How Do You Heat a House With Cold Air? Moving Heat Instead of Making It
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A heat pump delivers more heat energy into a building than the electrical energy it consumes, which sounds impossible and is not. It moves heat from outside rather than generating it, and a refrigerator does the same in reverse.
Why it beats a heater
An electric resistance heater converts electrical energy into heat at essentially one to one, which is the maximum possible if the energy is being created. A heat pump does something different, using electrical energy to move heat from a colder place to a warmer one, and the quantity moved can be several times the energy spent moving it. That is not a violation of anything, since energy is conserved and the heat was already there, and the electrical input pays only for the transport rather than for the heat itself. A well-installed unit delivers three to four units of heat per unit of electricity over a season, which is why replacing direct electric heating with one cuts consumption by roughly two thirds even before any change in how the electricity is generated.
How the cycle works
The mechanism uses a fluid that boils at a convenient temperature:
- •The fluid evaporates in the outdoor coil, absorbing heat from the air or ground even when cold
- •A compressor raises its pressure, which raises its temperature substantially
- •The hot gas condenses in the indoor coil, releasing heat into the building
- •An expansion valve drops the pressure, cooling the fluid below the outdoor temperature again
- •The cycle repeats, and reversing it turns the unit into an air conditioner
- •A refrigerator is the same machine arranged to cool the inside and dump heat behind it
Where the performance goes
Efficiency depends strongly on the temperature difference the pump works across, which is the single most important practical fact about these systems. Lifting heat from freezing outdoor air to a radiator at seventy degrees is a large lift and performance suffers, while lifting from the same air to underfloor heating at thirty five degrees is a much smaller one and performance is far better. That is why installations work best in well-insulated buildings with large emitters, and why simply swapping a boiler for a heat pump without changing the radiators disappoints people. Ground source systems draw from soil at a stable temperature year round, which avoids the worst conditions and costs considerably more to install. Cold climate units using multiple compression stages perform well at low temperatures.
The measure that is quoted
Performance is described by a ratio of heat delivered to energy consumed, and understanding the two versions quoted prevents a common confusion. The instantaneous ratio describes performance at a stated pair of temperatures, which is a laboratory figure at conditions that may not resemble a winter. The seasonal figure averages performance across a whole heating season including the coldest days and any electric backup used, which is what actually determines a bill. Manufacturers quote the first prominently and the second where required, and the gap between them is substantial. Field studies measuring installed systems have consistently found seasonal figures below manufacturer expectations, with the shortfall traced mainly to installation and control settings rather than to the equipment, which is why commissioning matters as much as specification.
The arguments about them
The disputes concern deployment rather than physics. Installation cost is high, particularly where a building needs insulation and new emitters at the same time, and the payback depends on the relative price of electricity and gas, which varies enormously between countries and determines whether running costs fall or rise. Electricity supply must cope with winter peaks if a large share of heating shifts to it. Installer skill is a real constraint, since a badly designed system performs far below specification and the resulting reputation problem is self-inflicted. Refrigerants used in the cycle are themselves potent greenhouse gases if released, which is driving a shift to alternatives. The underlying technology is not in question and its performance is measured routinely in the field.
The takeaway
The machine moves heat rather than creating it, so the electricity pays only for the transport and several units of heat arrive per unit consumed. A fluid evaporating outside absorbs heat even from cold air, and compressing it raises its temperature enough to release that heat indoors. Performance depends on the temperature difference, which is why large emitters and good insulation matter so much.