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After having new copper pipes and a tankless water heater installed I hear a loud banging noise about an hour after the hot water is used. There is an expansion tank already installed. Any suggestions for eliminating the noises?
You may want to install an air hammer arrestor (water hammer) at the furtherst end of the water run(s). Think of the air in your water pipes banging around inside there with no where to go. Finally it comes to the end of the pipe run and "shocks" the water with the air pressure. An m-80 firecraker under the water does the same thing. It creates an expansion of the surronding water and it needs a place to go.
Sometimes also inside the walls, the pipes have not been secured to the wall studs and if they are cooper will band around as well. The only way to fix that it to open up the walls and secure them. I don't recommend this unless it is in a place you can get to later as well. Wherever you install the water hammer should also be accessable since they do wear out as well. Good luck. Process of Elimination.
I read in another post about drilling small holes in the area (BE CAREFUL!) and then using "Great Stuff" spray to expand around the pipes and create an expanding foam/insulation barrier effect. It is not a bad idea. And to fix the holes in the walls would be easy. Just another idea.