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noisy power vented water heater PVC vent pipe in cold weather attic area

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Power vented hot water heater PVC vent pipe passing thru attic knocks loudly in cold weather.
asked in Heating and AC by john-dephillips (120 points)
edited by darekrudy
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1 Answer

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Hi John, 
This could be due to expansion of the WH’s PVC vent pipe. You can check its run at penetrations (ceiling, roof, any attic floor boards, etc,) and see if it’s tight at any of them. If it is tight, every time the warm exhaust gases start flowing through the pipe, PVC starts expanding and rubbing against those surfaces making noise. This could be happening as it is contracting / cooling down as well.
You should also check for tightness all WH’s PVC vent pipe joints (if there are any in your attic).
answered by darekrudy (21,730 points)