GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Compact heat pump water heater

Gwisejr | Posted in Green Products and Materials on

Based on the replies that I received, let me changed the question to finding a supplier of solar water storage tanks that are lowboy types. Same physical size as the former question.  I’d be interested in using something like the Nyle Geyser now the C-8 to heat a solar water storage tank. 


Any one know of a lowboy type heat pump water heater? My we have a conditioned crawlspace  ~ 36″ to the joist, ~44″ between joist.  The 43gal Sandon will probably work but i cant justify spending that much. I’m installing solar on this new build and it seems to me that the $$ is better spent on additional PV’s. Thank You.

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. joshdurston | | #1

    Too bad the Nyle Geyser isn't available anymore. It'd be perfect for you.

    1. Gwisejr | | #6

      I see that Nyle has something similar coming out, C-8.

      No info but seems to be similar.

  2. Expert Member
    Akos | | #2

    Typical residential electric water heater uses 10kWh/day. A heat pump would use 1/3 that, so the delta is 6.6kWh. My average is 5.5 sunhours/day, so I could offset the difference with 4 panels.

    Most likely, you are better off installing the extra couple of panels.

    You could also dig a pit in your crawlsapce to make space. A side benefit, if you have other equipment down there, it will make servicing it much more pleasant than trying to work in 3' height.

    1. Gwisejr | | #7

      Thanks for the suggestion but unfortunately, I poured a rat slab..

  3. user-2310254 | | #3


    Have you considered a Rheem Marathon electric heater? I had one in my last house, and it worked really well.


    The 50 gallon short model is 47.25 inches, so it is too tall. In any case, you should be able to combine an electric tank and solar to create a pretty efficient setup.

  4. walta100 | | #4

    If you have room for more panels you would have a more reliable system if you added panels and went with a standard electric water heater. It may cost less to install.

    No to refrigerant leaks, no air filter to keep clean and one less logic board to fail.

    When the water heater gets an error and starts beeping at 2 AM this spaces does not sound like a convenient spot to visit.


  5. Jon_R | | #5

    > the $$ is better spent on additional PV

    Which is better also depends on how the utility credits you for production. While hot water heating is more easily time shifted to when the sun is shining, it still isn't a perfect match.

  6. onslow | | #8


    I tend to agree with Walter on the Marathon being a better bet if you can't fit a more typical HPWH on the first floor with outside air available in a good climate choice.

    You didn't say where you are located for CZ or if there is no option for locating the heater in a garage or if you are in a warmer CZ. One important thing to know about HPWH in general is the rather wide variations in energy management, especially in regard to recovery rates. The most direct work around for lousy recovery rates is a large tank for fewer people.

    In a different thread, a poster provided a link to a very detailed NREL study of 5 HPWH. I have finally digested the whole 77 pages. While the information is now several years old, I think the general analysis holds up. A quick way to characterize all of them is Heat Pump Warm Eaters.

    I had been trying to figure out the validity of using HPWH in dry climates with longer heating periods. I have now concluded that due to frosting and icing problems, none of the units tested intend to utilize the latent energy of humid air in a major way. Any dehumidifiying that is accomplished is secondary to sensible heat harvesting. More revealing was the fact that Florida might not be a great place for some HPWH, while New Mexico would be great.

    If you live in a climate where you can provide substantial amounts of warm air that you are not paying to make warm, then the energy advantage to HPWH is substantial. IF you are stripping heat out of the home's enclosed envelope of air, as I would be doing for 6 months of the year, then the advantage is much lower. The solar hit for winter conditions with my situation would add to the energy problem if I was looking to use make up panels to balance the load. The load balancing Akos refers to would appear to only hold true for a pure heat pump in free air by my calculations.

    Fan size and their relationship to ducting the units was an additional concern I had which has been partially satisfied. One unit tested has a 500cfm fan, another 300cfm. It is important to note that 500cfm translates into 30,000cfm/hour. This is approximately 2ACH for a mid size home. Consider this when looking at knock on effects in your current design.

    As Akos has noted, 10Kwh a day can be expected to go into a water heater. This is about 45 gal of water rising 90F. This is about 34,000 BTUs that come from somewhere plus the energy penalty of the compressor. Winter temps of feed water for me can drop to 40F, so this number is a bit high. Check your water feed temp range and do the math for your usage.

    If the 34,000 BTUs are harvested from external air, then the cost to you is the pump run time at best. Not the case in some units due to energy management decisions embedded in the programming. For me in the winter, I would be stealing the 34,000 BTU from my house heat and be paying at best a 2.5Kwh(8500 BTU) price for the pump to steal them if I chose one of the tested units. During the winter (or summer here) humidity is not a problem. The other units may steal fewer BTUs directly from the air, but make up the difference by resistance heaters in the tanks. If the imbalance of stealing BTUs to resistance goes too far, the normal heater is quieter, less fussy, and generally cheaper like Walter says. (disclosure I have an 80 gallon Marathon. Yes it eats a lot of Kwh.)

    The Geyser unit appears to be a bottomless HPWH so to speak. You would need to hook it up to your existing water tank which negates much of the value in your situation. If you can find a large diameter insulated tank for the crawl space, perhaps it could be made to work. It may have an advantage of higher achievable water temps, but then you have the added complexity of blending valves to avoid dangerous hot water. The Geyser unit if placed in the crawl space would likely drop air temperatures significantly, so you would have to know or manage humidity levels to find whether frosting up the unit would be a risk.

    The COP of a unit whether a system COP or just the HP part is a measure of the efficiency of transfer of energy. A COP of 4 merely means the pump used one unit of energy to transfer four. A net transfer gain of three units. The system is blind to the source of the energy being transferred. It is not magical energy creation, which I suspect average lay persons think. If you like I can go into much deeper detail, but it does get tedious.

    Last thing to ask is your electric costs and your age. If you have 10 cent Kwh's, solar may be a poor way to spend money. I have 16 cent Kwh and due to qualifying for Medicare, a statistically dicey payback period. Solar sales people love to use insane energy cost multipliers to speed up the payback and use the panic of the disappearing tax benefit. Do stop and look at realistic tax liability for your future, as the time it may take to deduct actual liability may be much longer than you think. Or the fates think.

    1. joshdurston | | #9

      I think one of the main advantages of the geyser is the ability to put the heat pump and tank in separate locations. You could put a couple lowboy electric tanks together to the get the capacity you need in the crawlspace, and put the heat pump unit wherever it makes sense in your situation.
      Or retrofit an existing hot water heater into a hybrid.

      1. Gwisejr | | #10

        I'm thinking to use a solar water heater with the heat exchange loop connected to a Nyle Geyser/C-8. I'm on well water that although it has not been tested yet, I'm thinking it best to close the loop with distilled water between the heatpump and the tank. I'd be interested in a tank that has two heat exchanger loops and possibly run that to an actual solar collector. I'm looking at using a Sol-Ark inverter and tying the smart-load output to the Standard electric water heater and placing that in front of the heatpump water heater system as a preheater.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |