Heat Pumps: Moving Heat Rather Than Making It
The number that makes people suspicious is the one that matters. An electric heater is at best 100 percent efficient, because the most it can do is turn all its electricity into heat. A heat pump reaches 300 to 500 percent by the same measure, and the reason is that it is not a converter at all.
Where the Extra Energy Comes From
Air at zero degrees Celsius contains an enormous amount of thermal energy. Zero on the Celsius scale is an arbitrary reference, the freezing point of water, not an absence of heat; that would be absolute zero, 273 degrees further down. There is plenty of energy in cold air, it is just at a temperature too low to be useful directly.
A heat pump raises that temperature. It circulates a refrigerant with a very low boiling point through four components. In the outdoor evaporator the refrigerant is colder than the outside air, so heat flows into it in the ordinary direction and boils it into vapour. A compressor then squeezes that vapour, and compression raises its temperature well above the indoor temperature. In the indoor condenser the hot vapour gives up its heat to the house and condenses back to liquid. An expansion valve drops the pressure, the liquid cools sharply, and the cycle begins again.
Nothing here is exotic. It is the refrigeration cycle, described by Carnot in the 1820s and in every refrigerator since. The compressor is what the electricity pays for, and its work is what lets heat flow from cold to warm - the direction it does not go on its own.
The heat delivered to the house is therefore the sum of two things: the heat collected outside, which is free, plus the electrical work of the compressor, which also ends up as heat. That is why output exceeds input, and why nothing about it troubles the laws of thermodynamics.
Coefficient of Performance
COP is the ratio of heat delivered to electricity consumed. A COP of 4 means four kilowatt-hours of heat for one of electricity. Seasonal COP, which averages over a heating season including defrost cycles and part-load operation, is the honest figure for comparison and is typically lower than the peak value in a brochure.
The theoretical ceiling comes from Carnot again and depends only on the two temperatures involved. Pumping heat across a small temperature difference is easy; across a large one it is hard. A system lifting heat from 7 degrees outside to 35 degrees in an underfloor circuit has a theoretical maximum COP above 10 and achieves 4 to 5 in practice. The same unit lifting from minus 10 outside to 55 degrees for old radiators might manage 2.
This single relationship explains almost every practical recommendation about heat pumps. Larger radiators or underfloor heating lower the required flow temperature and raise COP. Insulation reduces the heat needed and allows a lower flow temperature for the same comfort. Ground source outperforms air source in winter because the ground a few metres down stays near the annual average temperature while the air does not, which is the same resource that shallow geothermal systems use.
It also explains why a heat pump beats a gas boiler even on a cold day. A condensing boiler reaches perhaps 90 percent. A heat pump at COP 2.5 delivers 250 percent. Even where electricity costs three times as much as gas per kilowatt-hour, the arithmetic is close, and it improves every year that the grid gets cleaner.
Cold Climates and the Objection That Persists
The standard objection is that heat pumps fail in cold weather, and it was once broadly true. Early units lost capacity rapidly below freezing and switched to resistance backup, which is expensive electricity used badly.
Two changes ended that. Variable-speed inverter compressors let a unit modulate output continuously rather than cycling on and off, which keeps efficiency up at part load. Enhanced vapour injection adds a second injection point in the compression stage, maintaining capacity at low ambient temperatures. Current cold-climate units hold a COP above 2 at minus 20 degrees and continue operating below minus 25.
The empirical answer is more persuasive than the specifications. Norway, Finland and Sweden have the highest heat pump penetration in the world - well over half of households in Norway - and they are not warm places. The technology works in cold climates because it has been engineered to, not because the physics was ever forgiving.
The remaining honest caveats are about buildings rather than machines. A house with high heat loss and small radiators sized for 70-degree water will need either fabric improvements or larger emitters, and the cost of that work, not the heat pump, is usually what determines whether a retrofit makes sense.
Why This Matters at System Scale
Heating is roughly half of final energy consumption in Europe and the majority of it is still burned fossil fuel. A technology that delivers the same heat with a third to a fifth of the input energy changes the size of the problem, not just its carbon content.
The multiplication compounds with a clean grid. One unit of electricity from a wind farm becomes four units of heat in a house, so a single wind turbine displaces several times its own output in gas. This is why heat pumps appear in almost every credible decarbonisation pathway and why they are treated as an electrification technology rather than a heating technology.
There is a grid consequence too. Electrified heating shifts demand into winter and into cold mornings, which is precisely when solar output is lowest in northern latitudes. Heat pumps therefore increase the value of generation that runs regardless of weather, and of storage that can bridge a cold, dark week.
They are also flexible in a way that helps. A well-insulated building is itself a thermal store: a heat pump can run harder when electricity is cheap and coast for an hour or two when it is not, without anyone noticing. That makes heat pumps one of the larger sources of demand-side flexibility available to a grid.
Frequently asked questions
How can a heat pump be more than 100 percent efficient?
It is not converting electricity into heat, so the word efficiency is misleading. It uses electricity to move heat that already exists outside into the building. The heat delivered is the outside heat collected plus the compressor's electrical work, which is why the output exceeds the electrical input without violating anything.
What is COP?
Coefficient of performance: heat delivered divided by electricity consumed. A COP of 4 means four units of heat per unit of electricity. Seasonal COP averages over a whole heating season including defrost cycles and part-load running, and is the number worth comparing between systems.
Do heat pumps work in cold weather?
Yes. Modern cold-climate units maintain a COP above 2 at minus 20 degrees and operate below minus 25, thanks to variable-speed compressors and vapour injection. Norway has the world's highest heat pump penetration at well over half of households, which settles the question empirically.
Why does flow temperature matter so much?
Because COP depends on the temperature difference the pump has to bridge. Heating water to 35 degrees for underfloor circuits is far easier than heating it to 55 for old radiators. Lowering the flow temperature through better emitters or insulation is usually the cheapest way to improve a heat pump's performance.
Is a heat pump the same as an air conditioner?
Mechanically, almost. Both run a refrigeration cycle that moves heat from one side to the other. An air conditioner pumps heat out of a building; a heat pump pumps it in, and most modern units reverse the cycle to do both. A refrigerator is the same machine cooling a box instead of a room.
