I heard a great idea from a great HVAC contractor - the two kind of go together don't they?
It is an easy answer to a common problem in our industry: the dirty sock syndrome. I have heard of many ways to avoid condensation forming over the winter in ducts and on coils, including but not limited to: operating your fan constantly, installing a fan cycling thermostat, and use of a UV light. Unfortunately, all of these solutions are not demand based, meaning they will operate whether they are needed or not. Don't get the wrong idea, these are solutions and will stop that smell first thing in the Spring. But, how much energy did it take to get you there, and how much was the total investment for the homeowner? What if I told you there was a simple fix that would cost you, the contractor, about $50? Got your attention?
In the Northeast we have a lot of boilers, and I mean a lot! Usually, this means a lot of ductwork must be added in the attic - for that all important conditioned air in the Summer. Unfortunately, this creates quite the "chimney effect" and phantom airflow issues all winter. Most homeowners are aware that they need to close the supply registers, but returns are often ignored since there is no easy way to close them. I used to use a white trash bag to cover the air filter, but you can see where this is leading when the homeowner forgets to call you for maintenance by late May. So, instead of closing off the registers, why not cycle the fan on a demand basis? Mold, mildew, and that dirty sock smell is created by condensation and standing water in the ducts and coils over the winter. In order for condensation to form, the relative humidity in the ductwork must reach 100% Rh, right? That is the humidity when it is raining outside, 100%. For sake of simple math, and to avoid the psychrometric conversation, I am going to leave out dew points and surface temperatures. The easiest solution there is to just insulate the ducts to code, so the temperature remains above the dewpoint of the air.
Anyhow, if you install a De-humidistat to close the Fan circuit, or 'G' on your Integrated Fan Control, then the fan will cycle on (in most cases on your lowest speed) prior to condensation forming in your ducts and coils. The key point here is that it will then shut off when the humidity reaches the acceptable level that you set it for. So, you may get a short span of cool air being blown around that zone, but doesn't that beat the poor indoor air quality and men's locker room stink in the Spring? Combine this with some of today's best IAQ products, and there is a winning combination for the Winter months. Not to mention less energy use (Kwh) to get you there!