[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.

[looofooo0]

First, sooner or later you have to replace your furnace with a heat pump. Second, modern furnaceses are condensing. Return temp should be as low as possible maximise condensing.

[Retric]

We agree return temps should be lower, but to determine how critical this is you need to out numbers on that. In steady state operation at maximum load a difference in return temperatures of 15f is ~1% efficiency. But steady state at maximum load is an extreme situation the average return temperature is well below that theoretical maximum thus an even wider difference is needed for a seasonal difference of 1%.

Redundancy is critical in areas that get really cold. That may eventually mean turning to hydrogen, but a backup gas furnace for a well insulated home really isn’t a major CO2 contributor. More relevant to the discussion it further reduces the impact of minor changes in efficiency or comfort.