High-Temperature Heat Pumps for Geothermal
Hello. We currently have a gas boiler, but we want to get off of fossil fuels and replace it with an electric option (solar will provide the power needed). However, we want to keep using our convectors, rather than converting to in-floor radiant or using forced air.
I don’t think we can get up to 180F, but we may be able to get away with 160F or 150F. I see two units in that range, a Nordic and a WaterFurnace:
https://www.nordicghp.com/product/nordic-products/high-temp-water-to-water/
https://www.waterfurnace.com/residential/products/geothermal-heat-pumps/504w11
Does anyone have experience with either of these units, and trying to use them for old hydronic convectors (rather than in-floor radiant)? I’d appreciate any suggestions, including whether a backup electric boiler can be integrated for particularly cold days. Thanks!
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Before going down the road of high temp heat pumps, check that you actually need it. In most older homes, even with minimal upgrades such as a bit of air sealing, attic insulation and storms, the original rads tend to be at least 2x over sized. This means that you can run them at much lower temperature as still have enough heat. If you have some fuel usage data, I would run through this:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Once you have the actual heat load from your house, you can check through your convectors size and see what temperature water they actually need to meet the heat load above. I've run old over sized cast iron rads with 120F water without issues.
PS. Hydronic heat pump in any shape or form is VERY expensive. In most cases replacing the rads with air source heat pumps is a much cheaper option that also provides AC.
You may be able to bring the temperature down with some modifications.
https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf
This is a good resource.
Even if you can't quite make it with some lower temp and existing radiators, a fan coil or two in the worse rooms might get you there. Most of the time they could be off.
You need a room-by-room Manual J (not a whole house load) to know if a room is adequately heated.
So I am making these upgrades now in my house.
I am also installing new Jaga Radiators.I’m planning to run them at a design temp of 140 degrees, but, when we insulate better dropping the temp down to 110.
Thanks everyone. We have made the improvements we reasonably can. My EDR calculations of our existing convectors suggest they may be fine down to the 160-150 range but not lower. I appreciate the suggestion, Ryan, for possibly putting in new radiators that have a higher BTU/h output.
I really am just wondering if anyone has experience with those two high-temperature geothermal heatpumps (or any others I am unaware of), as well as experience integrating geothermal heat pumps with an electric boiler backup. Thanks!
Try your question at https://www.geoexchange.org/forum
Member Doc Jenser has installed some of the Waterfurnace high temp units.
My house combines what people have been suggesting: I have a water-water heat pump (hydron module) running the original old baseboard convectors. With 110 F water, the heat output is about 1/4 rated. With envelope minor envelope upgrades, I got the load down to half of the convector rated output. Then I added panel radiators, not to replace, but to supplement the baseboard units. A good idea is to make a few of those fan convectors rather than radiators, and use those in the summer for cooling. (Insulate the pipes running to them really well to avoid condensation.)
One other option to consider is radiant ceilings. They can be cheaper than floors, and easier to retrofit. And they can even have comfort advantages, since when you are lying on the couch you get more radiant exposure from the ceiling than from the floor.
Subsequent envelope upgrades made the 110 F water unnecessary: I only need 100 F now.
But you may not need any of this, given the new systems that you linked. I'm not sure whether any of them have variable speed compressors but I would want that in a new system, and there are some low-temp water-water systems that have that available now.
I've considered adding an electric backup boiler, and it looked easy to do, but I prefer to have my electric backup be electric space heaters stashed in a closet, available if there were a problem, so that it's truly independent of the hydronic system. There's also the question of backup for grid outages, which is a different question.
Most high temp heat pump systems are two stage, which means there are two sets of compressors to get up to the temperature you want. This brings the effective COP of the system down quite a bit. Along with the high install cost, this also means higher operating cost. You are better off to install the extra radiant capacity and work with lower temperatures as suggested above.
A modification of Charlie's suggestion would be to get a low temp geo or air to water system to run the existing setup. Install a mini split in the open areas of the house to supplement the lost heat.
The radiators would still be used to even out the heat and give rooms better control but bulk of the heat could be done by the mini split. This would be much less intrusive and lower operating/install cost plus also provide AC.