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I want to electrify the heating in my house.

It's heated by a hydronic radiant system, fed by a propane boiler. Another loop from the boiler heats a hot water storage tank. The total floor surface area covered by the radiant heat is about 1000 sqft. Is it reasonable to do all this with a heat pump instead? (I'd rule out an electric resistance boiler as too inefficient -- electricity is relatively expensive in my area.)

I also have a mini-split AC system already installed. Would it be feasible to run that off the same outdoor unit as the heating equipment? (I don't want to use the mini-split itself for heat -- I have high ceilings, and the hot air from the indoor units doesn't really reach the lower half of the room.)

The climate is hardiness zone 5b, although in practice it doesn't go below 10 °F. I have no idea whether a ground-source heat pump is feasible on my site.

Note, I'm not planning on doing any of this work myself. I just want to be well-informed about what my options are as I start looking for installers.

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  • you seem to be asking three different questions – Jasen Dec 22 '20 at 22:41
  • Electric heat pumps are extremely efficient when in the heat pump cycle. However, when you start to use the resistance heat it is expensive. This occurs when the temperature drops and the heat pump can’t extract heat out of the air. This occurs at about 30 to 35 degrees, depending on the rating of the unit. Why use such a heating unit in a climate that goes to 10 degrees? – Lee Sam Dec 22 '20 at 22:44
  • You might consider geothermal. Same principle as a heat pump but rather than exchanging heat with air they exchange heat with a fluid pumped though a ground loop. There is a LOT more heat capacity in the ground than in the air. – jwh20 Dec 22 '20 at 23:05
  • When I built my house I looked into geothermal. As the system went thru various design revisions. The cost got ridiculously high. The amount of trenching was enormous. Admittedly I have a much larger space to heat, but still. I ended up going with a setup like you have: A propane boiler and heat-exchanger water heater connected to the boiler. @LeeSam Newer heat pumps, esp. the mini-splits can be very efficient even to quite low temps, but I have no idea how to connect them to an in-floor hydronic radiant system. There are experts here that might chime in soon. Great questions, BTW, + – George Anderson Dec 22 '20 at 23:43
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    One thing I've learned over the years is "simplify and separate". While it may seem cool to have a bunch of integrated systems, the problems are 1) few people (and getting fewer) that know what the Hell their doing, and 2) too many points of failure. IE: if your boiler goes down or your new heat pump setup goes down, not only will you have no yet, but no hot water. Think about an HP water heater, if practical in your area. If not, just go electric WH and call it a day. – George Anderson Dec 22 '20 at 23:49
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It's certainly possible - whether you can get someone in your area to install and service it, and whether you can afford it, are two entirely different questions.

Air source heat pumps that are designed for it can operate efficiently below 0°F - many mini splits qualify, and few whole house units do, for whatever reason. As such, they also work fine at 10F which you need.

Run-of-the-mill whole-house heat pumps (that are mostly air conditioners) do indeed crap out around 30F as Lee Sam comments - but there is better tech available if you can find a dealer and know what you are looking for.

The cold-weather air-to-airs are also generally far more efficient at cooling, as a bonus.

Getting that heat into water is a far more fickle beast - there are a few companies making Air-to-Water heat pumps, and at least one of those is a Canadian firm that claims operation to -25F - but they have no dealers in my area, and self-importing a system without a service person or dealer available was a bridge too far for my taste. So I purchased an air-to-air cold weather minisplit system with a servicing dealer in my area. For the moment, the hydronic tubing in my slab is idle. I'll probably install a wood-burning boiler for it eventually.

Water-to-Water Heat Pump (ground source hydronic heat pump) seems to be somewhat more common, but ground source tends to have high installation cost (relatively low operation cost, however, due to a stable source of non-varying temperature to work from.) It either takes a lot of excavation or drilling a couple of wells, and that runs to money. If that isn't a problem, it's probably the best way to do this, and certainly the most widely available heat pump to hydronic option.

Your mini-split is a system unto itself, and most likely cannot be combined with any other system, particularly an air-to-water one. It might be possible but likely not fiscally sensible to use a larger outdoor unit of its brand that can accommodate it and additional heads, but I'm not aware of any maker offering an air-to-water version of that type of setup.

The common, normal solution to high ceilings and winter heat distribution is to use fans to move the hot air down from the ceiling in heating season.

"Heat pump water heaters" in freezing climates depend on whatever the house heat is in winter, as none that I'm aware of can work with below-freezing outside air supply. So they save a bit on air conditioning in the summer, but make whatever your heating system is work harder through the winter (even if that's also a heat pump.)

If electricity is relatively expensive compared to propane in your area, making this change is unlikely to pay back, or might even cost you more to run.

[...and the usual boring stuff nobody wants to do 'cause it's not as sexy as a whole new system - if you look at your roof and you have icicles, you need more insulation. Insulation and air sealing tend to be the best bang for the buck in saving money on heating - even if you get a whole new system, less waste makes that cost less to operate, too.]

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  • 0f is actually a thing as is -15f on $$$. ground water heat pumps are by far the most efficient. most people already have 1 well the supply but you need a second discharge well to return the water into and it needs to be large enough in diameter/ depth that the flow will not over whelm the return well. The only issue with these units other than the return well is water tends to corrode heat exchangers so they don’t last as long but with the newer variable speed compressors and water to water the cost savings is huge and more than offsets the shorter service life including multiple water pumps. – Ed Beal Dec 23 '20 at 21:56

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