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Community and Q&A

Effect of gravity / thermosiphon recirculation system on Heat Pump Water Heater

RobKlinger | Posted in GBA Pro Help on

I apologize if this exact question was answered within other Q&A threads discussing pump driven recirculation loops, but I couldn’t find anything from searches.

I am considering installing a HPWH to replace a current NGWH with a gravity recirculation loop connected into the bottom of the tank.

I understand the effects of constant heat pump recirculation loops on a HPWH, but can anyone tell me if a gravity/siphon recirculation system would have the same effect or is it completely different?  If it is equal, then that answers if I need to excise this loop from our plumbing, but if it is different, if a professional could explain the potential effect it could have on my system, I would really appreciate it.

Thank you for your time and expertise.

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  1. Expert Member
    BILL WICHERS | | #1

    It will just look like more standby losses for the tank, similar to what removing some insulation from the tank would do. This isn’t usually a big deal, it’ll just make the unit cycle s little more frequently to make up for the additional thermal losses or the loop.

    You do want to be sure to do a good job of insulating the entire loop to minimize these losses. Note that if your HPWH has a heat trap on the hot water output port, you’ll need to defeat that since the thermosyphon loops usually don’t have enough flow to open up a heat trap.


  2. RobKlinger | | #2


    Thank you. So the effects that are seen with the elevated return temperatures via a always on, recirc pump do not apply here, as it is hot water (thermal energy) that is leaving the tank via the recirc loop path a the bottom of the tank? And that effect is the same as it would be with any siphon loop - standby losses, leading to increased firings to respond.

    As you stated, similar to reducing the insulation on the tank itself.

    The loop has been in place since 1993 when the home was built, but I will review the parts of the loop that could benefit from insulation - I do know that not all the loop can be insulated, as that neuters that loop by removing the T gradient that drives a thermosiphon.

    Thank you.

    1. Expert Member
      BILL WICHERS | | #4

      You still have “elevated return temperatures”, but they’re never higher than the temperature in the water heater. Note that a thermosyphon works by hot water leaving the top of the tank and returning to the bottom of the tank. You use a “swing check” check valve on the return line near the tank, with the check valve pointing TOWARDS the tank. This ensures that you don’t draw water from the bottom of the tank when there is demand for hot water. You can’t use a regular spring-type check valve here because the very slight flow in a thermosyphon loop can’t open such a check valve.

      You CAN insulate the entire loop, and it will work fine. Mine here at my own house is done exactly that way. Insulation isn’t perfect, so there is still a slight cooling effect, which keeps things circulating. You want to insulate the entire loop to minimize the losses. With a very short loop I suppose you might not get enough thermal difference to cause any water flow, but presumably if you want a recirculating system, you probably have a long enough run of pipe for it to work regardless of how well insulated it is.


  3. Expert Member
    DCcontrarian | | #3

    I learned recently that the reason that heat pump water heaters can't be used with recirculation is that HPWH are constructed differently from traditional water heaters. All heat pumps are dependent upon temperature deltas for efficiency; the HPWH is designed so that it pulls cold water from the bottom of the tank and heats it. In order to work efficiently the water in the tank needs to be striated. A recirculation system is constantly pulling water from the top of the tank and returning it to the bottom, which has the effect of mixing the tank and eliminating striation.

    So the question is, would a gravity thermosiphon do the same thing? I think it depends on where you plumb the return. Plumbing it into the bottom of the tank would probably result in mixing. Plumbing it into the top would probably work.

    1. Expert Member
      BILL WICHERS | | #5

      The heat pump can heat the water regardless, it doesn’t require stratification of the water in the tank for it to be able to work. You always get a little bit of stratification in any water heater with a tank though, which is why water heaters are always designed to draw hot water from the top and supply cold makeup water through a feed tube near the bottom (and that tube may be internal to the tank). This is the same reason why the heater is always near the bottom of the tank too.

      A thermosyphon loop provides a VERY small flow and will have much less effect on the stratification of the water in the tank compared to what a recirculating pump would do too.


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