Basics · 7 min read · Updated 2026-05-07
How heat pumps work — the physics in plain English
A heat pump is a refrigerator working in reverse. This guide explains the four-stage thermodynamic cycle, why COP can exceed 1, and what a 'natural' refrigerant changes.
The fundamental trick
A heat pump moves heat from a cold reservoir (outdoor air, soil, or groundwater) into a warm reservoir (your home's water circuit, or your indoor air). The trick that lets it deliver more useful heat than the electricity it consumes is that it isn't generating energy — it's relocating it.
The four stages in every heat pump:
1. Evaporation — a low-pressure refrigerant absorbs heat from the outdoor source and boils into a vapour. 2. Compression — an electric compressor squeezes the vapour. Pressure rises, and so does temperature. 3. Condensation — the hot, high-pressure vapour transfers its heat to your home's heating circuit and condenses back to a liquid. 4. Expansion — an expansion valve drops the pressure, the liquid cools sharply, and the cycle restarts.
The compressor is the only large electricity consumer. Everything else is just refrigerant moving around.
Why COP can be greater than 1
The Coefficient of Performance is useful heat output ÷ electrical input. A typical air-to-water heat pump operates between COP 3 and COP 5 in winter, meaning every kWh of electricity moves 3–5 kWh of heat. That doesn't violate any conservation law — the extra energy came from the outdoor air, soil, or water.
The two factors that drive COP:
- Temperature lift between source and sink. Cold outdoor + hot
radiators = hard work. Mild outdoor + underfloor heating at 35 °C = easy work, COP 5+.
- Compressor and heat-exchanger quality. Inverter compressors
modulate output instead of cycling on/off, dramatically improving seasonal efficiency.
SCOP, the number that matters most
A single COP measurement is one snapshot. Real homes face a range of outdoor temperatures across a heating season. SCOP (Seasonal COP) is a weighted-average COP across a defined climate's temperature distribution, calculated per EN 14825. EU energy labels and EPREL report SCOP for three reference climates: Warmer (Athens), Average (Strasbourg) and Colder (Helsinki). Always check that two SCOP figures you're comparing refer to the same climate zone.
Refrigerants — why R290 is taking over
The choice of refrigerant affects three things: efficiency, climate impact (Global Warming Potential), and safety class. The 2020s shift in EU policy (Regulation 2024/573) is driving manufacturers to natural refrigerants:
| Refrigerant | GWP | Class | Notes |
|---|---|---|---|
| R290 (propane) | 0 | A3 (highly flammable) | Best long-term, charge-limited |
| R32 | 771 | A2L | Modern HFC, dominant in mid-2020s |
| R454C | 148 | A2L | Lower-GWP HFC blend |
| R410A | 1924 | A1 | Phased out for new HPs from 2025 |
R290 wins on environmental footprint but its flammability forces engineering compromises (limited refrigerant charge → smaller indoor units or split designs with a buffer cylinder).
What's next
Once you understand the cycle, the natural follow-up reads:
- Air-to-water vs ground-source — which makes sense in your climate
- Choosing capacity — sizing methodology and why oversizing hurts
- Heat pump vs gas boiler — payback and CO₂ math