How does a heat pump work? Explained in plain language

A heat pump seems magical: extracting heat from cold outdoor air to heat your home. How is that possible? In this article we explain it with the fridge analogy, so in five minutes you'll understand what happens under the hood.

The fridge in reverse

A heat pump works on exactly the same principle as your fridge — but in reverse. In your fridge, heat is taken from inside and released at the back (feel how warm that black grille is). A heat pump does it the other way around: we extract heat from the outdoor air and release it indoors.

And yes — there is always heat in outdoor air, even in frost. Only at absolute zero (-273°C) is there no heat. A Dutch winter at -10°C is comfortably workable for a heat pump.

The 4 steps of the refrigerant cycle

Inside a heat pump a special fluid circulates: the refrigerant (e.g. R32 or propane R-290). It continuously cycles through four steps:

Step 1: Evaporate (outside)

In the outdoor unit the refrigerant has a very low temperature — colder than the outdoor air. As a result it absorbs heat from the air and evaporates (changes from liquid to gas). At -5°C outside the refrigerant may be -15°C, so 10°C cooler.

Step 2: Compress

The gas is pumped to a compressor, which compresses it under high pressure. Compression sharply raises the temperature — similar to a bicycle pump that warms up in use. The gas now becomes, say, 60–80°C hot.

Step 3: Condense (inside)

The hot gas flows to the indoor unit or a buffer tank. There it releases its heat to the air in the room (air-to-air) or to the central heating water (air-to-water). The gas cools and condenses back to liquid.

Step 4: Expand

The liquid flows through an expansion valve, where pressure drops sharply. The pressure drop cools the refrigerant strongly — back to -15°C. The cycle can begin again.

Why is it so efficient?

Here's the magic of a heat pump. The energy you put in (electricity for the compressor) delivers 3 to 5 times as much heat. You don't "create" the heat — you only move it. That costs little energy.

Compare with a gas boiler: that creates heat by burning gas. Its efficiency tops out at 107% (high-efficiency boilers). A heat pump reaches 300–500%. We call that the COP.

What are COP and SCOP?

COP (Coefficient of Performance)

COP is the efficiency at a single measurement moment. A COP of 4 means: 1 kWh of electricity produces 4 kWh of heat. COP is measured under ideal conditions (7°C outside, 35°C output temperature) and is therefore a "lab figure".

SCOP (Seasonal Coefficient of Performance)

SCOP is the average efficiency over an entire heating season. Much more realistic: on colder days efficiency drops, on milder days it rises. A SCOP of 4.5 means you get on average 4.5 kWh of heat per 1 kWh of electricity over the winter.

SCOP is the number you'll notice in practice. The higher the SCOP, the lower your electricity bill.

Types of heat pumps

Not every heat pump works the same. Three main types:

• Air-to-air: extracts heat from outdoor air and blows warm air into the room. Essentially an AC that also heats. No water piping needed.
• Air-to-water: extracts heat from outdoor air and supplies warm central heating water for radiators or underfloor heating. The most common type in Dutch homes.
• Ground-to-water: extracts heat from the ground (via a borehole or horizontal loop). More expensive, but the highest efficiency (SCOP 5.0+).

And cooling?

Many heat pumps can also cool — by simply reversing the cycle. The gas then releases its heat outside instead of inside. An AC is essentially an air-to-air heat pump that was sold cooling-only; modern models can do both.

Conclusion

A heat pump is no magic — it's a fridge in reverse that moves heat from outside to inside. Efficiency is high (3–5× as efficient as gas) because the system moves heat instead of creating it. With inverter technology it still works efficiently down to -15°C.

Want to learn more? Read about the differences between hybrid and all-electric or the 2026 ISDE subsidy.