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Water heater vent pipe leak

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The vent pipe from my hot water heater that is going out the roof is leaking.  Roofer looked at it.  Said he could not see any leaks.  In a rainstorm I climbed into the attic and observed drops of water coming from the pipe connection.  

No water on the outside surface of the pipe.  Roofer says the vent pipe itself may be cracked at the connection and the water that may dripping down inside may be coming out of the cracks.  Do you have any idea how I might seal this?
asked in Gas Water Heater by texmex (120 points)
edited by darekrudy
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1 Answer

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Hi Texmex,
There could be a few reasons for a leaking hot water heater vent pipe:
  1. Missing, improperly installed, or wrong type of a rain cap which allows for water penetration. There’s not much I can say about it because I don’t know if you have one or what type is it. If possible take a picture and email it to me.
  2. Corroded / cracked side wall of the vent pipe. You vent pipe should be double wall type and sometimes the exposed above the roof section corrodes, cracks, or partially disintegrates and starts leaking inside. If this is the case, the top section of the pipe should be replaced / it’s first joint is usually beneath the roof surface / in your attic.
  3. Rain (or storm) collar on the vent pipe became loose and leaking. In most designs the collar sits on the vent pipe, a few inches above the roof surface and flashing. By design it provides water resistant seal but with temperature fluctuation, contraction and expansion of the vent pipe, it sometimes leaks. People use silicone around it but it rarely provides permanent solution.

most common areas of leakage on B-vent pipe above the roof

Patching doesn’t really work on vent pipes because they serve such an important function in discharging exhaust gases to exterior. Replacing compromised part does the trick and lasts much longer but you are probably going to need a heating contractor to determine what’s causing the leak.
One more thing you could do is go into your attic (be careful) during the daylight and check for any light penetration around the vent pipe at the roof penetration level. Just to make sure that your roofing contractor did a good job on checking the flashing.
Let me know if there's anything else I can help you with. Just click "comment" below. 
answered by darekrudy (21,730 points)
Thanks for your comments.  Hopefully the following observations can help pinpoint the cause.  I have been to the attic several times and once sat there for 15 min during a rain storm.  There is definitely no daylight.  No water dripping along the sides.  Water drops appear at the first joint only - pipe surface above it is dry.  So seems like water is leaking from the inside of the pipe.

If I have to replace the upper section of the pipe wouldnt I need a heating contractor and roofer?


If there's no daylight visible around the vent pipe you'll have to research those 3 possibilities I've listed above. All three of them and especially first two can be responsible for the internal leakage. 
Vent pipe replacement might involve roof flashing removal (recommended if old) and depending on how was the flashing originally installed, section of the roof covering surrounding roof penetration area might need to be removed as well. 
Also, based on your geographical location ice & water shield re-installation might be required around the flashing so the heating contractor may not be willing to take the responsibility for flashing related problems (in the future) if he’s the one to perform both vent pipe and roof flashing installation. 
It pretty much depends how skilful he is…, you might need a heating and roofing contractor.