Hello I have had this on going problem with one furnace in a building of 5 units 5 furnaces. Every year in July and Aug. when the temp gets to 85-90 at specifically 4:15-4:30 PM a sewer smell comes out of one furnace in the building. We have a number of furnace guys look at it and an air specialist. The checked the drains by the furnace the vent pipe on the roof and no one can figure out this smell! Has anyone experienced this problem? Tim:(

Smells are elusive. Good luck finding the problem.