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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: 3,828
    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 ..........
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,143
    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.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • 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.
  • slazinger_7
    slazinger_7 Member Posts: 1
    I know this thread is several years old, but I have had a rather perplexing water hammer problem that I'm REALLY hoping might be solved by the suggestion in this thread. Although, our setup is a bit different:

    We have a Rinnai tankless with an internal re-circulation pump (Model RUR199IN). The below picture shows our setup:

    I added some annotations to better show the incoming cold (blue) and hot water (red) flows, along with where the re-circulation pump is located.

    We have had constant water hammer after we installed the Rinnai. Historically, we have used Rinnai's "Control-R" feature to run the re-circulation pump on demand (mostly before someone takes a shower). Today, I happened to schedule the re-circulation pump to run all day, and I noticed this morning that the water hammer on the hot water side had basically gone away. Even when I rapidly turned the cold water side on/off, the hammer was almost inaudible.

    Tonight we ran laundry after the schedule turned the re-circulation pump off - we heard the water hammer again. I got the bright idea to run the re-circulation pump for 20 minutes, and the hammer seemed to go away (I'm assuming the laundry was still running).

    Doing some Google searching landed me at this thread, and the check valve immediately after the blue water filter being the culprit seems like it might make sense. Although, I'm not really sure what the answer is to fix the problem because our set-up is different than Bruce's, in part because the Rinnai has an internal pump.

    Any suggestions?

    The much LONGER backstory is that about a week after we installed the Rinnai (about 3.5 years ago), we had to evacuate a major wildfire - we were out of the house over a year. At install, my understanding is that the Rinnai re-circulation pump was running 24/7, and I don't seem to recall any hammer at that time, but granted that was years ago and it was only in operation a couple days before we had to evacuate.

    The contractor we hired to do the fire remediation work at the house also did some additional plumbing work related to adding a new bathroom. We had major workmanship issues with the contractor, and eventually fired him before the bathroom was finished. Part of the plumbing was installed, but never fully tied back into the system as a result (one of the pipes on the right in the picture is capped).

    When we moved back in, the Rinnai wasn't working right. The service tech came out and fixed the issue by flushing air from the lines, and replacing a faulty pressure regulator the contractor had installed. Although, when the service tech was talking with Rinnai, we were told the re-circulation pump should not run 24/7, and they sent us the Control-R. I just set the system to on-demand heating, and we usually only ran it in advance of taking a shower.

    We also had a new washer/dryer installed around the same time, and the washer was making the most water hammer noise.

    All in all, we had a number of plumbing related items to troubleshoot all around the same time.

    We have tried a couple things, including flushing the system and replacing the water filter. That helps for a while (maybe a day or two), but then the hammer returns. I also tried installing arrestors on the hot and cold water sides of the new washer, but that didn't do anything to stop, or even reduce the hammer. I also replaced the black expansion valve, but that didn't seem to do anything either. We have had maybe 5 plumbers out to the house over the years, and no one has had any idea what to do.

    It would actually seem to make sense that the hammer is being caused by the check valve mentioned above, so I'm hoping someone on the forum has a good answer for us as I would love to get this fixed!