hot water pipe knock

There are a number of reasons why pipes knock. One of them is excessive pressure. Anything over 60 psi is enough to cause pipe knock. In addition faucets that have cartridges often cause pipe knock, because they stop the water too fast. When you get water flowing in one direction and you shut it off quickly, all the forward motion is has no place to go. It is absorbed in the piping with banging.

One way to change that is to install a pressure reducing valve on the inlet of the house to reduce pressure to 60 psi. Another way is to change out the faucets to compression type faucets. Finally installing air chambers under the sink will help to absorb the motion of the water. Since the air can be compressed and the water cannot, when the water runs up agains the closed valve, it is diverted into the air chamber compressing the air and absorbing the shock.

Good Luck

Raymond VinZant Plumbing Prof.

I have this very same problem in my new home when I run hot water in my kitchen sink, or even if I pour hot water from a pot down the sink, the walls in front of the sink start knocking loudly. My house is still on warranty and the maintance guys can't figure it out. Keep me posted if you find an answer and best of luck, I know how annoying it is!!

Same problem here-14 year old home-I recently replaced cold water supply to a new water heater-used 3/4 in steel compression supply.
Shortly thereafter, everytime we turned on upstairs tub hot water, the cold water supply under the house going to the water heater would staccato knock! It was noisiest upstairs but could be easily felt in the crawl space below the first floor.
Consequently-replaced the expansion tank on the water heater & installed a PRV----achieved absolutely nothing-still bangs!