Smelly heating vents

[J.C.]

We have a perplexing problem. When the heater turns on (but not the A/C) two of our bedrooms get this sickly sweet odor that comes from the heat registers. The furnace is in the garage, and the heating/cooling ducts run under our 1 story house in the crawlspace. We had the entire system professionally inspected in Spring 2010, all ductwork was hooked up correctly and the entire heating/coolong system as well as the dryer vent were cleaned out. New furnace filters were installed. Over the summer, when the A/C would run, but not the heat, the smell was not evident. We also had a pest control company seal up all outside entries to the crawlspace and they didn’t find any recent (within the past few years) evidence of “critters” under the house. The smell is in two adjacent bedrooms, and is stronger in the one that is closer to the outside A/C unit. But since the smell doesn’t occur when the A/C is on, I don’t think the A/C is the problem. Any suggestions?

[Ian Gills]

I'd say the odd smell every now and again is nothing to worry about, especially given all the checks you have had done and the fact it does not whiff with the AC on.

I get the smell of burnt dust once in a while. Especially if the furnace has been out of action for a little while.

It's one of the joys of living in America with expensive forced air heating rather than efficient water-heated radiators.

[J.C.]

Thanks, Ian, for the quick reply. The smell is really strong. Almost so that one wants to leave the room. I am familiar with the burning dust smell one can encounter when firing up the furnace after it's been idle over the summer. This odor happened at the end of last winter, it is what prompted us to have the inspection and cleaning done. Now it's a little cool outside, and with the furnace is kicking on overnight the smell is back.

[jadnashua]

If the drains are not properly vented, and a trap is sucked dry, the pressure differences could be sucking sewer gasses into the system and blowing them around. During the summer, the a/c condensation drain is constantly filled with water and the trap (which should be there) is full of water, so it can't draw sewer gasses in. If this trap and possibly an air gap is not made properly, in the winter, since there may be no condensation to reprime the trap, it dries out.

While (I'm sure) there could be other reasons, I'd check that.