Can we heat a home with cold air? Sounds impossible?

svetza5minut · In depth · approximately 6–8 minutes to read

It is freezing outside, yet the house is warm. And the machine beside it gets energy for heating from that very same freezing air. At first, the idea sounds a little like trying to dry a towel with water.

A heat pump, however, follows ordinary physics. Its most interesting feature is not the large outdoor fan. It is a substance circulating inside a sealed loop: refrigerant. We can make it colder than the winter air in one part of the loop and hotter than the heating water in another.

How? By changing its pressure and using its ability to turn from liquid into vapour and back again.

Even at −5 °C, air can serve as a heat source

When we call air cold, we usually mean cold compared with our bodies. Physics does not care whether we need a coat. What matters is the temperature difference.

Consider this illustrative example:

The air is ten degrees warmer than the refrigerant, so it can transfer heat to it. Just as a warm mug warms colder hands, cold air can warm an even colder substance. OpenStax uses this temperature example in its physics textbook. It is not a prescribed setting for every heat pump. 1

Zero on the Celsius scale marks the freezing point of water at ordinary pressure. It does not mark the disappearance of thermal energy. Air below zero still has energy to give.

Strictly speaking, air has internal energy, some of which can be transferred as heat. In everyday explanations, we shorten this to “even cold air contains heat”.

What is that cold liquid?

Refrigerant is a working fluid chosen because it can evaporate and condense over a useful range of temperatures at suitable pressures. Examples used in domestic heat pumps include R290, which is propane, and R32, a synthetic refrigerant. The particular substance depends on the equipment. Propane is not burned here: it circulates as an energy carrier. 2

Calling refrigerant a liquid throughout the cycle is misleading. In some places it is liquid, elsewhere it is a liquid–vapour mixture, and it enters the compressor as vapour. Chemically, it remains the same working substance.

It is also not the water flowing through the radiators or underfloor heating. In an air-to-water heat pump, these circuits are separate. Heat passes through a metal heat-exchanger wall; refrigerant and heating water do not mix.

During normal operation, refrigerant is not consumed like fuel. It returns to the beginning and is used again. 2

How can a liquid boil below zero?

We associate boiling with 100 °C because that is approximately where water boils in an open pan at ordinary atmospheric pressure. It is not a universal rule for every substance and every pressure.

Lower pressure means a lower boiling temperature. That is why water boils at a lower temperature high in the mountains than at sea level. Refrigerants have different properties from water and can boil well below zero at suitable low pressures.

Inside the outdoor heat exchanger, refrigerant can therefore absorb energy and evaporate at, for example, −15 °C. It does not first have to reach zero or room temperature. During the liquid-to-vapour transition, much of the energy it receives goes into that change of state rather than a sharp temperature rise.

This is why evaporation is so useful: the refrigerant can take in heat while remaining very cold. For a pure substance at approximately constant pressure, boiling itself occurs at approximately constant temperature. 3

So what keeps making it cold?

This is the crucial question. Put a container of liquid at −15 °C outside in air at −5 °C and the air will gradually warm it. Without another process, their temperatures will move closer together. A single container of cold liquid would not keep a house warm indefinitely.

In a heat pump, refrigerant continuously circulates between high-pressure and low-pressure sections. The compressor draws vapour from the low-pressure side and delivers it to the high-pressure side. The expansion valve restricts flow between them and meters refrigerant into the evaporator. Together, they maintain the pressure difference. 4

When liquid refrigerant passes through the valve to a lower pressure, some of it rapidly evaporates. The energy needed for this change comes from the refrigerant stream itself, cooling the resulting mixture. A cold mixture of liquid and vapour then enters the outdoor heat exchanger, ready to absorb more heat. 3

There is no second freezer hidden inside the machine. Cooling is part of this circulating process. Electricity is essential, especially to drive the compressor, which keeps the process running. After shutdown, pressure and temperature differences gradually diminish; refrigerant is not “permanently cold”.

One circuit, four stops

1. Evaporator: collecting heat. A fan brings outdoor air across the heat exchanger. The refrigerant inside is colder, so it absorbs heat and evaporates. Having given up energy, the air leaves colder than it arrived.

2. Compressor: raising pressure and temperature. An electric motor drives the compressor, which compresses the vapour. Doing so requires work and increases the refrigerant’s energy and temperature. The vapour must become hot enough to heat the water in the heating system.

3. Condenser: delivering heat. Refrigerant transfers energy to the water through the heat-exchanger wall. As it does so, it condenses back into liquid. The warmed water circulates through the home’s heating system.

4. Expansion valve: returning to low pressure. Refrigerant passes through a restriction, pressure falls, and a colder mixture of liquid and vapour forms. The circuit repeats. 5

These processes happen simultaneously in different parts of the operating machine. The animation shows them in sequence to make them easier to follow. Its colours represent temperature; real refrigerant does not turn orange when it warms up.

What temperature is the refrigerant actually at?

There is no single temperature for the whole machine. It varies with position in the circuit, outdoor conditions, the required water temperature, the refrigerant used and the heat pump’s controls.

The −15 °C in our example refers to evaporation on the cold side. It is not automatically the temperature after the compressor or at the valve inlet. As conditions change, so does the operating point.

Two rules are enough to remember: to collect heat, refrigerant must be colder than its source; to deliver heat, it must be hotter than the water it is heating.

How can one kilowatt-hour turn into four?

The four do not come from electricity alone. If a heat pump uses 1 kWh of electricity and extracts 3 kWh of energy from the air under particular conditions, it can deliver approximately 4 kWh of heat to the house.

1 kWh of electricity + 3 kWh from the air = 4 kWh of heat for the home.

The ratio of heat delivered to electricity consumed is the coefficient of performance, or COP. In this illustrative case, COP is 4. No energy appears from nowhere, and no law of physics is broken. There are two energy inputs, not one. 1

COP 4 is not a promise for every day. Nor are we assigning it automatically to the −5/−15 °C example above. Seasonal indicators describe performance across a heating season; a favourable measurement at one operating point does not tell us the result for the entire year.

Why does winter make the job harder?

The larger the temperature difference the heat pump must overcome, the harder the transfer becomes. Both the weather and the house’s requirements matter. Heating water to, say, 35 °C is generally less demanding than heating it to 55 °C when other conditions are equal. Low-temperature heating therefore suits heat pumps well, although a properly designed radiator system can work too. 6

In damp weather, frost can build up on the outdoor heat exchanger. The machine must defrost it, affecting electricity use and heat delivery. Performance depends on the model and operating conditions, not just one outdoor temperature reading. Some operating conditions also call for a supplementary heat source. 7

Air-to-water is not the only option

The first part of the name tells us where the heat comes from; the second tells us what receives it.

In a typical closed-loop ground-source system, a separate water–antifreeze mixture circulates through the buried collector. It transfers heat to the refrigerant through a heat exchanger; the two are not the same fluid. 8

No type is best for every house. What interests us here is their shared principle: collect energy from a colder environment and use electricity to move it somewhere useful.

You probably already have a related machine at home. A refrigerator removes heat from its interior and releases it into the kitchen. A heat pump uses the same idea for heating — this time, the warm side is the one we particularly want.

Cold air is not air without energy. And refrigerant is not a magical liquid: we control its temperature by circulating it, compressing it and allowing it to evaporate.

The world gets more interesting when you understand it. Follow svetza5minut.

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Sources and further reading

This article describes a conventional vapour-compression cycle. Numerical examples are illustrations, not heating-system designs or specifications for a particular product.

1. OpenStax, College Physics 2e: Heat Pumps and Refrigerators — underlying physics, energy balance and temperature illustration; open textbook, CC BY 4.0.
2. Vaillant: Refrigerants in heat pumps — working fluids, refrigerant examples and the sealed circuit.
3. Danfoss: Refrigeration — an introduction to the basics — pressure, phase changes and expansion.
4. Danfoss: How thermostatic expansion valves work — refrigerant metering and valve operation.
5. Viessmann: How does a heat pump work? — overview of the operating cycle.
6. Energy Saving Trust: In-depth guide to heat pumps — heating systems and performance.
7. U.S. Department of Energy: Measured Performance of a Low Temperature Air Source Heat Pump — air-source operation and defrosting.
8. U.S. Department of Energy: Heat Pump Systems — system types and energy sources.

Additional source on ground loops: PNNL, Building America Solution Center: Ground-Source Heat Pumps.