ITs horrible... I really think its making me ill... had a guy check the furnace about 4 yrs ago... its a Carrier high efficiency. I was told it could be the heat exchanger but I don't read anything that says it has this putrid smell. Its like a sewer smell. I need help... I really think I have been ill for 3 yrs because of it and no one can find the smell. Others have smell it too and call it sewer, but we are not on sewers. We are on septic and its OK.