoring such routine maintenance items as clean filters, coils and ducts. Hit the thermostat and air comes out of the ducts. Nothing to it. That attitude costs.
Apart from the dusty ducts, you didn't mention whether or not the whole air-handler itself, including the coils, have been cleaned lately. I suspect not, so that would be the place to start. Wads of the type of dust you see in the return duct could and probably are lodged between the fins of the coils and get wet during air-conditioning operation from the system condensing moisture out of the air. Turn the system off and let it sit a while and mildew sets in. Turn it back on and the mildew dust-bunnies act like an obnoxious air-wick releasing the mildew smell. This is known in the industry as the “dirty sock syndrome” because that’s what it smells like.
Unless you are a very advanced do-it-yourselfer I don't recommend tackling the job yourself. Call your service provider and repeat to them what you've told me and what I've told you. Have the system cleaned and the coils vacuumed or air-blasted. Clean and make sure the whole air handler chassis gets a good going over.
Check the condensate pan located under the air conditioning coils that catches the dripping condensate to be sure it's clean and does not let water stand in some part. I have seen mold swamps form in these pans.
Cleaning the whole duct distribution system is a good idea if the ducts are seriously dirty. Take a flashlight and a hand mirror and examine as many of both the return and supply ducts as you can get to. Pull the registers up from the floor and unscrew air return grilles from the wall.
If you have a good vacuum cleaner with a wand and brush attachment, you can do a pretty passable job of duct cleaning yourself. Hiring a professional duct cleaning service will give you a whistle-clean job but be prepared to spend a couple of hundred dollars for it.
Changing system air filters is your job so do it at least at the change of seasons-- works out to four times a year. Many people think air filters on forced air systems help keep the house from getting dusty. Not true. They are there to protect the equipment. A seriously dirty air filter will restrict air flow and cause the system not to preform properly and in some instances even cause it to freeze up. One of the main functions of air-conditioning is dehumidification and air speed across the coils is controlled to cause that to properly happen. A clogged filter can and will disrupt that and problems arise. The warm up we had last week is an early wake up call for air system maintenance. Summer’s coming so be ready for it.
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