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Recirculator Issues

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My house had an oil fired water heater with a recirculator pump on the hot water line. The feed line is 3/4 and the return 1/2, with the pump on the return just before it enters a T with the cold water supply. The 3rd leg of the T was the inlet to the water heater. The pump ran 24/7.

I replaced the heater with a hybrid electric model, and immediately noticed that I now had 2 new issues:

1. When I ran hot water on a couple of the faucets, it would quickly turn luke-warm to cold and stay that way. I eventually figured out that cold water was being drawn back up the recirculation return and overpowering the hot. I suspect it all worked fine before because the oil fired tank was simply an open tank with 1-1/4" inlet and outlet, and offered minimal resistance to hot water flow. I don't know what the interior of the new tank is like, but it has more resistance. Enough so that the easiest path is now up the return, even with the pump running!

To fix this, I added a swing check valve to the return. It solved the backflow issue, but now I'm often getting a repeating water hammer when the hot water is running. It definitely happens with the recirculation pump off, but I haven't verified if it happens when the pump is running. This ties into issue #2.

2. The hybrid electric water heater does not play nice with the recirculation system. See this TSB for more details: https://rmc-cdn.s3.amazonaws.com/site/rheemdotcom/resources/tech-bulletins/Heat+Pump+w-recirculation+1331.pdf

I plan to solve this by either adding an aquastat on the return to only run the pump when the water has cooled, or a timer to periodically run it instead of constantly, or both. I thought of just ditching the system entirely, but without it my kitchen sink takes a solid 2-3 minutes to get even marginally warm water. There's also no easy way to remove the loop as it is buried in walls and I don't want a bunch of stagnant warm water in the system.

I'm not sure what to do for #1 and I'm open to other suggestions for #2.

Thanks in advance!

Comments

  • Larry Weingarten
    Larry Weingarten Member Posts: 3,335
    edited September 2022
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    Hi, I think you could solve both problems by first, replacing the swing check with a "silent" spring check. Then best would be to install a demand controller for the pump. This will activate the pump only when you want to, greatly cutting down on heat loss from the plumbing. This matters with a heat pump heater as they have a very slow recovery in heat pump mode. Constant recirc can easily overpower a heat pump, forcing the heater to use resistance heating.

    Yours, Larry
  • hillridge
    hillridge Member Posts: 9
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    Thanks, do you have an example of the spring checks? I assume they have to be pretty weak in order to allow the circulation to work, as the pump doesn't seem to have high flow. How does a demand controller work?
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,335
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    Hi, Here is just one supplier of a silent spring check: https://www.stayflowproducts.com/check_valves/bronze_threaded_check_valve/ What's important is that there not be metal to metal contact, but metal to a resilient material, like rubber or a soft plastic. And, here is a link to the original demand control: https://gothotwater.com/ The technology is licensed to others as well.

    Yours, Larry
  • hot_rod
    hot_rod Member Posts: 22,258
    edited September 2022
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    Should be a low lead brass valve also NSF 61 certification
    A check with a .25- .50 psi “pop” is what you want. Look for a brand with the Neoprl brand conical type check. Commonly used in small recirculating circulators and hydronic models. No letter thsn a 1/2” check so small flow rates can fully open it.

    Insulate as best you can on any exposed hot lines. This reduces heat loss and lowers the gpm or time required with on demand systems

    Saves $$ on operating and AC costs, possibly
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • HVACNUT
    HVACNUT Member Posts: 5,864
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    Looks like Rheem doesn't even recommend a recirc loop for their heat pump. And definitely not in Energy Saving Mode.
  • pedmec
    pedmec Member Posts: 988
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    Did you confirm that the check valve is the source of the banging? I"m having a hard time imagining a check valve on the recirc line causing banging as you would need alternating pressure differentials on both sides of the check valve for it to open and close repeatedly thereby creating the banging noise. my question is whether you have a another check valve on the cold water line like a rpz which when discharging does cause the type of banging you describe. it also has a drain attachment which might cause you not to notice that its discharging.
  • hillridge
    hillridge Member Posts: 9
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    (meant to post this days ago, checked the page and it's still sitting as a draft)
    Thanks for the suggestions.

    I'm certain the hammer is due to the check valve. If I leave the pump running, the hammer goes away. If I shut off the pump and close the ball valve on the recirc line, it also goes away. It's only when the line is open without the pump running that it happens. My guess is that the pump running provides enough flow for the valve to slowly close instead of slamming shut.

    I think switching to a silent one will solve the issue if I change the setup to only recirculate when the water has cooled off.

    In the meantime, I changed the water heater mode to "heat pump", which seems to do fine with the recirculator constantly running, and I put the pump on a timer that shuts it off from 11pm to 5am to save energy when there is unlikely to be any use.

    I suspect I may need a new cartridge on the pump. The body gets hot enough that I can't hold onto it for more than a second or two, and it sounds a little grumbly when running. It draws a constant 0.75A.
    hot_rod
  • Larry Weingarten
    Larry Weingarten Member Posts: 3,335
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    Hi @hillridge As long as you're fiddling with the plumbing, do make sure you have adequate air elimination installed, so the pump cannot get air-bound and run dry. It sounds like that happened and has damaged the pump.

    Yours, Larry