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.
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.