Why is my furnace blowing exhaust out the water heater vent?
Running a furnace and water heater simultaneously can overload their shared vent pipe. Because the furnace uses an exhaust fan and the water heater does not, the furnace pushes its exhaust backward through the water tank and into the room. This blows hot air into the house and randomly trips the tank's high limit switch, leaving people at a loss for how to stop it even after opening windows for more air.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
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Why is my furnace blowing exhaust out the water heater vent?
- 1Installing carbon monoxide detectors near the utilities.Did this help you?
- 2Checking chimney and exterior vents for physical blockages.Did this help you?
- 3Blowing out the vent pipes with a leaf blower to clear partial clogs.Did this help you?
- 4Calling a professional HVAC technician to perform a combustion analysis.Did this help you?
- 5Opening a nearby window to let more air into the mechanical roomDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I'm at a loss on what I can do to fix it. I've tried opening a window to let more air in but idk what else to do”source ↗
“The old one did the same thing. The furnace has a fan on the exhaust. The hot water tank does not. So anytime the furnace is running it will push exhaust through the hot water tank and likely cause the hi limit to trip on the tank cover randomly.”source ↗
“But hot air is definitely felt around the hot water heater vent when both are running.”source ↗
“Less to no hot air coming out of the hot water vent. Same when furnace only. Both seem to overload the pipe.”source ↗
Where this came up
People with this problem also raised
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- 4Why is my apartment heating vent blowing painfully hot air?
- 4Landlord installed a dangerous makeshift furnace exhaust vent
- 4How to clean soot off things after a fire