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?

Click to reply button
Inspiration banner


Post a reply as Anonymous

Photo must be in JPG, GIF or PNG format and less than 5MB.

Reply choose button


Post new button or Login button

To test the boundaries of small-footprint living, interior designer Jessica Helgerson moved her family to a 540-square-foo... Filling an underutilized area beneath the stairs is a smart way to save space. Doing so with a stash of wood, however, is ... The Audubon Society inspired wallpaper in this Adirondack-styled mudroom will get you in the outdoor mood. Grab your coat ... Chalkboard paint opens up endless possibilities for customizing your dresser time and time again. Use chalk to label the c... A fireplace in the bathroom creates the ultimate setting for relaxation. Homeowners often choose electric or gas over wood... This roomy boot tray made from punched metal stands up to all the elements. Station it in your mudroom or at your back doo... There’s nothing like a new set of cabinet hardware to refresh a room. The possibilities are endless: Go modern, rustic, or... FLOR tiles are an affordable way to customize a carpeted floor covering for any space. Make anything from runners to wall-... Chalkboard paint features prominently in this elegant yet unpretentious headboard design. Add a new message daily to reflec... Salvaged boards in varying widths and colors make up the dramatic accent wall in this attic space. The high-gloss white of... The indecisive homeowner need not fret over choosing one (or even two) cabinet colors. The kitchen cabinets in this artist... Incorporate nature into your lighting scheme by securing a dead tree in a concrete mold and draping your pendant lamp from... Simple and striking, a couple of pieces of "lovingly used" furniture creates a special kind of charm. A weathered chair fo... First dress up your metal shelves with a coat of paint in an accent color that complements your kitchen decor. Then arrang... Dark wood shelving and a matching upholstered bench keep this closet sleek and refined. The large window brightens the sub...
Follow banner a
Newsletter icon Flipboard glossy Facebook Twitter Pinterest Youtube Rss icon