The cooling season is almost over. Whew! Soon the evening or morning will come when the heating cooling gets switched at the thermostat for occasional heating duty in the evening and cooling during the day.
You will very likely smell an odor from the ducts. At any other time of year I would recommend a service appointment right away, however not in this case.

During the summer we have used the furnace blower and the heating cooling fan and ductwork for cooling only. During this time a layer of household dust and possibly a mold bloom have formed on the electric heaters of a heat pump or on the heat exchanger of your gas furnace. The ductwork near the furnace plenum and the interior of the air handler or furnace cabinet will also have this collection of dust and molds.

The high temperature low relative humidity will first cook the mold spores on the heating coils or exchanger and dehydrate the furnace insulation, this will deactivate the mold spores for lack of a way to absorb food for reproduction. This smells musty. The mold and dust are in the air anyway, this is not unusual and no cause for alarm.

If you have been running a UV light or household air purifier all summer this can be puzzling. These air cleaning devices seem to work okay in the lab, but not perfect in practice. Actually the UV lights I encounter do not have anywhere near enough intensity to interfere with the ability of these airborne organisms to reproduce. The volume of air a portable unit can clean is also limited to its immediate area. Both can provide an improvement in air quality. As far as they can reach they will help, but not cure this yearly odor.

If your furnace sits in unconditioned space as a garage or crawlspace or even some (RIR) basements the effect is more noticeable, less so in attic units and even less in more modern well sealed duct systems. 

So.. should the furnace be serviced. If the smell is not musty but electrical burning, or if the smell is apparent in heating and in cooling. If you sense a smoke like thickness to the air (some say it smells hot) then its time to call the serviceman.

If.. the furnace odor is at the beginning of the heating season and just smells like an old book. Leave the fan set to ON and run the heat normally, turn the thermostat back to FAN AUTO in a couple of days and see if the odor persists.

I can't really say whether it needs checked without checking it of course, but I get about two dozen calls at the beginning of each heating season for a smell that is normal and doesn't occur in cooling operation. After a little questioning about the nature of the furnace smell and when it occurs my advice is very often to make a personal note about when it starts and see if it goes away on its own, before you buy a service repair appointment .

Peace of mind.. if this little tidbit is all you needed, you're welcome. If you want a safety inspection anyway, or if you've lived in your home for some years and you know this is not normal furnace operation in your home then call me up and I'll look it over for you.  I promise you a safety inspection and peace of mind.

P.S. If you ask for, or if I see the need, a monoxide test at the heating register or in the ductwork is free for the asking since I'm there anyway and it only costs me a little time.

Question? Email me from the Get in touch page on my site.

Save your money,
Ed Cline