Water, not air.

That water circulates to the emitters.

Radiators, underfloor pipe or fan coils.

A hot water cylinder is usually served too.

It occupies the position a boiler occupied.

Because emitters were sized around a boiler's output.

A boiler reached temperatures a heat pump cannot.

The same radiator gives less heat at lower flow.

Room heat loss still has to be met.

Capacity is rarely what blocks the job.

A room-by-room assessment.

Each room's heat loss is calculated.

Each emitter's output at the new flow is checked.

Shortfalls are identified before anything is bought.

Replacing a boiler like for like skips all of it.

Enlarge or replace the emitters.

Use underfloor heating where the fabric allows.

Specify a high-temperature machine.

Often a combination of the three.

Each has a different cost and disruption profile.

Efficiency, and sometimes refrigerant choice.

Reaching higher flow costs more work.

Seasonal performance is lower as a result.

It may avoid replacing every emitter.

That trade can still be worth making.

Giving the machine enough water to work against.

A small system volume causes short cycling.

Cycling wastes energy and wears the compressor.

Buffer volume lets it modulate steadily.

It also assists defrost operation.

It lowers flow temperature as conditions ease.

Full flow temperature is only needed at the extreme.

Efficiency improves at every lower temperature.

Most of the season is spent well below design.

Much of the seasonal saving is won here.