How does a heat pump work in winter?

Step 2 of 15 · Predict

At −15 °C it still heats

On a mild 7 °C day the model's heat pump, at a medium compressor speed, gives about 7.4 kW of heat for about 1.4 kW of electricity. That ratio, the coefficient of performance or COP, is about 5.2: five units of heat for each unit of electricity.

A common guess is that in a cold snap it stops, or quietly switches to plain electric heating. Neither happens. Drop the air to −15 °C and the same unit still gives about 3.9 kW for about 1.2 kW of electricity: a COP of about 3.1. That is still three times what an electric heater gets from the same electricity. The air blowing out of the unit leaves at about −19 °C, because the heat pump has taken heat even from −15 °C air.

Step 4 of 15 · Explain

Low pressure lets the refrigerant boil below −15 °C

The key is boiling. Boiling doesn't mean hot; it means 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, but at only about 70 °C on top of Mount Everest, where the pressure is lower.

The model's refrigerant is propane, which boils at −42 °C at normal air pressure (NIST). The compressor keeps sucking the evaporator coil down to a low pressure. At −15 °C outside, that pressure is only about 2.6 bar, about two and a half times normal air pressure, and there the propane boils at about −18 °C: a few degrees colder than the air. Heat flows from the air into the boiling refrigerant, and boiling swallows that heat, the same way a spray can gets cold while you spray.

On the mild 7 °C day the pressure in the coil is about 5.3 bar and the refrigerant boils at about 3.5 °C. The heat pump simply runs its cold side at a lower pressure when the air is colder.

Step 5 of 15 · Try it

The colder the air, the lower the COP

What the cold does change is the cost. The compressor has to lift the heat from the temperature at which the refrigerant boils to the temperature at which it gives the heat to the heating water, about 38 °C for floor heating at 35 °C. The colder the air, the bigger that lift, and the more electricity each kilowatt of heat takes.

In the model, at the same compressor speed, the COP is about 5.2 at 7 °C, 4.7 at 2 °C, 3.8 at −7 °C and 3.1 at −15 °C. It falls steadily but never reaches zero. A real air-to-water heat pump's datasheet shows the same curve: 5.31 at 7 °C, 4.46 at 2 °C and 3.16 at −7 °C air, all with water at 35 °C (Viessmann Vitocal 250-A), and a test lab measured about 2.9 at −10 °C on a sister model (WPZ Buchs).

Step 7 of 15 · Explain

Less heat just when the house needs more

Not only the COP falls; the heat output does too. At the same compressor speed the model gives about 7.4 kW at 7 °C but only about 3.9 kW at −15 °C. The reason is the vapour the compressor sucks in: cold vapour at low pressure is thin, about half as dense at −15 °C as at 7 °C. Each turn of the compressor moves only about half as much refrigerant, like a pump working in thin mountain air.

Meanwhile the house loses heat faster the colder it gets. The model house needs about 8.4 kW at −7 °C and 10 kW on the coldest days, −12 °C and below. At medium speed the heat pump gives only about 5 kW at −7 °C, and the rooms would only reach about 16 to 17 °C.

Modern heat pumps solve this with an inverter that changes the compressor's speed. Turned up to about 95 Hz, the model's compressor delivers about 8.7 kW and the rooms reach 20 °C again. The price: the electricity doubles to about 2.6 kW and the COP drops from about 3.8 to 3.4. Working harder costs efficiency.

Step 9 of 15 · Explain

On the coldest nights a back-up heater helps

At −15 °C even full speed isn't enough in the model: the heat pump gives about 8.2 kW, the house needs 10 kW, and the rooms settle around 18 °C. The outdoor temperature below which the heat pump alone no longer covers the house is called the balance point; for this model house at full speed it is about −10 °C.

Below it, a real system switches on an electric back-up heater, which turns 1 kWh of electricity into just 1 kWh of heat. The model doesn't show it. It matters less than it sounds, because such nights are rare: in Fraunhofer ISE field tests in existing houses, the back-up heater used only 1.3 to 1.9 % of the heat pumps' electricity.

Step 13 of 15 · Try it

Frost forms on damp days around freezing

The fins of the outdoor unit are colder than the air, because the refrigerant inside boils below the air temperature. When they are below 0 °C and the air is damp, the water in the air freezes onto them as frost. A layer of ice blocks the air and makes the heat pump work harder.

A common belief is that icing is worst on the coldest days. It isn't. Frost is heaviest around 0 to +5 °C in humid air, above about 70 % relative humidity, and rare below −10 °C, because very cold air holds very little water (Zhu et al. 2015; Bosch). In the model, at 2 °C and 90 % humidity, frost forms; drop the humidity to about 70 % and it stops. At −15 °C the model forms no frost at all.

Step 14 of 15 · Try it

Defrosting: the cycle runs backwards for a few minutes

Every air source heat pump defrosts itself. A four-way valve turns the refrigerant loop round for a few minutes: the fans stop, hot gas from the compressor flows into the outdoor coil and melts the ice, and the water runs off. The heat for that comes from the house's heating water. In the model the heat to the house goes negative, about −6 kW, for the four minutes the defrost lasts; then the valve flips back.

Real units defrost for about 3 to 8 minutes every 30 to 90 minutes in damp weather around freezing. Over a whole winter that costs about 5 % of the energy in simulations, more in some measurements, and on frosty days the daily COP can drop by 15 to 20 % (Università di Bologna; E3S 2019).

Step 11 of 15 · Explain

Ground and groundwater stay warm in winter

Outdoor air is coldest exactly when the house needs the most heat. The ground isn't: below about 10 m it stays near 10 °C all year (C.A.R.M.E.N.). A ground source heat pump pumps a fluid, brine, through a borehole often around 100 m deep. In the model the brine comes back at about 0 °C even when the air is −15 °C, and the COP is about 4.4 instead of 3.1. Groundwater from a well is about 7 to 12 °C all winter; in the model it gives a COP of about 5.5 at −15 °C (Bundesverband Wärmepumpe).

Field tests show the same over a whole year: air source heat pumps in existing houses reached a seasonal performance factor of 3.1 to 3.4, ground source ones 4.1 to 4.3 (Fraunhofer ISE). The trade-off is the cost: a borehole or wells cost much more to build and need a permit.