How does a heat pump work?

Step 2 of 15 · Explain

Four parts in one closed loop

The outdoor unit in the garden holds the whole refrigerant circuit. Only water pipes run into the house, to the circulation pump in the cellar and on to the underfloor heating. One cable brings electricity into the box; most of it drives the compressor, a little the fans and the control.

Inside, a refrigerant runs round a closed loop and passes four parts. In the evaporator, the finned coil behind the fans, the cold refrigerant takes heat from the air. The compressor squeezes it, which makes it hot. In the condenser, a stack of steel plates, it gives that heat to the heating water. The expansion valve lets the pressure drop, and the refrigerant is cold again.

Step 3 of 15 · Predict

Electricity drives the compressor, the air supplies most of the heat

In the model at 7 °C outside, the unit draws about 1,400 W and gives about 7,400 W of heat to the water: a COP of 5.2, close to the datasheet's 5.3. About 6,100 W of it come from the outdoor air, and about 1,300 W are the work of the compressor and the water pump, which also ends up in the water. The fans' roughly 150 W leave with the air that blows out of the unit.

Step 4 of 15 · Predict

Cold air still heats a colder refrigerant

A common objection is that air at 7 °C, or at −7 °C, has no heat left to give. But cold only means less heat, not none. Heat flows from warmer to colder, so all the heat pump needs is something colder than the air.

That something is the refrigerant, here propane. In the evaporator, the finned coil behind the fans, it boils at about 3.5 °C. The 7 °C air blown through the fins warms it and leaves the unit about 4 degrees colder; the boiling propane takes up about 6,100 W.

Step 5 of 15 · Explain

Boiling doesn't mean hot

Boiling only means that a liquid turns into gas, and the temperature at which that happens depends on the pressure. Water boils at 100 °C at normal air pressure, at about 70 °C on top of Mount Everest and at about 120 °C in a pressure cooker. Propane, sold as camping gas, boils at −42 °C at normal air pressure; it stays liquid only inside its pressurised canister.

In the evaporator the pressure is set so that the propane boils at about 3.5 °C. Boiling swallows a lot of heat, which is why sweat cools your skin and a spray can gets cold when you use it. The boiling propane takes that heat from the air blowing through the fins.

Step 8 of 15 · Try it

Without low pressure there is no cold side

The expansion valve keeps the pressure in the evaporator low. In the model you can open it fully: within about 15 seconds the boiling temperature climbs to 7 °C, as warm as the air, and the heat taken from the air drops to almost nothing. Low pressure is what makes the refrigerant boil cold; without it, a heat pump can't take any heat from the air.

Step 10 of 15 · Explain

One lap: evaporate, compress, condense, expand

The refrigerant leaves the evaporator as cold vapour, below 10 °C. The compressor squeezes it, and squeezing a gas heats it, like the air in a bicycle pump: it comes out at about 54 °C, hotter than the 35 °C heating water.

In the condenser, a stack of steel plates, the hot vapour gives its heat to the water and turns liquid; it leaves at about 34 °C. That is still warmer than the water coming back from the floor at about 30 °C, which it warms first.

Then the expansion valve, a narrow opening, lets the pressure drop. At the low pressure about a fifth of the liquid boils off at once and takes the heat for that from the rest, which cools to a few degrees above zero. So the compressor makes the refrigerant hot, and the valve makes it cold. The cold mixture enters the evaporator, and the lap starts again.

Step 13 of 15 · Predict

Without the compressor no heat flows

If the compressor stops while the fans and the water pump keep running, you might expect the fans to blow at least some heat into the house. They don't: the heat output drops to 0 W at once. Only about 180 W of electricity is left for the fans, the pump and the control.

Without the compressor the refrigerant is never squeezed, so it never gets hotter than the water, and the propane in the outdoor unit simply takes on the temperature of the air. Heat never flows from cold to warm on its own. The compressor is what lifts it from the 7 °C garden to the 35 °C water: it is the pump in heat pump.

Step 15 of 15 · Quiz

Below zero it keeps working

At −7 °C outside, the pressure in the evaporator drops further, so the propane boils at about −10 °C, still colder than the air. Heat still flows in: about 3,700 W from the −7 °C air. At the model's moderate compressor speed that makes about 5,000 W of heat for 1,300 W of electricity, a COP of 3.8.

On a cold day the house needs more than that, so the compressor runs faster. The propane then boils colder still, at about −13 °C, and the COP falls: to about 3.3 in the model, and to 3.16 on the datasheet (9.7 kW of heat for 3.07 kW). That is still more than three times what a fan heater gives. The datasheet unit keeps heating down to −20 °C outside.