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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.


  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 2,975
    edited December 2018
    That vertical swing check valve is causing the water hammer. When you turn on the hot water, water will flow out of the return line, slamming that check valve closed.

    Remove it and install a spring check valve after (not before) the pump.

    I have the same thing happening at my house, but I've been really busy with work and the grandchildren and trying to keep Loretta happy and harmonica classes and ..........
    Often wrong, never in doubt.
  • Larry Weingarten
    Larry Weingarten Member Posts: 2,295
    Hello, @Alan (California Radiant) Forbes nailed it. That swing check mounted as it is has very little closing pressure. I'd remove it and that old gate valve and put a spring check down below the pump. ;)

    Yours, Larry
  • brucerpeterson
    brucerpeterson Member Posts: 4
    Can you explain why it only happens when the pump is off? Obviously the check valve will be open when the pump is running. I'm thinking out loud here... When you turn on the hot water, then that will create space that the cold water will try to fill so there will be flow on the cold water line into the heater. Some might try to flow back up the return line but with a check valve in the line, the line would be filled with water already so I don't see how there could be any more flow in the backwards direction. Obviously I'm missing something.

    Also, you said the location (and type) of the check valve should be changed. In looking at diagrams I found online by Googling, I see that some people put them before the pump and some put them after. I thought that I even saw at least one diagram with a check valve both before and after the pump but I may be mistaken about that. In any case, what's the reasoning for putting the check valve after the pump?

    Thanks for you help.
  • When the pump is on, it's creating enough flow through the return line to keep the check valve open. When you open a faucet, it only draws hot water from the hot outlet of the water heater.

    You verified that it's the check valve when you turned off the gate valve just after it, taking the check valve out of the formula.

    As far as the location of the new check valve, pumps do not like any device that restricts flow on their suction (inlet) side. They don't mind it on the discharge side.
    Often wrong, never in doubt.
  • brucerpeterson
    brucerpeterson Member Posts: 4
    All right. Sounds like I have a plan. Thanks all for your help.
  • brucerpeterson
    brucerpeterson Member Posts: 4
    Just got it done and it does seem to have fixed the problem. Thanks for your help.