Q. Our home was built in 1985 and has a heat pump that we had replaced recently. The system does not have a humidifier. When we start our system after it has been idle for several days we notice what seems to be a mildew odor coming from the ducts. Aft er a while the odor disappears.

I have sprayed Lysol 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. We lived in our other house for about 20 y ears and never had the ducts cleaned. We had gas heat there.

I checked the condensing part of the system and there is not any standing water. 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 suggestion s?

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 o n the coils themselves or somewhere along the duct work.

Gas heating systems with air conditioning are not immune to this sort of thing but, unlike heat pumps, tend 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 year s old that have never met a service technician. Many folks apply the "if it ain't broke, don't fix it" principle to heat pumps mistakenly ignoring such virtues as cleanliness of filters, coils and ducts.

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 on the side of the return duct get past the filter and probably lodge 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 an d let it sit a while and mildew sets in. Turn it back on and the mildew dust-bunnies act like an obnoxious air-wicks releasing the mildew smell. I’ve seen it referred to in the heating and cooling trade as the “dirty sock syndrome”.

Unless you are one of those very advanced and dauntless do-it-yourselfers I don't recommend tackling the job yourself. Call a heating and cooling service technician and tell 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. Make sure the heating coils get looked at, too.

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 primordial swamps form in these pans. Many pros suggest pouring some blea ch through the condensate drain lines to keep them clear-- if you can get to it easily I’d say do it.

Cleaning the whole duct distribution system is a good idea if the ducts are seriously dirty and this is the time of year it makes the most sense to me to do it. During the heating mode the dust will dry out but add cooling and you can moisten some and it can stink. The problem is compounded if you have pets. Cats and dogs give off both dander and fur and a good bit of it ends up in the air distribution system.

Take a flashlight and a hand mirror and examine as many of both the return and supply ducts as you can reach. Pull the registers up from the floor and unscrew the grilles-- you’d be surprised at what you might find in there. You name it--toys, coins, so cks, food-- if it can fit down there I’ve found it.

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 shell out a few hundred doll ars for it.