Sewer smell only when the range hood runs? How negative pressure pulls gas through bad drain work

An exhaust fan pushes air out of the house, and the same amount has to come back in somewhere. If the drain, waste and vent piping has an open joint, the house draws sewer gas through it. Hawthorne Plumbing, Heating & Cooling lists strong exhaust fans pulling gas through weak trap seals among the usual causes, along with dry traps, blocked vents, loose toilets and cracked pipes.

In this duplex, described on Reddit's r/Plumbing on Nov. 8, drains from both units meet at homemade tees held with hose clamps above the garage. That junction is the first suspect, but the cheap checks come first: run water in every fixture and make sure each toilet sits solid. Gurgling drains point to a blocked vent that siphons traps. For a peppermint test, a helper pours roughly 2 ounces of peppermint oil down the roof vent, then a gallon of hot water, while someone inside walks the rooms. A smoke test, blown in through a cleanout or vent, shows leaks behind walls. Never hunt for sewer gas with a flame: methane burns.

The code rule has changed. The 2009 through 2015 editions of the International Residential Code required makeup air for any range hood over 400 cfm, building code consultant Glenn Mathewson wrote in Green Building Advisor. Since the 2018 edition, the rule applies only when a natural-draft fuel-burning appliance sits inside the home's envelope. That appliance is the second reason to care: the same suction can backdraft an atmospheric water heater into the house.

Makeup air reduces the pull, but the leak stays. The fix is replacing the clamped junction with proper fittings and repeating the smoke test.