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Unpleasant odor from gas furnace return air intake

12 years ago

We live in the San Francisco Bay Area and have an unusual (for us) problem with our 15-year-old Rheem gas furnace. Recently, and before our delayed winter rains kicked in, we noticed an unpleasant odor coming from the two air intake openings for the furnace, which is located in a tall (4 - 5 ft. high) crawl space beneath our 40-year-old 2-story home. It was more obvious to my wife than to me, but I could smell it if I put my nose the intake grill (that is, the source of the return air into the furnace). I replaced the inexpensive filters that covered the openings of the two intake vents. I notified our local gas utility (PG&E) that we thought we smelled a gas leak, but their technician checked the furnace and every other gas appliance and found no leaks whatsoever. I also went beneath the house to inspect the entire furnace and its surroundings. There were rat droppings all around the furnace and in several places where they had apparently nested in torn fiberglass insulation, so I vacuumed all of this unpleasant material away. I never did see any rats, so I don't know how long their droppings had been there. The furnace ducting seemed to be fully intact (rigid insulated sheet metal ducts to all floor and wall heater vents, soft flexible ducting for the return air intakes) but since the furnace had not been serviced since its installation I arranged for a very well-respected air duct cleaning service to clean out all the ducts with HVAC and clean the furnace fan as well. Even after all this, the odor is still there! It smells faintly of a dead animal but I have not seen one anywhere and there seems to be no way a rat could access the furnace to hide or get stuck inside it or the ducting; the air duct cleaning people found nothing wrong, either. Most mysteriously of all, the warm air entering the house heater vents from the furnace does NOT stink, so the odor is not spread while heating the house -- only when the heater is idle does the odor drift in through those return air intakes. Our only recourse so far is to use scented candles to cover up the smell! Any suggestions?

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