A heat pump runs a vapour-compression cycle: a refrigerant evaporates (absorbing heat from the cold source), is compressed (raising its pressure and temperature), condenses (releasing that heat to the warm sink), and passes through an expansion valve (dropping pressure to start again). The phase change carries most of the heat.
Efficiency is a ratio, not a percentage: the coefficient of performance COP = heat moved / work in. Because you're only paying to move heat, COP is typically 3โ5 โ one unit of electricity delivers three to five units of heat.
The ceiling is thermodynamic: the bigger the temperature lift (sink minus source), the lower the best-possible COP. A heat pump is a Carnot engine run backwards, and real machines reach roughly 40โ55 % of that ideal. The model below shows lift eating COP.
1 · Move heat, don't make it
An electric heater is 100 % efficient and that is its problem: every joule of electricity becomes one joule of heat, and no better. A heat pump breaks past 100 % โ not by cheating thermodynamics, but by not making the heat at all. It gathers heat that already exists in the outdoor air, the ground, a river or a waste stream, and pumps it uphill to where you want it. The electricity only drives the pump, so one unit in can deliver three, four or five units of heat out.
The trick is that heat flows downhill on its own โ from hot to cold โ but we want it to go uphill, from a cold source to a warm room. To reverse that natural direction you need a working fluid that can be made colder than the source (so heat flows into it) and then hotter than the sink (so heat flows out of it). A refrigerant, cycled between two pressures, does exactly that. A refrigerator, an air conditioner, a chiller and a building heat pump are the same machine โ they differ only in which side you care about, the hot or the cold.
2 · The vapour-compression cycle
Four components in a loop, connected by pipe carrying refrigerant. Two of them are heat exchangers (the evaporator and condenser); one adds energy (the compressor); one throttles pressure (the expansion device). Trace the refrigerant around once:
Interactive — the refrigeration cycle, drawn
Live P&ID| Stage | Component | What happens |
|---|---|---|
| 1 → 2 | Evaporator | Cold low-pressure refrigerant boils, absorbing heat from the source (air/water/ground). It leaves as a low-pressure vapour, slightly superheated. This is the "cold" side โ a fridge's freezer, an AC's indoor coil. |
| 2 → 3 | Compressor | The vapour is compressed to high pressure. Compression raises its temperature well above the sink temperature. This is where the electrical work goes in โ the only energy you pay for. |
| 3 → 4 | Condenser | The hot high-pressure vapour condenses back to liquid, releasing its latent heat to the warm sink (the room, the hot water, the cooling tower). It leaves as a liquid, slightly subcooled. This is the "hot" side โ a fridge's back coils, a heat pump's indoor unit. |
| 4 → 1 | Expansion valve | The liquid is throttled through a small orifice, dropping to low pressure. The sudden pressure drop flash-cools it โ some flashes to vapour and the rest chills to below the source temperature, ready to boil again. |
The heavy lifting is done by phase change, not by warming the fluid. Boiling a liquid absorbs a large "latent" heat at constant temperature; condensing it releases the same. That is why the refrigerant runs at two roughly-constant temperatures (cold in the evaporator, hot in the condenser) rather than sliding continuously โ and why a small mass flow can shift a large heat duty.
Where the heat exchangers come in. The evaporator and condenser are just heat exchangers โ and everything in that guide applies: counter-flow, approach temperature, and the fouling that quietly raises the lift the compressor has to fight. A fouled condenser is one of the most common reasons a heat pump's COP drifts down in service.
3 · COP: efficiency as a ratio
Because a heat pump moves heat rather than converting fuel, its performance is a ratio that can exceed one โ so we never call it "efficiency". We call it the coefficient of performance:
COPheating = COPcooling + 1. The same machine is a slightly better heater than it is a cooler.
A COP of 4 means one kilowatt of electricity delivers four kilowatts of heat: three of them pumped up from the free source, one from the electricity itself. That is why heat pumps beat any combustion boiler on primary energy and carbon โ and why the cooling world quotes the same idea as EER (energy efficiency ratio) or the seasonal SEER/SCOP, which average performance over a year of varying conditions rather than one design point.
4 · The Carnot ceiling — lift sets the limit
No heat pump can beat a thermodynamic ceiling set only by the two temperatures it works between โ the same limit Carnot found for engines, run in reverse. In absolute temperature (kelvin):
This is the single most important idea in the whole series, because it explains almost every design and maintenance decision downstream: keep the lift small. Every degree you can raise the source, or lower the sink, buys COP directly. It is why ground-source beats air-source in a cold snap (the ground stays warmer than winter air), why oversizing the heat exchangers pays back in efficiency, why a fouled condenser or a plugged filter hurts so much โ and why cooling towers, the next-but-one guide, exist purely to hold the sink temperature down. Try it:
Interactive — COP versus temperature lift
Live modelReal COP as the lift grows
Where the delivered heat comes from
5 · Superheat & subcooling — the two vital signs
Two small temperature measurements tell you almost everything about a running cycle's health and charge, and they reappear in the monitoring guide as leading fault indicators:
- Superheat โ how many degrees the vapour leaving the evaporator is above its boiling point at that pressure. A few degrees of superheat guarantees the refrigerant is fully boiled (all vapour) before it reaches the compressor โ because liquid does not compress, and slugging liquid into a compressor destroys it. Too low superheat risks liquid carry-over; too high means the evaporator is starved (often undercharge or a stuck expansion valve).
- Subcooling โ how many degrees the liquid leaving the condenser is below its condensing point. It confirms the refrigerant fully condensed (all liquid) before the expansion valve, and is the primary indicator of refrigerant charge. Low subcooling usually means undercharge; high subcooling means overcharge or a condenser that isn't rejecting heat (fouled, low airflow).
Together, superheat and subcooling are how a technician charges and diagnoses a system without opening it โ and, streamed continuously, they are exactly the kind of signal a condition-monitoring platform trends toward an alarm long before a failure.
6 · Heating, cooling & the reversing valve
Because the cycle is symmetric, one machine can do both jobs. A reversing valve (a four-way valve) swaps which coil is the evaporator and which is the condenser, flipping the direction heat is pumped. In cooling mode the indoor coil is the evaporator (absorbing heat from the room); in heating mode it becomes the condenser (releasing heat to the room), and the outdoor coil takes over as evaporator. Everything else โ compressor, expansion device, refrigerant โ is unchanged. That is the whole secret of a reversible air conditioner or an air-source heat pump: one loop, a valve, two seasons.
The only asymmetry is the source. In summer the outdoor air is a convenient hot sink to dump into; in a hard winter it is a cold, sometimes frosting, source to scavenge from โ and the colder it gets, the bigger the lift and the lower the COP, exactly as the model shows. Managing that winter source is why ground-source, water-source and hybrid systems exist, which is where the next guide picks up.
Key takeaways
- A heat pump moves heat, it doesn't make it โ so it delivers more heat than the electricity it consumes.
- Four components, one loop โ evaporator, compressor, condenser, expansion valve โ with the refrigerant's phase change carrying the heat.
- COP is a ratio, typically 3โ5; heating COP = cooling EER + 1 (the same machine heats slightly better than it cools).
- Lift sets the ceiling โ COP falls as the sink-minus-source gap grows; real machines reach 40โ55 % of the Carnot ideal. Keep the lift small.
- Superheat and subcooling are the running cycle's vital signs โ protecting the compressor and revealing the charge.