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Community and Q&A

Heat Pump for Domestic Hot Water and Hydronic Floors

bucksbear | Posted in Mechanicals on

Putting the question upfront – I have a potential design to use a heat pump water heater as the supply for both domestic hot water and a lightly-loaded hydronic floor heat system. I’d love to hear ways that my idea is bad or suggestions for improvements. Details are below.

I am doing a renovation and small second-story addition on our 1870s-era house in Western Massachusetts (Climate Zone 5). The house is expanding from 2050 to 2400 s.f. I am moving the house from an oil furnace and 50 gallon electric hot water tank (each unit is 10 years old) to a ground source heat pump and heat pump water heater for the house’s heating, cooling, and domestic hot water needs. I pay a high rate for electricity (now up to $0.27 / kWh for 2022!), but I am comfortable with the higher expense with the goal of moving away from fossil energy at home. We are also adding a solar array. For the heat pump in particular, while it is expensive upfront, the substantial federal and state credits and rebates make the payback period roughly 5 years (!). The system is sized to serve our full heating and cooling needs for the whole house – around 60k BTU from a 5-ton system (based on a full Manual-J conducted by the GSHP company).

In the renovated and added spaces (kitchen, two bathrooms, and primary bedroom – 900 s.f. total, spread over two floors), I am also getting hydronic floor heat installed, mostly as a luxury feature rather than as a primary source of heat, which will be served by the heat pump. We have 8 foot ceilings for most of the house, except for a cathedral  ceiling for the new addition (up to 14 feet at the peak for the 450 s.f. primary bedroom and bath). The new space will have an R49 roof with R23 walls, so it should be reasonably tight, while the old space has R15 walls and I will hunt down air leaks where possible. We are also planning to insulate the basement walls and attic floors, and all ductwork and the HPWH will be in insulated spaces. These decisions are all made, though I could definitely understand counterarguments for air source heat pumps or minisplits, against hydronic floor heat for a bedroom, and other aspects.


My question centers on the potential to use the same heat pump water heater for both the domestic hot water supply as well as for the main source of hot water for the hydronic floor heating system that serves part of our house (again, 900 s.f. out of 2400 total). My reasoning is that HPWH systems are more energy efficient than a typical electric hot water heater (gas is not an option) and that there will only be limited load on our hydronic floor heat system. However, HPWHs have slower recovery times. I have a good friend who is an experienced plumber who proposed an idea that uses an 80 gallon HPWH for domestic hot water (50 gallons is likely enough for our DHW needs, but the incremental cost of the larger unit is manageable). Then, coupled with this and serving the hydronic loops would be a 40 gallon HTP SuperStor that would “run in reverse” and mostly just maintain water temperature with the bulk of the heating coming from the HPWH. The SuperStor has very low standing heat losses and can essentially buffer the hot water produced by pushing the HPWH a little harder, particularly in times of low hot water demand. Then, at peak times (showers, dishwasher, laundry, etc. – we’re a family of four), there will always be hot water available for both the HPWH and floor heating. This will also isolate the water used for the floor loops from that used for domestic hot water in order to address concerns about pathogens like legionella. I recognize that this plan does not save much on capital costs – getting the SuperStor is essentially the cost of buying a separate electric water heater – but if the plan works, it should save by replacing what would be the operating costs of the hydronic floor hot water  supply heating element with only the additional HPWH operating costs. Again, with 27 cents per kWh electricity, operating costs are significant. The dream would have been a water-to-water ground-source heat pump system, but that option was too expensive upfront.

 My questions for the GBA community: is this idea for using the HPWH to provide the heat supply to a hydronic floor heat system reasonable? Do you have any ideas for alternatives or improvements?

Thanks in advance for reading my screed.

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  1. paul_wiedefeld | | #1

    Many geothermal heat pumps which heat air also have desuperheaters that heat water - is that an option? I think that's the most elegant solution and seems like it can't be more expensive than installing a HPWH and an indirect.

    You're using a heat pump water heater to
    1. cool an indoor space
    2. heat an indoor space
    3. then possibly you plan to reheat the indoor space using the ground source heat pump correct? This probably works out to around the same efficiency as a resistance tank for maybe 5-10x the price.

  2. Expert Member
    Akos | | #2

    5 tons for 2400sqft of house with any insulation in the walls is way oversized. I would run through the calculations here based on the previous fuel usage of the existing structure:

    Since the existing structure will be the bulk of your heat loss, you can add on a about 12-15 btu/sqft for a new 2x6 structure to get a feel of where a proper manJ should be.

    The biggest cost of geo is the the loops, there is no point in oversizing it if not needed. This also means the equipment will get much longer runtimes which is good for efficiency and comfort.

    As for floor heat from a HPWH. Most units have a 4000BTU heat pump, that is good for a large bathroom or two but that is about it. Even there, you'll get much longer runtimes on the heat pump and it might not last. Putting on a timer so the floor heat only runs when people are home will help.

    If you want more BTUs, you can look at a Sanco2 unit which has a much larger compressor. It is a split system with the heat pump outdoors so you are not stealing capacity from the GSHP.

    A de-super heater is only a couple thousand BTU as well, so you can't run much from it. At the cost of a indirect plus complexity, it is probably not worth it.

    For a new construction like this, your best bet is a electric resistance matts. The ROI on anything hydronic is probably never.

  3. BirchwoodBill | | #3

    The system sounds over sized. In Zone 6a, we have Hydronic heat which runs between 75F to 95F using a heat exchanger off the DHW tank. Roughly 30k BTUh for 2400 sq ft. The system is micro-zoned so the tile areas heat most of the house.

    My boiler is over sized at 80k, I would look at an Air to water heat pump…. Artic, Nordic, SpacePak they can also do cooling and dehumidification.

    The decision for hydronics is always comfort and Good IEQ.

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