Dirty Sock Syndrome
The term for what you are smelling is "Dirty Sock Syndrome" and you are not alone. Luckily there is a cure for "Dirty Sock Syndrome". You probably have a heat pump, not a furnace. This smell is much more pronounced with the use of heat pumps rather than furnaces, but still can form in furnaces.
With advent of higher and higher efficiency indoor coils they tend to trap water for longer periods of time as the fins between the coils are closer together. Bacteria grows on them and when this first started happening back in the 80's some stop gap measures were discovered. Unfortunately these measures where only temporary.
In the past some of the cures for this dirty sock like smell were:
Chlorine Cleaning: This has been banned and is not very beneficial to your aluminum coils.
Bio-Cleaners: There are a number of bio cleaners on the market that will clean your coils and kills whatever lives on their surfaces, but like all cleaners they do not solve the problem. They temporarily treat the problem. The dirty sock smell will reoccur.
Drain Pan Additives: These work, but only for the time that the chemical additives are active.
Changing the Indoor Coil Out: A pretty extreme step, but the conditions that caused the problem in the first place are not addressed so the problem comes back at a later date.
This problem is directly linked to mold and bacteria growth on the coil during the air conditioning season, but comes back during the heating season. The reason that it comes back during the heating season on heat pumps is because of the defrost cycle. When the heat pump enters into defrost mode the indoor coil can drop below the dew point and moisture can form in the coils. This moisture reactivates the mold, mildew and bacteria starting the dirty sock smell process all over again.
On forced air heaters this can reoccur when the moisture levels become high enough. This normally happens when a humidifier has been installed and the humidity levels are set too high. During a normal humidification process there will not be enough moisture running through the coils to reactive the mold, mildew and bacteria when the furnace is pumping out 140 degree heat.
There are only two ways to remove this problem from your system in the long run. The first way is to install a UVC Technology device. Simply put, an ultra violet light that shines on the coils. Ultra violet light is a natural bacteria killer. The second way is too install an electronic air cleaner that lets off enough ozone to inhibit the growth of bacteria and a good enough filtration (purifier) to prevent organic matter from getting to the coil in the first place so that bacteria will not form.
When running into this situation the first step would be to clean the coils and furnace with an EPA approved biocide for residential ducts. The second step, taken at the same time, is to install an AccuClean air purifier. Normally this should take care the problem immediately. In extreme situations, or reoccurring situations, a UVC light should be installed that shines directly on the evaporator coil.
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