How to patch an inaccessible vent pipe
Full replacement is impossible when a damaged vent pipe sits hidden between a floor and an exterior wall, forcing a repair from the inside instead. Standard exterior patching products often crack repeatedly within months, leaving homeowners stuck searching for a durable solution that holds up on hard-to-reach connections.
What the numbers show
Counted from the threads quoted below. How we count
- The most recent one is from October 2026.
- This comes up 17.2× more often in r/hvacadvice than across the site as a whole.
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?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Attempting to patch the pipe in place without replacing the damaged section.Did this help you?
- 2Asking online communities for alternative products to recommend to the contractorsDid this help you?
- 3Using Xypex fast setting crystalline quartz patchDid this help you?
- 4Cleaning out the hole and using hydraulic cement on both sidesDid this help you?
- 5Removing the failed caulk and applying Quikrete patch with spray paint finishDid this help you?
- 6Doing the repair work yourself instead of relying on hired contractorsDid this help you?
- 7Calling a chimney or HVAC professional to inspect and provide a quoteDid this help you?
- 8Attempting to patch or replace the stack as a seasoned DIYerDid this help you?
- 9Trying to patch the joint with a rubber couplingDid this help you?
- 10Having a technician cut out the section and re-seal it properlyDid this help you?
- 11re-caulking the cement and flashing jointDid this help you?
- 12covering or filling in the upslope valley so water sheds to each sideDid this help you?
- 13sealing the area as a testDid 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.
“Can it be patched or replaced easy without digging into the roof as an seasoned diy-er or needs hvac professional due to carbon monoxide concerns?”source ↗
“So basically, replacing this coupling is really not an option because there is no access it is right in between the floor and exterior wall. I need to patch this somehow.”source ↗
“This was found this way when I got the house”source ↗
“Noticed water at the bottom of the furnace. Only appears when furnace is run, not A/C. Assumed a clogged condensate line but that is fine. Followed the water up and it appears to be coming back down the exhaust but leaking out at this joint.”source ↗
“Can it be repaired with a simple rubber coupling if the coupling fits in the hole through the sheet metal of the furnace top?”source ↗
“Had a power vented hot water heater put in last year and the product used on the exterior keeps cracking.”source ↗
“The company has come out to redo this twice now but there surely has to be a product that doesn’t crack within 3 months.”source ↗
“Frustrating that I have to recommend something to the people that are supposedly the experts but here I am.”source ↗
“Have a small leak in a clay s-tile roof at a pipe vent penetration.”source ↗
“Short of pulling the existing tiles are there some simpler things can that be tried here - for example, re-caulk the cement/flashing joint or cover/fill-in the upslope valley so water sheds to each side?”source ↗