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Water hammer when hot water recirculating pump is off
I've owned this house for 34 years and since day 1 it has had water hammer whenever the hot water is turned on and the recirculating pump is off. Everything works fine when the pump is on. The pump is on a timer in order to save some energy so I have the time set to be off from 11 PM to 7 AM. Usually this works fine but if someone gets up early or is up late and uses the hot water then there's an annoying water hammer and I have to get up and manually override the timer so that the pump is on. After all these years of living with this I've finally decided that I would like to fix this if possible. I know, I probably should have done this years ago...
I just did a small test to see whether there was water hammer if the valve on the return line was closed and I verified that there is no water hammer in that case. I've attached a file showing the setup I have. Basically, the return line feeds back into the cold water line into the heater. The return line is 1/2" but goes to 3/4". Then there is a check valve, the Grundfos pump controlled by the timer, and finally a T into the cold water line going into the heater. That line also has a shutoff.
Any thoughts as to what's happening and how I could go about fixing it? Thanks.