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?