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Plumbing in the Home/Hot Water Pipe Knocking When not in Use


First of all, thank you for having this wonderful service web site.

The copper hot water pipe knocks when the water is not running. It stops pretty fast when I open a hot water faucet anywhere in the house and or leave it just a bit open. Stops immediately when I open the hot water faucet closest to the water heater.

The knocking started after the house furnace was replaced and I remember the technician had to close the main water valve. I have also noticed that the water pressure is higher than it was before.

This is a list of actions I have taken in an attempt to stop the knocking:
1. Closed the main water valve, drained the internal hot and cold water pipes and reopened slowly the main water valve. (Didn't noticed a difference in the pipe knocking#
2. Reduced the hot water temperature level of the water heater (Didn't noticed a difference in the pipe knocking).
3. Closed the main water valve 35% and have noticed the knocking is less loud and less frequent.

I am wondering if I should reduce the water pressure using the pressure water valve but I have never done this and I don't know if it is safe to do this on my own.

Your advice is appreciated.
Thank you

Dear Richard,  The knocking in the pipe is being caused by the expansion of the pipes from the increased pressure.  The pressure is increasing due to the water being heated... it may be that a pipe has become loose from a fastener or a new pipe added and in contact with a hot water pipe.

What I would suggest is to add a pressure tank to the system....  Extrol pressure tanks are required in many cases anyway, and they serve to balance the pressure in the system.  They are not hard to install, but you may need a plumber to do the soldering.

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Thomas Ambler


I am experienced at troubleshooting and solving all types of plumbing problems. Issues related to specific manufacturers fixtures and specifications though are beyond my field of expertise. I have dealt with problems in both rural and urban areas, and with water supply and drainage systems. Plumbing issues can be very obscure... such as odors, stains or occasional seepage. I have even had to chase rats from a customer's sewer line.


I have 25 years experience dealing with plumbing systems (which also includes hot water and steam heating systems). My main expertise is in diagnosing and solving problems. I have worked in construction and installation of new systems but do not regard myself as an expert in that area.

Asbury Park Press, Seattle Times, Denver Post.

I completed high school and one year of college. I served 4 years in the Marine Corps and have education in electronic and electrical troubleshooting and repair. I have attended classes on installing and repair of oil burners, and I have EPA universal certification for air conditioning and refrigeration. I am now enrolled as a full-time student at Everest college to re-train for a new career. Any donations of financial support would be greatly appreciated.

Past/Present Clients
All of Denny's Restaurants in NJ, Several Burger King stores, I have done work at two different homes for Bruce Springsteen. I have also worked for Jim Johnson (of J&J) and his family, Whitney Houston, Steve Forbes and many others.

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