How a Heat Pump Moves Heat Instead of Making It
A heat pump uses electricity to carry heat from a cold place to a warm one. Here is the four-step refrigerant cycle and what its coefficient of performance means.
In this article
A heat pump heats a building by moving heat from outside to inside, rather than by burning fuel or making heat from electricity directly. A typical household unit delivers around four units of heat for every one unit of electricity it uses, according to the International Energy Agency (IEA). This article explains the cycle that makes this possible, using the IEA and the Energy Education encyclopedia from the University of Calgary.
Moving heat, not making it
The IEA says a heat pump uses technology similar to a refrigerator or air conditioner and extracts heat from a source such as outside air, the ground, nearby water or waste heat from a factory. Energy Education states the same idea and notes that heat pumps require an input of energy to move the heat. That is how they stay within the laws of physics: electricity drives heat from a cold place to a warm one, which does not happen on its own.
The main parts
The IEA names a compressor, which moves a refrigerant through a refrigeration cycle, and two heat exchangers, one taking heat from the source and one passing it to the place being heated. Energy Education describes the same four working parts in its heating cycle: an evaporator, a compressor, condenser coils and a pressure-reducing valve.
The heating cycle in four steps
- Evaporator: liquid refrigerant absorbs heat from outdoor air and becomes a gas.
- Compressor: it raises the pressure and temperature of that gas.
- Condenser coils: the hot gas flows through coils indoors, gives up its heat and turns back into a liquid.
- Pressure-reducing valve: the liquid passes through a valve that lowers its pressure and temperature, and the cycle begins again.
The refrigerant can absorb heat from cold outdoor air because, after the valve, it is even colder than that air. The compressor then makes it hotter than the room, so the heat flows into the room.
Running the cycle in reverse
Energy Education says that in cooling mode, the evaporator and condenser swap roles and the refrigerant flow reverses. Heat is absorbed from indoor air and released outdoors. The IEA agrees that many heat pumps can also cool spaces in summer. Neither page explains the mechanical part that reverses the flow, so this article does not describe it.
Coefficient of performance
Efficiency is described by the coefficient of performance, or COP. Energy Education defines it as the heat delivered divided by the electrical work put in. A higher value means less work is needed for each unit of heat moved. The page also says a theoretical limit to COP is set by entropy and the second law of thermodynamics.
The IEA says a typical household heat pump has a COP of around 4, so its heat output is about four times its electrical input. It also says current household models are three to five times more energy efficient than gas boilers and can be cheaper to run. These are figures for typical models, and the IEA does not give them for every climate or home.
Delivering the heat
The IEA says that in buildings heat can be delivered through forced air or through hydronic systems, meaning water-based systems such as radiators or underfloor heating. Heat pumps can also supply hot water through a tank. In industry, they supply hot air, water or steam. The IEA adds that large commercial, industrial and district-heating units need higher input temperatures, which can come from industrial processes, data centres or wastewater.
What the sources do not say
Neither source gives details of how performance changes in very cold weather, and the Energy Education page gives no typical COP values. Readers choosing a system for a real home should consult an installer or the manufacturer's data for their climate. This article is a general explanation of how the technology works.
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