Heat Pumps for Hydronic Baseboard Heat?

Yes - with caveats. The Idronics journals cover this really well, I’d start there. 

I've been doing a lot of reading about this as well. I agree with @Hot_water_fan - there is a great Idronics issue about this. There are several air-to-water heat pumps (theoretically) available, although finding someone to install/service them will depend on location. Arctic, Aermec, Spacepak, and Enertech make them, amongst others. There are 'cold-climate' as well as inverter-driven models available, but as others mentioned, they usually top out at like 130-135F as the maximum water temperature. Several of them also have integrated electric boilers, and can supplement as needed.

For retrofitting a house with hot water baseboard, it's really important to actually understand the heating requirements for your house. 180F seems to have been the standard assumed supply water temperature for many hydronic systems, but many systems are also grossly oversized. Baseboards will still put out heat at lower temperatures, but the output is reduced as the delta between baseboard temp and room temp lowers. There are also 'high output' baseboards (with bigger/more fins), panel radiators and fan coil units that could all be installed to replace/supplement existing baseboards if more output at lower temps were needed. After doing a lot of measuring, my house has enough baseboard that it could probably handle 110F-120F water just fine.

Good to hear Pat! Try using this for the heat loss: https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

Various sorts of baseboard. Fintube needs relatively high temperature compared to cast iron or steel panels. The first is convection while other ones radiate somewhat.

I seriously doubt that heat pumps are viable for baseboard heating unless you fill the entire house with baseboard. Anyone who thinks they can get 130 deg out of a heat pump (don't forget all it is is a reverse air conditioner) you head pressure will be out of sight. Yes you may be able to run it at that high temp for a while. How long do you think the compressor will last. I would give it 5 years at best. Think about what your asking the machine to do

spacepak's solstice system a2whp does not require any anti freeze.

Spacepak makes both. Nordic makes a split one as well.

I believe the spacepak does require glycol, though there may be a unit I am unfamiliar with that does not.

from the manual for the solstice extreme




full manual here
https://www.literature.mestek.com/dms/SpacePak/ILAHP Manual (ILHP2-0722).pdf

Spacepak makes both. Hope that clears it up!

What is low temp baseboard?? Baseboard is baseboard either CI or fin tube it will work at low temp the question is do you have enough wall space that will take enough baseboard to do the job. Most homes do not

Here is an example of a low temperature fin tube

Both Jaga and Runtal have some mini fan boosted fin tube assemblies made specifically for radiant temperatures 
120 SWT seems to be a sweet spot for A2WHPs

https://smithsep.com/wp-content/uploads/Heating-Edge-2-Literature-2.pdf

WMno57 said:

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Peakedtoosoon said:

"Don't forget your outdoor A2WHP will need Glycol"
Not if you install a split system, where the heated water is contained within the home.

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yellowdog said:

spacepak's solstice system a2whp does not require any anti freeze.

https://www.literature.mestek.com/dms/SpacePak/SIS2-0421_2.pdf

"If unit is operating in a cooling capacity then the
water system must contain a mixture of inhibited
glycol and water with thermal protection sufficient
for the coldest expected temperature for the
installation. The inhibitor level can degrade over
time, and may need to be adjusted periodically.
The inhibitor is essential to prevent the glycol from
accelerating corrosion of metal components in the
system. The glycol and inhibitor levels must be
checked regularly (no less than once annually).
The minimum allowable concentration of glycol is
10% by volume in all installations."

Interesting find WMno57. Note that is for cooling mode, not heating. Typical chilled water supply temps are 4C-7C (39F-45F), somewhat above the freezing point. Anybody know is it typical for commercial chiller systems to use glycol? Does the Spacepak have poor control of LWT so they want extra safety margin?

Would a good intermediate step towards eventually going to a HPWH be to replace the standard hi-temp fin-tube baseboards with new low temperature, fin-tube baseboards? I can do the plumbing myself. Would this be worth it, since I could then lower the supply temp of the current boiler down to 150F? House is a ranch - 1 heated floor - back loop is 35.66 ft of fin-tube, front loop is 38.83 feet.

Do you have easy access to the underside of the floor? Personally I'd be inclined to add staple-up radiant to the floor rather than removing and replacing existing baseboard.

Hi

Did you ever go forward with an install?

I'd like to see their outputs at 100 degree SWT. Then I could run it with radiant ceilings and no mixing valves.

@DCContrarian excellent, thanks! Makes for a large panel, but worth it to, in my case, run condensing all the time.

a wall can be turned into a supersized panel radiator , ideal for HP temperatures also

I'd supply the wall with 120F, a good operating temperature still for

HPs. And within the comfort range for sheetrock.

20 delta for 110AWT. The multiplier for the wall radiant is .8. I come up with 32 BTU/sq. ft.

Ceilings are a bit lower btu/ sq. output as you get some temperature stratification up against the ceiling.

I think that should be delta T divided by 0.71? I've used a slightly different constant for years — 0.6.

I would wait for the Daikin Altherma 3 due out 1st qtr 2025 is what I got from supplier. Sounds like certification is done. Or maybe Mitsubishi will finally release theirs, it would be my 1st choice.