It partially depends on your climate. I’m in Portland, Maine, so for us, we have a natural gas furnace with radiators, but they’re heavily supplemented by a few heat pumps. For us, the temperature frequently gets very low in the winter, sub 10°F, and this is the point when heat pumps aren’t as efficient. An ideal set up in a small home could be heat pumps running off solar and a woodstove for when it gets really really cold.

Figure out the minimum flow (supply) temperature that can meet your building heat loss on the design day. If you have enough radiation, you might be able to get by with 135°F flow.

I have baseboard in the attic and cast iron rads on the main level. I have the flow set to 126°F until the outside air temp drops below 30°F and then I increase by 2°F for every 4°F change in OAT. So at 2°F OAT, I have 140°F flow.

That’s enough to maintain temp in my fairly poorly insulated 1920s house near Boston. It’s enough to slowly recover temp as well.

140°F is a stretch for air to water heat pumps, but you could reasonably use an A2W for most of the year and supplement with a 9kW (31KBTU/hr) electric boiler as emergency supplemental heat and use that a couple days per year.

What killed it for me (and the reason I have a fairly new gas boiler installed) is the upfront cost was utterly uncompetitive.

If you have a condensing boiler, set the high-limit or flow temp to 130-135°F and see how your house handles it on the cold days. You can measure and calculate all you want, but nothing beats trying it.

This guy is a great resource if you want to understand what goes into a proper A2W system install. In Europe, it seems by law they are required to do heat loss calculations to size an install. In the US, it seems to be a little more loose. https://www.youtube.com/watch?v=0IvGe4JZLSU

I saw a video recently from a ... less sophisticated place in the US... where the HVAC installer literally held up a piece of paper from the street with different sized houses on it, and sized the HVAC based on which of the houses best "covered up" the actual house when looking through the semi-transparent paper. Just astonishing. No consideration at all for the build quality/insulation details, no adjusting for the distance from the road, just a "welp, this looks like a 3-ton job".

That's insane. Proper sizing is paramount, it's also a good way to surface that 5k in insulation improvements might be the better first step to consider vs installing a larger system just to keep up with losses.

What works for me in the northeast with an old house is a high velocity small duct air handler/distribution system (unico) coupled with a Bosch cold climate heat pump, with an oil furnace backup. This is forced air, so not exactly what you are looking for and I get that the heat from those baseboards is a better quality/less drying form of heat. I could upgrade to add a whole house humidifier to address this a bit. I use the heat pump most of the year, down to an outside temp of about 25F, although Bosch says I should be good down to 5F. The oil furnace runs basically Jan/Feb (but not as much this Feb as it's like 50F today). I've also extensively sealed/insulated the house (although there's always more of that to be done in an old house). It's a pretty good solution. I'd love for the air to water heat pump tech to work better, but I got tired of waiting.

As others comments said, it depends on the climate : The problem is that the power/performance of the heat pumps decreases with the cold weather and the necessary increase in the temperature of the heating water, which obliges to keep the boiler for the cold weather. This is a problem for the heat pumps without probable solution and little progress to wait. The solution is to insulate the house very well so that its needs can be covered by the heat pump with less hot water. Divide the losses by 2 at least.

So CO2 heatpumps are starting to become available in the US which can run higher temperatures. But most likely the best option would be to switch to radiators that don't require as high of temperature to operate. Something like underfloor heating or panel radiators.

The other option that is common in New England is to keep the baseboard oil heat and get mini splits. Then run the mini splits for heat and if they cannot keep up switch to the oil heat.

> From what I've read, that's not quite feasible yet, since current systems can't produce a high enough temperature.

Is it because baseboard hot-water systems have limited exchange surface surface and thus need very high temperatures to do anything?

Yep - exactly the issue. They've started shipping much more efficient baseboard systems which run at lower temps by adding more fins, adding CPU fans to increase heat transfer across the plates, etc. but the correct answer is to usually invest in air sealing/insulating the house which will require fewer BTUs in the first place.

I think this is possible. I will ask to a teacher of hvac-learning and come back to you asap. But basically you can heat water instead of air with a heat pump.

Solar hot water would probably be a better fit for your situation.