[sokoloff]

Baseboard heaters are often sized such that very high temps are needed (because that's what cheapest/lowest labor/least space used), but they don't have to be sized that way. In the attic bedroom, we have baseboards around the entire perimeter on two walls and same in the bath. I run the attic zone on the same water temp (outdoor reset controlled to be quite low) as the rest of the house (mostly large cast iron rads, one cast iron convector). Good insulation and air sealing in the attic means that the attic zone calls way less than the downstairs.

[looofooo0]

Which is not exactly efficient. 40C or less is desirable.

[sokoloff]

My return water temps are 115F (46C) on a P98 design heating day, and obviously cooler on warmer than design days. Cooler is always better, but "baseboards require 180F [82C] water because that's what's on the spec sheet" is a commonly-held but mistaken belief.

[looofooo0]

This is bad. Return water should be less 35 degree max. Actually 30 degree after the heat pump would be ideal: https://www.flow30.de/

[jillyboel]

[citation needed]

[looofooo0]

Ah this is school knowledge of thermodynamics: the smaller the delta the more efficient the heat pump. For human comfort look at the iso7730. Also the system is self regulatory with such low temps.

[Retric]

Heat pumps and furnaces behave very differently here.

For a furnace you’re talking fractions of a percent difference in efficiency across a wide temperature range so by far the most critical issue is total heat losses to the outside. A heat pump’s efficiency is far more variable making total losses to the outside less important.