The Heat Pump

Why the heat pump matters

Almost every other way of making heat — a gas furnace, an electric coil, a wood fire — converts some fuel or work directly into thermal energy, and the best you can ever do is turn 100% of the input into heat. A heat pump breaks that ceiling by refusing to make heat at all. Instead it moves heat that already exists in the outdoor environment, and the compressor work only has to pay for the pumping, not for the heat itself. That single reframing — pump, don't burn — is why a well-matched heat pump delivers three to five times more heat than the electricity it draws.

• Space heating and cooling. A single reversible unit replaces a furnace and an air conditioner, heating in winter and cooling in summer with the same hardware.
• Domestic hot water. Heat-pump water heaters reach a COP of 2.5 to 4, versus 1 for a resistance tank.
• Decarbonization. Electrifying heat with a COP of 3.5 cuts primary energy and, on a clean grid, the carbon of building heat by well over half.
• Industrial process heat. High-temperature heat pumps recover waste heat and lift it to 80–160 °C for drying, distillation, and pasteurization.
• District heating. Large ammonia and CO₂ heat pumps draw from rivers, sewage, and data-center waste heat to warm whole neighborhoods.

How it works, step by step

A heat pump is the same four-component loop as any vapor-compression refrigeration system; the only difference is which side of it you point at your living space. Follow one parcel of refrigerant — a fluid such as R-32, R-290 (propane), or R-410A — around the loop:

1. Evaporator (absorb). Cold, low-pressure liquid refrigerant enters the outdoor coil at a temperature below the outdoor air. Heat flows the natural way — from warm air into cold refrigerant — and boils the refrigerant to a vapor. This is the low-grade environmental heat, Q_cold, entering the cycle.
2. Compressor (add work). The compressor draws in the low-pressure vapor and squeezes it to high pressure. Compression does work W on the gas and, because compressing a gas raises its temperature, the vapor leaves hot — often 70–90 °C, hotter than the room it will heat.
3. Condenser (reject). The hot, high-pressure vapor flows through the indoor coil. Because it is now hotter than the room, heat flows out of the refrigerant into the indoor air, condensing the vapor back to liquid. This delivered heat is Q_hot = Q_cold + W.
4. Expansion valve (drop pressure). The warm high-pressure liquid passes through a throttling device — a thermostatic expansion valve or electronic expansion valve. Its pressure drops sharply, some of it flash-evaporates, and the temperature plunges back below the outdoor air. The cold liquid re-enters the evaporator and the loop repeats.

The magic is entirely in the pressure–temperature relationship of the refrigerant: at low pressure it boils cold enough to pull heat from freezing outdoor air, and at high pressure it condenses hot enough to warm your house. The compressor's job is only to maintain that pressure difference.

The governing equation: coefficient of performance

The efficiency of a heat pump is measured not by a percentage but by its coefficient of performance, which can and should exceed 1. In heating mode:

COP_heating = Q_hot / W

where:

• Q_hot — heat delivered to the warm space, in joules (J) or watts (W) of thermal power
• W — electrical work input to the compressor, in joules (J) or watts (W)

By the first law of thermodynamics, energy is conserved around the loop, so the heat delivered is the heat absorbed plus the work added:

Q_hot = Q_cold + W

Substituting shows why COP always beats 1: COP = (Q_cold + W) / W = 1 + Q_cold/W. The "+1" is the free-heat bonus that resistance heating never gets. The absolute ceiling is set by the reversible Carnot cycle, using absolute temperatures in kelvin:

COP_Carnot = T_hot / (T_hot − T_cold)

• T_hot — absolute temperature of the warm space (condenser), in kelvin (K)
• T_cold — absolute temperature of the cold source (evaporator), in kelvin (K)

Two facts fall straight out of this. First, the smaller the temperature lift (T_hot − T_cold), the higher the ceiling — which is why ground-source units, with their warm 10 °C earth, beat air-source units in winter. Second, as the outdoor air gets colder, T_cold drops, the lift grows, and the ceiling collapses. Real machines reach roughly 40–60% of the Carnot value, so a Carnot ceiling of 8 might yield a real COP of about 4.

In cooling mode the useful output is the heat removed from the room, so the metric is COP_cooling = Q_cold / W, exactly one less than the heating COP for the same machine. (The industry expresses this as SEER or EER in cooling and HSPF in heating.)

A worked example

Consider an air-source heat pump on a 0 °C day heating a house held at 21 °C indoors. The refrigerant must run a little colder than outside and a little hotter than inside, so take the evaporator at T_cold = −8 °C = 265 K and the condenser at T_hot = 35 °C = 308 K. The Carnot ceiling is:

COP_Carnot = 308 / (308 − 265) = 308 / 43 ≈ 7.2

A good real unit achieves about 50% of that, so a real COP ≈ 3.6. To deliver 6 kW of heat to the house, the compressor draws W = Q_hot / COP = 6000 / 3.6 ≈ 1.67 kW of electricity — and pulls the remaining 4.33 kW straight out of the freezing outdoor air. A resistance heater delivering the same 6 kW would draw the full 6 kW, using 3.6× more electricity.

Outdoor air	Temp lift (approx.)	Typical real COP	kWh electricity per kWh heat
+10 °C	25 K	4.5	0.22
+2 °C	33 K	3.6	0.28
−7 °C	42 K	2.6	0.38
−15 °C	50 K	2.0	0.50
−25 °C	60 K	1.3	0.77
Resistive coil (any)	—	1.0	1.00

The reversing valve: one machine, two jobs

The elegant trick that makes a heat pump a heat pump — and not just a fixed refrigerator — is the four-way reversing valve. It sits in the vapor line at the compressor discharge and, by sliding an internal spool, re-routes the hot high-pressure gas to either the indoor or the outdoor coil.

• Heating mode: hot gas goes to the indoor coil (condenser, rejects heat inside); the outdoor coil is the evaporator (absorbs heat from outside).
• Cooling mode: energize the valve's solenoid, the spool shifts, hot gas now goes to the outdoor coil (condenser, dumps heat outside); the indoor coil becomes the evaporator (absorbs heat from the room). You have an air conditioner.

The compressor and expansion device stay put; only the direction of heat flow flips. The same reversing action is what lets a heat pump run a defrost cycle: on humid near-freezing days, frost builds on the outdoor evaporator, so the controller briefly reverses to cooling mode to warm and melt the ice off the outdoor coil, then reverses back.

Common misconceptions and failure modes

• "COP over 1 breaks physics." No — the machine moves pre-existing environmental heat and adds work on top; it does not create energy. Q_hot = Q_cold + W is just the first law.
• "Heat pumps don't work when it's freezing." There is still abundant heat in −15 °C air (it is 258 K, far above absolute zero). Cold-climate inverter units with vapor/economizer injection hold a COP above 2 at −15 °C and still deliver useful heat below −25 °C.
• "Bigger is better." An oversized unit short-cycles, wearing the compressor and never reaching efficient steady state. Correct sizing to the design heat load is what protects the seasonal COP.
• Low refrigerant charge. A leak drops evaporator pressure, starves the compressor, and cripples both capacity and COP; it is the single most common field failure.
• Excessive temperature lift. Pairing a cold source with a high-temperature radiator system (e.g. 65 °C water) forces a huge lift and can push COP below 2. Low-temperature distribution — underfloor loops or oversized radiators at 35–45 °C — is what keeps a heat pump efficient.
• Auxiliary strips masking a fault. If backup resistance heat runs constantly, the electricity bill looks like a plain resistance heater — a sign the heat pump is undersized, iced up, or low on charge.