[flakeoil]

The water temperature which you deliver to radiators are not defined by capacity of the heatpump, but how hot the radiators can be for safety/comfort reasons. If the radiators are too hot people could burn by touching them or stuff like platsic chairs would melt. Also the piping in the walls and floors cannot support too hot temperatures.

The temp for water used in radiators 60-70C is easily achievable by an air-top-water heat pump. It does not depend on the energy source, gas/oil/electricity.

[adrianN]

The heat pump runs with much better efficiency at <50 degrees.

[ZeroGravitas]

Condensing gas boilers similarly run more efficiently at lower temps.

If the water returning to the boiler isn't below 54C then there will be no condensing at all, and the advertised 90%+ efficiency won't happen till the return value is more like 46C.

That translates roughly to max winter temp of 65C leaving the boiler and lower when lesss heating is required.

This can be tweaked by the end user and save 10-20% on heating bills.