02:04PM | 11/15/05
Member Since: 11/14/05
1 lifetime posts

I recently put in a new 92% furnace and high performance water heater in my house this spring. Both are exhausted horizontally through (separate) PVC pipes.

My problem is that when it is cold outside and the furnace and/or water heater turn on - the exhaust pipes make a knocking sound. I believe this is due to expansion/contraction of the pipes due to the temperature difference. The pipe is not really that hot, but the knocking increases as it gets colder outside. You can feel the pipe when it knocks. Doesnt happen in summer at all.

I do know that the noise is coming from the exhaust pipe of the water heater - I have a hole in the ceiling to prove it. It doesnt always happen at the exit from the house, though most of the noise appears to occur closer to the exit than closer to the HVAC/heater.

The pipes were initially mortared in the brick by the contractor. When the knocking initially occurred, the mortar was removed, the space filled with insulation and the knocking went away. However, we did not want an exposed entry into the house for critter/bugs, so the contractor caulked the opening with silicone caulk.

The knocking still existed but was muffled. (I think the outside temp was also aiding the situation as it was getting warmer outside). Now the knocking it occurring again, and it is getting louder as it gets colder.

The intake and exhaust both have screens to prevent birds/animals from getting inside. Never had this equipment with snow on the ground, so I can say that the issue I am having has not been caused by it

I am looking for ideas to get rid of or greatly slience the knocking noise. I have thought of putting foam around the pipes where they run through the supports. This way if it is vibration induced, the foam should silence it.

Thanks (and sorry for the long read),



08:58PM | 10/19/13
Did you solve this problem? How so.

I am having the same issue.


10:58AM | 03/23/14
I have this same problem and the knocking is driving my nuts.

Is there a resolution to this? An HVAC guy said to insulate the pipe going through the attic?



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