A homeowner's late-night question about whether duct tape could safely cover a rusted-out water-heater flue drew a wave of warnings about combustion gases and led to a decision to shut the system down.
Here's what to know
The concern came from a post in Reddit's r/hvacadvice forum, where the original poster showed a metal vent pipe with a rusted hole.
The OP said they had been told the spot 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.
Warnings were blunt. "This s*** can be deadly," one commenter wrote.
Another user added: "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 worked out that the furnace was likely on a different vent line. After the OP said the white PVC was "connected to my furnace," another user replied: "Good. You can run your furnace then at least."
A theory from the comments section was that the metal vent may have started failing after the furnace was moved off it, leaving the water heater as the only appliance using that run. As one person 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 rerouted to PVC, the remaining metal flue can stay behind to serve just the water heater. Commenters treated the corrosion as something worth taking seriously, even if rust by itself does not mean a disaster is imminent.
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What can be done?
The homeowner followed the safety advice and turned off the gas appliance tied to the damaged flue.
People in the thread also mentioned replacement paths that would avoid relying on the old metal flue. One suggestion, from the last commenter, was a power-vent water heater that vents "out the wall," and they said a heat pump water heater could also make sense if the basement was large enough.
Someone else added that if the metal line served only the water heater, a suitable replacement unit might allow the homeowner to abandon the vent. That would avoid having to patch or rebuild the vent.
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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 can I learn more?
Other homeowners have run into similar issues.
• Homeowners learned that a vent that's "just a little short" can still let carbon monoxide spill indoors.
• After a furnace swap, an open water heater flue raised deadly gas concerns.
• Another homeowner noticed evidence of backdrafting when melted plastic appeared above the heater.
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