GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Recirculating Hot Water Line Impact on Heat Pump

kb167 | Posted in Mechanicals on

After adding a recirculating hot water line to my ao smith hot water heater, it seems like the heat pump is running 24/7. Plumber installed a grundfos auto adapt pump. Before the recirculating line was added, it almost never ran. Is this bad for the hot water heater? Any tips to help the heat pump? I was thinking of wrapping the pex piping. Thanks for any tips.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    You absolutely need to insulate ALL of the pipe in the recirculating loop. If you have uninsulated pipe in that loop, you’ve basically connected a looong radiator to your hot water heater, and the heater will have to run to make up for all the heat loss in that “radiator”.


  2. kb167 | | #2

    Thanks for that thought. Curious if anyone has any specific knowledge regarding heat pump water heaters. Was a recirculating line a bad idea? Is it ok for the pump to run that often?

  3. Expert Member
    Akos | | #3

    You don't want to run your heat pump 24/7, these were not designed for that and might end up frosting up.

    As Bill said, you must insulate your hot water supply and return lines. Un-insulated pipes loose around 40btu/linear feet, typically recirc loop of around 60 feet, you are looking at 2400btu which is a good chunk of the capacity of your heat pump.

  4. chrisjri | | #4

    Does the auto adapt pump run 24/7? Or is it on a timer?

    Where is the return water entering the water heater? I'm not a plumber but am thinking of doing a recirc loop. I would remove the drain spout, put in a tee so the return water enters the water heater at the bottom of the tank.

    1. kb167 | | #5

      The auto adapt learns your schedule and pumps water to accommodate those times. My plumber installed the loop exactly as you described.

      1. Expert Member
        Akos | | #6

        Is the pumps sensor installed properly? Too close to the tank and it might think there is hot water use all the time.

        Also check the pump is in auto adapt mode, sometimes plumbers leave them in always on.

        1. kb167 | | #10

          It's pretty close to the tank, but that is what the install diagram shows. It is in auto adapt.

          1. Expert Member
            Akos | | #13

            Check the pipe where the sensor is mounted. Say half an hour after a hot water draw, the pipe there should be cold. If the pipe is hot, then move the sensor further away from the tank. The sensor should be on the HOT WATER side not on the return.

            The auto adapt uses this sensor to figure out demand, so if it is always hot, it will think there is hot water usage and run the pump too much.

  5. jackofalltrades777 | | #7

    I unplugged my hot water recirculation pump. On a short run, it makes no sense to run it and it wastes energy/electricity. Within 3 seconds, I have hot water, so it's stupid to run the pump. Code required it but I simply unplugged it.

    Depending on you water line runs, you can always disconnect the pump. Especially in the summer when instant hot water is not needed. You will definitely wear out your heat pump water heater if you keep the recirc pump running 24/7. Probably shorten the water heater life by half.

    1. kb167 | | #12

      What makes you think it will shorten the water heater life by half? No disrespect intended, but are you qualified to make that claim? I am obviously concerned about my investment so I want to understand if that is a guess or a pro's response. Thanks!

      1. luislarrea | | #20

        He meant water heater pump, he mentions it in the first half of his sentence. Running the pump 24/7 vs timer will wear it out in half the time.

  6. fourforhome | | #8

    I plugged my pump into a wifi smart plug which I can control and schedule from my phone. It is scheduled to turn on when we get up in the morning - all other times are on demand.
    It can be triggered using Alexa, Siri, or Google and the macro I programmed into it automatically turns it off in 5 minutes.
    All total, we run it less than 30 minutes a day.

    1. kb167 | | #11

      Great idea!

  7. brad_rh | | #9

    I have a long recirc loop and have the supply line insulated, but not the return line. I use a push button demand system, so the pump only runs when we tell it to. The HPWH doesn't seem to run too much. They do run longer than a gas or resistance electric unit.
    Is the auto-adapt logic in recirc system working well or running too much? I think in order to save money and energy with a recirc system it needs to be on demand.

  8. onslow | | #14


    One additional point to add. Check the model number of the Grundfos and verify that it has an internal flow check valve in it. Also check that it is installed in the correct orientation for system flow if it does have the internal check valve. The vane design of the pump impeller apparently allows water to flow through pump. This may have something to do with the very low pressure head it is meant to deal with. I will assume you have power on 24/7 to the pump, so it may "learn" your usage habits in Auto mode. There is a mode which will force 24 hour circulation, but that lights up to tell you.

    The plumber who put in mine probably saved a few bucks by getting the one that seems to lack the check valve, which resulted in creating the very long radiator others described in earlier comments. After a few months of insane electric bills, I first unplugged the recirc pump figuring that would answer my question about what was causing the high usage. However, I finally determined that the heater was running almost constantly thanks to the thermo-siphon effect created by the recirculating loop. If the pump had the internal check valve, the looping should have been stopped. My biggest issue to fixing the perceived situation was finding a very low pressure check that would work with the low pressure head of the pump. I have one now, but other more pressing matters have distracted me from putting it in. Plus I don't have the expanding tool for the pex. So I don't have confirmation of the fix yet.

    I also admit to be a bit perplexed by the flow direction to be resisted by the check valve and placement relative to the pump to avoid the thermo-siphon effect. The universally unclear direction sheet for the pump never once clearly indicates which side of the pump is connecting the return part of the loop and which side goes to the cold return leg of the water heater. If anyone can explain the diagram on their page 12 of the UP 10-16 PM instruction sheet, please share. The placement of the sensor right at the top of the heater on the hot pipe seems odd at best. Prior experience tells me that location will always be hot which seems pointless to tell the pump. If it is some how detecting flow of water, why put it on the hottest standing location? I admit to being baffled by the non-instructions, which is why I trusted the plumber to know what he was doing. Apparently, he didn't either.

    I do have the deepest inaccessible parts of the loop insulated quite thoroughly and the effect of insulation is quite pronounced. The water coming to the bathroom faucet can sit in the line for almost 40 minutes and still present at the faucet as very warm. It only lasts for the volume of the insulated sections in the walls and second floor, but enough to be a gift when brushing my teeth.

    I do intend to finish the insulation of the lines where expose in the basement. Just when is open until other project get done first. Anyway, go nuts with insulating, it helps a lot.

    1. Expert Member
      BILL WICHERS | | #15

      A low-flow check valve won’t stop a thermosyphon from operating, since the check valve has to “point” in the direction of flow of the pump, and that’s the same direction the thermosyphon would flow. If you need a low-resistance check valve, you want what is known as a “swing check” valve. This type has a swinging gate instead of a spring-operated valve mechanism. Note that swing check valves work best for low flow rates when installed horizontally, but be sure they aren’t tilted too much or they won’t work properly.

      You would normally want to sense the hot water temperature at the farthest fixture on the recirculating loop. If the sensor can’t reach that far, you need to sense the return line temperature at the water heater.

      If you want to stop the thermosyphon effect when the pump isn’t operating, you can usually accomplish that with a spring-type check valve, or use a thermosyphon preventer on the water heater itself (many newer units have one built in, they look like a plastic iris-like device inside the water pipes).


  9. Expert Member
    Dana Dorsett | | #16

    >" If you have uninsulated pipe in that loop, you’ve basically connected a looong radiator to your hot water heater, and the heater will have to run to make up for all the heat loss in that “radiator”."

    That's not only following the laws of physics, insulating the recirculation loop to a minimum of R3 (both supply & return) is a code requirement under the IRC (and many local plumbing codes.)

    Unlike IRC code, the laws of physics are self-enforcing.

    >" Is this bad for the hot water heater?"

    Well, yeah. Things are designed to operate at some reasonable duty cycle, and a nearly 100% duty cycle isn't that reasonable. Every revolution of the compressor & blower puts some amount of wear on the bearings & seals- nothing lasts forever. Running it at a higher duty cycle inevitably shortens the equipment life.

  10. Expert Member
    BILL WICHERS | | #17

    Adding to what Dana said, it’s worth pointing out that it is NOT necessary to leave the return line uninsulated “for the system to work”. I’ve seen this info out there, especially regarding thermosyphon systems. You should insulate the ENTIRE loop, supply AND return. If you want to start a thermosyphon going, just run the hot water at the far end until it gets hot, then shut it off. A fully insulated recirculating loop works just fine, only with a slower flow which is actually desirable.

    You only what a little bit of flow if the loop runs all the time since higher flow = higher heat losses. The only time you want a high flow is when you have a beefy pump on a fast-acting on-demand system.

    Note also that the loop legs can be different size pipes. Mine is a 1” supply (makes for little pressure drop if someone opens a second faucet), but only a 1/2” return line. Works just fine, and it’s cheaper and less materials too.


  11. billwilljulz | | #18

    I plan to install a heat pump water heater (Rheem Proterra) in my new home with a recirculating pump and dedicated return line. I came across your question and this article regarding Rheem heat pumps with a recirculating pump:
    It certainly raises a lot of questions for me at first glance but hopefully it helps clarify for others.

  12. nathan808 | | #19


    I know this is an ancient thread but would love to hear any feedback on the Proterra with the recirc. Thanks

    1. billwilljulz | | #21

      Originally, I wanted a recirc line in my home with a water heater. I REALLY wanted the Proterra until I found the article linked in my above post. The article states that basically it won't work in heat pump mode, which defeats the purpose of a heat pump water heater. There may be other tank options out there that do work. I have since changed my thought processes to multiple tankless units (one feeding kitchen, one or two feeding two bathrooms) located inside said rooms for instant hot water.

  13. brad_rh | | #22

    I think Rheems concerns in that article are moot if it's an on demand (push button) recirc system. An on demand system is also the most efficient

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |