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We're trying to go all-electric/solar and reduce reliance on fossil fuels. We currently have an 11 year old propane tankless combi-boiler that supplies hot water and also circulates water through a hydronic baseboard heating system connected to a thermostat (starting to slowly fall apart). We also have Mitsubishi heat pump split units that we use as our primary heat source in the winter (the boiler is more to keep the house above a minimum temperature to protect the pipes). These are in great condition.

Given our goal, I was thinking of installing an electric heat pump water heater to provide hot water for faucets, shower, wash, etc.. Because that wouldn't serve as a boiler, we would not be supplying any hot water circulation for the baseboards, and just rely on the splits for heating the house. When reading the documentation, it seems that they can perform at capacity when well under minimum temperatures we experience here in the winter.

My question are, is it ok to do this, and what problems might I run into?

Additional details:

  • The house is 1300 sqft with OK insulation, but we'll be sealing and insulating any gaps/thermal bridges.
  • We live in the northeast in climate zone 5 in this Energystar map.
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  • I've heard feedback from people in this same climate zone saying that their heat pumps struggle in coldest weather. You could find out directly: keep the hot water heat off for a winter, see how the heat pumps alone hold up.
    – aquaticapetheory
    Commented Jun 16, 2023 at 13:28
  • Please clarify "the boiler is more to keep the house above a minimum temperature to protect the pipes." Are the boiler and circulating pumps actually running in winter for that purpose? Or do you mean that you are relying on the hydronic system for backup in case the heat pumps fail or become inadequate due to unusual low temperature?
    – MTA
    Commented Jun 16, 2023 at 14:10
  • @aquaticapetheory That varies considerably with the minisplit. With the Mitsubishi H2i I have no troubles in climate zone 6 on that map. However, I opted not to go heat pump on the water heater because that adds load to the heating heatpumps, (so they'd have to be oversized to meet the heating + water heater load) and adds complexity (and a large price increase) with a poor warranty to the water heater. And outside-air heat-pump water heaters are mindbogglingly expensive with poor warranties.
    – Ecnerwal
    Commented Jun 16, 2023 at 15:44
  • IMHO heat pump water heaters are much more complex and expensive than traditional tank type WHs and don't have the recovery rate of tank type or tankless, but since you already have a tankless propane fired WH just keep it, it will use very little propane just for hot water (DHW) production, not nearly enough to have even the tiniest impact on climate change. Like Ecnerwal said, they work by extracting heat from where the are installed. If in unconditioned space, like a garage, it might be acceptable, but you might be able to toss out your fridge as it turns your garage into a giant frigde
    – George Anderson
    Commented Jun 16, 2023 at 16:33
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    ...ran out of space in my previous comment: It takes very little power to run propane fired tankless water heaters, furnaces, boilers. So if you want redundancy, a small generator could power the tankless and the boiler. Many experts have predicted less and less reliability on the electric grid and rolling blackouts as the utilities attempt to convert to green energy. In going all electric, doesn't necessarily mean you're going "green" you need to look into what generates your electricity, In the NE I believe a lot of it is coal or natural gas.
    – George Anderson
    Commented Jun 16, 2023 at 16:40

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Not that it really matters to this answer, but I heat entirely with mini-splits in a climate that gets to -15F (actual with the heat pumps installed and working fine - though sucking a lot of power to do it) or -20°F ("zone-wise," and I've seen it, but not when the mini-splits were installed nor indeed for over a decade at this point. My mini-splits are rated to -15F - last I knew, if you need to go colder you need to look to ground-source heat pumps. And if we had that more than we usually do, they might be worth looking at harder for more efficiency in those conditions, due to the steady 50°F or so input temperature they give. But ground-source wells are expensive.

If you'll be leaving the boiler system idle, you either want to blow the pipes out so they are dry, or fill with an antifreeze solution, since baseboard radiator pipes tend to be located near or in the outside walls and are not always terribly well insulated. With regular heating load this is not a problem, but when sitting idle they are prone to freeze up (a common problem for woodstove users in cold weather.)

The heat for water heating (in freezing climates) with the more affordable hybrid heat pump water heaters comes from the heat in the house, so the heating heat pumps will be working harder to keep the house warm while the water heater makes water hot and the house colder. Those units generally have a minimum intake temperature of 37°F or so.

If you are looking at one of the far more expensive outside (sub-freezing for part of the year) air to hot water units, that problem goes away, but at that point having wells drilled or soil loops installed for ground source heat pump water (and house) heating makes a lot of sense, unless the prices have come way, way, down since I last looked for them.

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    My techs suggested running the hydronic heating system periodically to reduce freezing risk.. though most of my pipes are inside the insulated envelope, a friend did have a radiator pipe freeze a few years ago despite the csystem being in operation.
    – keshlam
    Commented Feb 6 at 15:04
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Don’t do it! Hydronic heating has a different effect on the space. If your house isn’t fully insulated and sealed it will be a drafty nightmare if you go full air-to-air heating. I ripped out my boiler and put in a Solar Assisted Hot Water Heat Pump (which works great for domestic hot water actually) but my house is Chilly and my bills are $$$ because of how my home was built (80s) keep the propane, think of your home as a hybrid. That will get you the best combination of efficiency and cost effective heating!

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    – Community Bot
    Commented Feb 6 at 14:20
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    There are indeed issues with heat distribution between floors in minisplit systems. There are ways to manage that. The energy savings are huge. It's not a perfect technology but it is worth considering; talk to people in your area who have installed these in similar houses to get their opinions and operating tips.
    – keshlam
    Commented Feb 6 at 15:10

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