Rusted water heater flue prompts warnings about dangerous gas leaks

"Shut off any gas-burning devices that use this flue until repairs can be performed."

A homeowner asked late at night whether duct tape could temporarily cover a rusted-out water heater flue, and the question quickly drew warnings about combustion gases and ended with the system being shut down.

What happened

The issue surfaced in a post on Reddit's r/hvacadvice forum, where the original poster shared a photo of a metal vent pipe with a rusted hole.

The poster said someone had told them the damaged area could be taped over for the time being until an HVAC contractor could replace the part. "Any second opinions? We have our furnace off as well because I'm a scaredy cat," they wrote.

The responses were direct. "This s*** can be deadly," one commenter wrote.

Another user said, "You're not going to have anything catch fire, but obviously if the flue isn't sealed, combustion gasses are a major concern. Shut off any gas-burning devices that use this flue until repairs can be performed."

Commenters also concluded that the furnace was probably on a separate vent line. After the original poster said the white PVC was "connected to my furnace," another user replied, "Good. You can run your furnace then at least."

One theory in the thread was that the metal vent may have begun deteriorating after the furnace was moved off it, leaving the water heater as the only appliance using that vent run. As one commenter put it, "That's [a] single wall vent. That stuff gets HOT, and look at the clearance to combustibles lol. That's why it's rotted; it's cooling and condensing."

When a furnace is switched to PVC venting, the old metal flue can remain in place for the water heater alone. Even so, commenters treated the corrosion as a serious issue, though rust by itself does not necessarily mean an immediate disaster.

What can be done?

The homeowner followed the safety advice and turned off the gas appliance connected to the damaged flue.

People in the thread also suggested replacement options that would avoid depending on the old metal vent. The last commenter recommended a power-vent water heater that exhausts "out the wall," and said a heat pump water heater could also work if the basement is large enough.

Another user added that if the metal line served only the water heater, a proper replacement unit might allow the homeowner to eliminate the vent altogether, avoiding a patch or rebuild.

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"It's completely off. I don't plan to turn it back on until after HVAC guy fixes it," the homeowner wrote.

Where to learn more

Other homeowners have faced similar problems.

• Homeowners learned that a vent that is only slightly too short can still allow carbon monoxide to spill indoors.

• After a furnace replacement, an open water heater flue raised deadly gas concerns.

• Another homeowner noticed signs of backdrafting when melted plastic appeared above the heater.