A heat pump extracts heat from outside air (or ground, or water) and uses a refrigeration cycle to raise it to a useful temperature for the home. The same basic cycle used in a fridge, run in reverse, is what heats the water flowing to your radiators or underfloor heating.

A monobloc unit contains the full refrigeration circuit outdoors, with water pipes running into the house. A split system has an indoor unit too, with refrigerant pipework crossing the wall. Each has trade-offs around installation complexity, refrigerant handling and indoor space.

R290 (propane) has a much lower global warming potential than R32 and is increasingly common in newer monobloc units, though it comes with its own handling and installation considerations due to its flammability. R32 remains widely used and well understood by installers.

Almost all modern heat pumps use inverter-driven compressors that vary their speed to match demand, rather than simply switching fully on or off. How far a given model can turn down — its minimum modulation — affects how well it avoids cycling in mild weather.

Manufacturer data is published at standard rating conditions like A7/W35 — 7°C outside air, 35°C flow temperature. These figures are useful for comparing models on equal terms, but the property’s actual design conditions will usually differ, so the rated figure is a starting point, not the final answer.

Different manufacturers make different trade-offs around noise, minimum modulation, defrost strategy and footprint. None of these differences change the underlying design process — heat loss, flow temperature and emitters still need calculating regardless of which model is eventually chosen.

How Does a Heat Pump Work?

Heat Loss & Heat Pump Sizing

Hot Water & Cylinders