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

Hybrid hot water heater for space heat and hot water in small A.D.U.

jyasord | Posted in General Questions on

I am designing a 750 sq. ft. Accessory Dwelling Unit (A.D.U.).  It will be well insulated with SIP panel construction, have extensive passive solar gain, and utilize a slab radiant heat system.  There is extensive documentation on gas tank and tankless systems, but I would like to make an all-electric fossil-fuel-free house. We will use PV panels for most of our electricity, though we will be connected to the grid.  I want to use a hybrid electric water heater to heat the home and hot water, likely in an open direct system with a timer to circulate fresh water in the tubing. We will have only one bathroom, no tub, and 1 low flow shower.  Is there a reason this isn’t being done/won’t work? We will have a backup wood stove for the coldest snaps and could use a solar preheating unit, though I’d prefer not to so the system is as simple as possible. Opinions, Ideas? thanks

Sorry for leaving out geography.  The house will be located in Reno,  I have contacted and they have suggested a theme tankless unit suited to space heating.  Sounds like the hybrids don’t put out enough heat and are counter-productive.

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. this_page_left_blank | | #1

    I have an open direct system. Will this be seasonal house? Trying to figure out why you'd incorporate a timer, as it's generally not necessary.

  2. jyasord | | #2

    Websites have suggested a timer so the water doesn’t get stagnant, especially in the summer. The house will be used year round by two people. The alternative to the tank system, is tankless electric. There seems to be very little documented use of these despite being reasonably priced. I’m looking to keep systems simple, affordable and electric.

  3. this_page_left_blank | | #3

    There's a simple way to plumb it so fresh water is drawn in every time you use hot water. You can see it at or Anyone suggesting a timer as the primary method of avoiding stagnation doesn't know what they're doing.

    1. jyasord | | #13

      Climate zone will be Reno, NV. Zone 5b on your map, though i’ve Seen it referred to as zone 4 on other maps. Recommended space heating recommendations are around 45btu’s per sqft.

  4. GBA Editor
    Martin Holladay | | #4

    First of all, we need to know your climate zone or geographical location to give you good advice.

    When you use the term "hybrid electric water heater," I assume that you are referring to a heat-pump water heater that also has an electric-resistance heater for backup. This kind of electric water heater can't be used for space heating. More accurately, the only way it can be used for space heating is if the heat-pump function is disabled and it works in electric-resistance mode.

    The vast majority of heat-pump water heaters sold in the U.S. and Canada (the single exception is the Sanden) have a compressor located on top of the tank. A heat-pump water heater lowers the temperature of the room in which it is located -- it cools the air -- and uses that heat to raise the temperature of the water in the tank. If you tried to use a heat-pump water heater for space heating, it would be working furiously to cool the air, and the same time that you tried to distribute the hot water and heat up the room you just cooled. The net result would be an efficiency equal to an ordinary electric-resistance water heater.

    If you are planning an all-electric house, the usual way to provide space heating is with a ductless minisplit. Fortunately, that approach matches your goal, since you wrote that you would "prefer to have the system as simple as possible."

    1. this_page_left_blank | | #7

      There is some merit to running the heat pump mode in the cooling season, and switching to electric in the heating season. Depends on the climate and how good a deal you can get on the water heater, but you're right that the mini split makes more sense in the vast majority of cases.

      1. jyasord | | #14

        I’ll start researching the ductless mini split, will it efficiently heat water for the slab radiant flooring? I haven’t seen it mentioned in hydronic heating use.

        1. GBA Editor
          Martin Holladay | | #15

          You are building a tiny house (750 square feet) and you have told us that you "prefer to have the system as simple as possible." So you don't want hydronic heating or a radiant system in your slab. All you want is to be warm and comfortable.

          A single ductless minisplit will easily supply space heating and cooling for this small building. Forget the hydronic system. (For more information on why, see "All About Radiant Floors.")

          If you don't take my advice, and you prefer a complicated heating system, you can make hot water for a hydronic system with a boiler that burns natural gas or an electric-resistance boiler. It's also possible to use an electric-resistance water heater. Or you could buy an air-to-water heat pump -- but at that point your system becomes quite complicated and expensive, and you won't find many contractors who will be able to service it. For more on this topic, see "Air-to-Water Heat Pumps."

  5. Expert Member
    Akos | | #5

    I've looked at the ducted heat pump water heaters for a small space, but they just don't put out enough heat.

    If you look at something like the Rheem unit it has a 4200btu heat pump, if you can get your heat load down to bellow 6 btu/sqft, might work for you.

    1. Expert Member
      Dana Dorsett | | #9

      How do you know they put out enough heat? Without a good estimate on the heat load number (and at what outdoor temperature) the whole discussion is somewhat pointless.

      It's sort of like asking if a bicycle will be the right choice for transportation, where the particulars of the application could easily call for anything from a pair of running shoes to a 12 ton dump truck and beyond.

      So the question for jyasord is, how much heat is enough heat, and at what indoor & outdoor temperatures?

    2. this_page_left_blank | | #10

      That is about 1.25kW, which is shockingly small (about 1/4 the output of a standard electric water heater). That isn't even enough to supply domestic hot water, let alone also heat the house.

  6. GBA Editor
    Martin Holladay | | #6

    I'm not sure what you are talking about. Rheem heat-pump water heaters lower the temperature of the space where they are located; they can't be used for space heating (unless, I suppose, you install the water heater in a garage).

    1. Expert Member
      Akos | | #8


      Their Professional Prestige series can be installed with supply and exhaust ducted to the outside of the building.


  7. walta100 | | #11

    What you are proposing is bordering on a perpetual motion machine.

    You want to take the energy from the air in the room use that energy to heat water so you can take the energy from the water and put it back into the room air. You see it is a closed loop any energy you lose to the outdoor is gone until the outdoors is warmer than the indoors.

    Because your water heater has a back up heating element that will be replacing all lost to outdoors, so a $100electric baseboard heater will have the same operating costs as your $5000 loop.

    If you want lower operating costs you need a heat pump that can take energy from outdoor and bring it indoors like a minisplit, PTAC.


    1. Expert Member
      Akos | | #12


      If the water heater heatpump is ducted outside (like the Rheem unit), it is taking the heat energy from outdoors not the room, exactly the same way split system works. No perpetual motion here.

      The only problem is the 4500 btu capacity. Not going to be able to heat much more than a small shed with that amount of heat especially if typical water usage is taken into account.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |