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Community and Q&A

Tankless backup for a heat-pump water heater

user-616070 | Posted in Mechanicals on

I currently have a GE Geospring 50 gal heat pump water heater (now discontinued) that has worked pretty well. However, my toddler will be using more hot water as she ages and we’re finishing the basement to rent out, most likely to one person. Lately, the water heater has been maxing out after two showers (2.5 GPM x 10 min x 2 people=50 gal). I’ll be retrofitting a 1.5 gpm adapter which will help, but I’m still nervous about having a tenant and running out of hot water.

To add capacity, I’m considering both a solar water heater and a tankless unit. My main question is: Would it work to put the tankless electric heater in a series in front of the heat pump tank so that the tankless kicks on only when the tank starts dropping in temp? I want to use the heat pump tank most of the time since it’s efficient. I’m not interested in replacing it with a larger unit since the cost would be very high and I can’t get another rebate.

Are there tankless units designed for this or that may be better for this use? Any tips on sizing the tankless unit on a backup basis for 4 users?

I’m in Seattle, zone 4a, ~52F incoming water temp.


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  1. GBA Editor
    Martin Holladay | | #1

    Just make sure that the tankless water heater you buy is able to sense incoming water temperature and only turns on when necessary.

    I have the Rinnai RL75iP, and it does a good job. (It's $849 when you order it online.) When the Rinnai tankless water heater is fed hot water from my solar thermal system, it senses the incoming water temperature and only fires for a very short time.

  2. Expert Member
    Dana Dorsett | | #2

    An electric tankless isn't as sensitive to incoming water temps as fossil burners, but they are REALLY abusive to the grid infrastructure, and one of the LEAST green options there is!

    To fill a tub with 110F water at a not-exactly fast 4 gpm rate with 52F incoming water takes 28,000 watts of power. That's a gia-normous load (6x that of a standard electric water heater), enough to make the wires jump and the transformer serving your house heat up.

    The fact that it's intermittent and thus not adding up to a lot of ENERGY due to the low duty cycle (typically less than 10 minutes at a time) doesn't matter. The infrastructure to support that intermittent load still has to be purchased and maintained, raising the fixed-cost portion of all residential ratepayers' bills just to have enough capacity for the few minutes per week that it might actually be needed.

    An earth & grid friendler capacity boost would be to install a standard electric water heater in series with the heat pump water heater (HPWH), feeding the output of HPWH into the cold feed of the standard tank. It'll still suffer the standby loss of a standard water heater( some of which is recovered by the HPWH), but it won't be doing much actual water heating until both tanks are nearing depletion.

    If showering capacity is the issue (sounds like it is), an even greener option is to install the biggest (tallest & fattest) drainwater heat recovery heat exchanger that fits. A 4" x 48" or larger can recover more than 50% of the heat that's going down the drain, and put it into the incoming water stream. So a 105F water heading down the drain, the 52F incoming water is instantly heated to 75-80F (depending on flow rate) before it enters the water heater, and to the cold side of the shower. That has two effects: Less hot water is needed to mix with ~78F water to get 105F at the shower head, so less hot water is being drawn, and the water entering the water heater only has to be raised 27F rather than 53F by the water heater, which means as slower depletion time and a shorter recovery time. The net effect is to turn the "apparent capacity" of at 50 gallon HPWH into a 65-75 gallon water heater, at least in showering mode. It does nothing for tub fills, since the drain isn't flowing when filling the tub. See:

    Gravity film type drainwater heat recovery units have to be installed vertically to work, so you need a section of vertical drain tall enough and accessible to be able to do this. It doesn't have to be next to the water heater to be effective- there's only a couple of gallons in 100' of 3/4" pipe, but for maximum effectiveness the output of the heat exchanger has to feed both the shower and the water heater.

  3. GBA Editor
    Martin Holladay | | #3

    Sorry -- I missed the fact that you were considering a tankless electric water heater instead of a gas-fired tankless heater.

  4. walta100 | | #4

    Electric tankless whole house water heater as a retrofit into an existing house is almost never a viable option as any unit large enough to serve the whole house will require more power than then is likely to be available from any existing panel and service.

    When you add the tankless water heater, to the new electrical panel, with new meter base and heaver wires all the way to the pole, the number is more than most people will go for.


  5. NormanWB | | #5

    I had a Bosch electric tankless once. I needed 120 more amps to run it, so going this route may require you to not just upgrade the panel, but get a larger feed from the pole. That can be pricey. What about another HPWH or a standby tank fed via a circulation pump from the HPWH?

  6. user-616070 | | #6

    Thanks everyone! Glad I posted here before buying an electric tankless unit. I have 200 amp service, enough space in the panel, and was thinking of smaller unit since it'd be for backup. However, Dana's points about the grid damage make sense. I think at this point I'll try to lower flow rates and look into the second tank recirculation idea.

  7. GBA Editor
    Martin Holladay | | #7

    If you decide to add an electric-resistance water heater (with a tank), you don't need a pump. Just install the new water heater downstream from your existing HPWH, and set the thermostat on the electric-resistance water heater 10 degrees F lower than your main heater.

  8. Expert Member
    Dana Dorsett | | #8

    What Martin said- keep it simple, no recirculation, feeding the standard water heater set to 120-130F with 130-140F from the HPWH. A recirculation system would only increase the standby loss for a MINISCULE improvement in apparent capacity over just running the tanks in series.

    Insulate all of the hot water distribution plumbing with R3 foamy pipe insulation, including the 10' of cold feed nearest the HPWH, and the nearest 10' of temperature & pressure overflow plumbing. IRC 2015 doesn't require insulating hot water distribution plumbing smaller than 3/4" (IRC 2012 did, if it's over a certain length), but that doesn''t mean it's not a good idea.

  9. user-616070 | | #9

    Thanks Martin and Dana.

    Speaking of simplicity, and assuming the new hot water needs can be limited with flow restriction, what are your thoughts on a "Tank Booster" mixing valve? Basically turning up the thermostat on the HPWH to increase load capacity. This would negate the need for any new tank.

    Also, I just finished insulating all hot water lines that are accessible and plan to insulate the cold water from the foundation to the HPWH (partially to dampen sound).

  10. NormanWB | | #10

    I would think the mixing valve could be used for those times when you need the extra capacity. I would not use it when the HPWH can handle the entire demand with a lower thermostat setting, since doing so would increase your power consumption.

    As far as Dana's and Martin's idea of an additional water heater versus a standby tank with a recirculating pump, that seems like a workable solution at a potentially lower cost.

  11. Expert Member
    Dana Dorsett | | #11

    Thermostatic mixing or tempering valves between water heaters and distribution plumbing to sinks, tubs & showers is now required by code in most locations. Some of those pre-plumbed tank booster units can be quite flow-restrictive, so choose carefully- a larger thermostatic mixing valve is usually going to work out better from a max-flow perspective. Turning up the storage temperature to 150F or higher then mixing it down to 115F with a thermostatic mixing valve can provide greater apparent capacity.

    The down side to that is that a substantially higher storage temperature increases standby losses (not by more than adding a second tank though), and lowers the heat pump's raw efficiency.

  12. user-616070 | | #12

    I guess the main question is: Is it cheaper in terms on energy consumed (long term) to turn up the thermostat on the HPWH and use a mixing valve. OR, add an additional standard electric resistance tank and run in a series.

    I understand the the standard electric tank will consume less energy than normal since the water is almost always preheated (except in a high consumption period), and therefore only needs to keep it at the thermostat temp rather than heat up from the ground main temp.

  13. Expert Member
    Dana Dorsett | | #13

    Turning up the existing HPWH will be cheaper in total energy consumed, due to the lower standby loss of 1 tank vs. 2 (even at the elevated temp), and the higher efficiency (than a resistance element) of the heat pump, even at higher temps.

    A drainwater heat exchanger would actually lower the energy used, usually well into double-digit percentages for homes where most people shower rather than tub-bathe.

  14. user-4053553 | | #14

    Why not wait and see if there is a problem at all, if the new tenant showers in the evening or an hour or two away from you there may be no problem at all.
    I would raise the temp when they move in and see what happens, if there is no problem then you got away with what you have, if there is be friendly and see if a schedule can be worked out and if neither work then look at changes

  15. user-616070 | | #15

    Totally agree Alan. I'll start with flow restriction on the showers, then turning up the thermostat, then start building on these options if problems present. I'm also considering solar hot water tubes regardless so hopefully no new additions to the system will be necessary.

    Thanks again to everyone on the forum, especially those who commented, and big thanks to Martin and Dana for sharing their wealth of knowledge with the community!

  16. brad_rh | | #16

    As already suggested, reducing the flow rate and turning up the temperature, (and the drain water heat recovery) would be the 1st things to try. One other thing you could do, which 'may' not use any more energy than adding an electric WH, is change the mode of the geospring to hybrid instead of heat pump.

  17. nqc2k | | #17

    Any updates on how this turned out for you?

    I’m looking to use a Geospring water heater to feed an electric tankless water heater. I wonder what water temperatute on the Geospring will be the most efficient when feeding the tankless. (Geospring was just collecting dust in the corner, figured I’d see if I can save on electricity in trying )

    1. Expert Member
      Dana Dorsett | | #18

      Describe where the Geospring is installed, and what your hot water use patterns are like. Why do you think you need the tankless at all?

      In general setting the Geospring to a higher temperature (140F is good) and tempering down to 115F or 120F would be more efficient overall than setting the Geospring to some lower temperature and finishing with the tankless at 115-120F. No matter how high you set the Geospring's temperature it's output will always be more efficient than any contribution from the tankless, unless you have CRAZY low daily hot water use. If you need more capacity, maxing out the temp of the Geospring is the first and most efficient thing to do.

      As an independent but perhaps relevant fact, stored hot water between 80-110F is a prime breeding temperature for legionella. Storing water at lower temps than 120F to reduce standby losses has consequences.

    2. user-616070 | | #19

      My solution didn't end up including an additional water heater tank or tankless unit. I added 1.5 gpm flow restrictors on all shower heads and added a drain water heat recovery pipe to the drain for the most used shower. As an aside, this thing is awesome. I currently use the HPWH in heat pump mode only so I have some room to increase output by changing to the hybrid or high demand mode.

      Regarding the electric on-demand unit, see above for Dana's grave warning about how harsh it is on the grid. I would only consider one of the electric on demand units for an individual fixture that can't be efficiently connected to the HPWH.

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