A Heat Pump Delivers More Heat Than It Consumes

An electric heater turns each unit of electricity into one unit of heat. A heat pump routinely delivers three or four, which sounds impossible until you look at what it is doing.

The electricity moves heat rather than making it

A resistive heater converts energy into warmth directly, and cannot exceed one for one. There is nowhere for additional heat to come from.

A heat pump instead runs a refrigeration cycle in reverse, collecting heat from outside and releasing it indoors. The electricity powers the transfer, not the heat itself.

The heat delivered is therefore the sum of what was gathered outdoors and the energy used to move it. The ratio between output and electricity is the efficiency figure quoted.

Cold air still contains usable heat

The counterintuitive part is extracting warmth from air that feels freezing. Cold is not an absence of heat, only a lower quantity of it.

The working fluid is expanded until it becomes colder than the outside air, at which point heat flows into it naturally, exactly as it always flows from warmer to cooler.

Compressing that fluid raises its temperature above the indoor air, so heat now flows outward from it into the house. The cycle repeats continuously.

Efficiency falls as the gap widens

Moving heat across a small temperature difference is easy. The colder it is outside and the hotter you want the radiators, the more work the compressor must do.

Performance is therefore not a fixed number. The same unit might deliver four units of heat on a mild day and closer to two during a hard frost.

This is why heat pumps are paired with larger radiators or underfloor pipes. A lower water temperature over a bigger surface keeps the gap narrow and the efficiency high.

Defrosting is an unavoidable interruption

The outdoor unit runs below air temperature, so moisture condenses on it and freezes. A layer of ice blocks airflow and the unit stops collecting heat effectively.

The system responds by briefly reversing itself, sending warm fluid outside to melt the ice. During that period the house receives no heat and the electricity is being spent backwards.

Frequency depends on humidity as much as cold. Damp weather near freezing causes far more defrost cycles than dry weather well below it.

The house sets the outcome

Because output temperature drives efficiency, insulation matters more with a heat pump than with a boiler. A leaky house demands hotter water, which is precisely what the system is worst at.

Steady low-level running also beats intermittent blasts, which is the opposite of how many people operate a gas boiler.

The technology is old and well understood. What changed is the expectation that heating equipment should be optimised around the building rather than sized to overwhelm it.