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Electric boiler and water heater sizing and manufacturer help

Mickey Hoang | Posted in General Questions on

I’m looking for help deciding a electric heating system that can also do domestic hot water for a 2250 sq. ft. house with radiant floor.  The load for heating is 25,000 Btuh (including safety factors).  For domestic water, it’s a 2-bathroom house with kitchen and washer.  Should I get a boiler and a separate water heater or can I combine them into one?  Do I need an indirect DHW tank if I combine them?  I also looked at air-to-water heat pump but most of these max out at 100MBH heating so I don’t think it’ll work for domestic water unless I install two (don’t want to do that).
I also looked at mini split but decided against it for aesthetic reasons.  Also we’re not cooling the house.
Thanks for your help.

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  1. Paul Wiedefeld | | #1

    Air to water plus an indirect would work, you would have storage so you don’t need a high capacity heat pump. Size for the heating load, not the DHW. I don’t see a particularly compelling reason to use an indirect over an indoor heat pump water heater, you don’t gain efficiency, save any space or increase reliability. I doubt it would save money either. Another option would be to use a electric tank resistance heater and use the air to water heat pump to preheat the incoming water with heat exchanger. Like an indirect, but with backup and separability.

    1. Mickey Hoang | | #3

      I didn't think about indoor heat pump water heater. What's the cost compare to indirect? Doesn't the heat pump water heater require more space to draw heat from? That makes the space it's located in colder, right?
      How would I size the indirect DHW or the HPWH?

      1. Paul Wiedefeld | | #4

        Not sure on the price difference - I think it would be insignificant if I had to guess. It will require more space or a louvered door for air flow. Sizing the HPWH is easy - you have about 3 sizes to pick from. They have slower recovery than gas, which might lead you to an 80 gallon tank. You can Lee the water at a higher temperature and use a mixing valve to bring it down to 120 when it leaves the tank to store more. Using a drain water heat recovery unit could also improve real capacity during showers. Sizing the indirect depends on which air to water heat pump you choose - the Nordic brand has an internal desuperheater so you could use a preheat tank followed by an electric resistance tank. In general, an air to water heat pump will have higher capacity than a HPWH, so you can go with a smaller tank.

  2. DCContrarian | | #2

    Chiltrix will do what you want. Indirect hot water tank piped as a zone on a hydronic system. It can do cooling too but I think it's a bit of a kludge the way they did it, but you said you don't want cooling anyway.

    It's just really expensive, by the time it's all done it's going to be 2-3 times as much as going with an indoor HPWH and air-air heat pump heat. But if that's how you want to spend your money who am I to stop you.

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