Vapour Compression Cycle
How the vapour compression cycle works in a heat pump water heater — the four stages and the refrigerant state at each point.
A heat pump water heater does not generate heat — it moves heat from the ambient air into water. The vapour compression cycle is the mechanism that makes this possible. A refrigerant circulates continuously through four components, changing state (liquid ↔ gas) and pressure (low ↔ high) as it goes.
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Vapour compression cycle diagram
The four stages
1. Evaporator — absorb heat from air
The refrigerant enters the evaporator as a cool, low-pressure liquid. A fan draws ambient air across the evaporator coil. Even at low air temperatures, the refrigerant is cold enough that heat flows from the air into the refrigerant. The refrigerant absorbs this heat and evaporates, leaving the evaporator as a warm, low-pressure gas.
2. Compressor — raise pressure and temperature
The gaseous refrigerant is compressed. Compressing a gas raises its temperature — the refrigerant leaves the compressor as a hot, high-pressure gas, at temperatures well above the target water temperature. This is what allows heat to flow from the refrigerant into the water in the next stage.
3. Condenser — transfer heat to water
The hot refrigerant passes through the condenser, which is in thermal contact with the water (either wrapped around the tank or via a heat exchanger). Heat flows from the refrigerant into the water. As the refrigerant loses heat, it condenses back into a liquid. It exits the condenser as a warm, high-pressure liquid. This is the only stage where useful heat is delivered to the water.
4. Expansion valve — drop pressure and temperature
The warm liquid refrigerant passes through the expansion valve, which rapidly reduces its pressure. The pressure drop causes the refrigerant to cool significantly, returning to a cool, low-pressure liquid ready to absorb heat from the air again. The cycle repeats.
Refrigerant state summary
| Stage | Component | State | Pressure | Temperature |
|---|---|---|---|---|
| 1 → 2 | Evaporator | Liquid → Gas | Low | Cool → Warm |
| 2 → 3 | Compressor | Gas → Gas | Low → High | Warm → Hot |
| 3 → 4 | Condenser | Gas → Liquid | High | Hot → Warm |
| 4 → 1 | Expansion valve | Liquid → Liquid | High → Low | Warm → Cool |