Heat-pump cycle, COP and system modes

Core idea

An electrically driven heat pump uses work to transfer heat from a colder source to a warmer heating system. In its vapour-compression cycle, the evaporator absorbs source heat, the compressor adds work, the condenser releases heat and the expansion device reduces pressure before the cycle repeats. Useful heat output includes both the energy collected from the source and the electrical contribution.

What to recognise

• COP compares useful heat output with electrical input at stated operating conditions.
• SCOP is a standardised seasonal energy ratio; compare ratings for the same climate and temperature application.
• A smaller temperature lift usually helps COP, so suitable emitters and lower heating-water temperatures matter.
• Outdoor temperature, cycling and defrost can affect available output and electricity use.
• For a stated heating duty, monovalent operation uses the heat pump alone; supplementary heat requires a defined combined-source strategy.

Apply it

If a unit supplies 8 kW of useful heat while taking 2 kW of electrical power within the same measurement boundary, its COP is 8 ÷ 2 = 4. The figure describes those conditions. Source temperature, flow temperature, controls and auxiliary electricity can change the result, so check which pumps and fans the stated input includes.

COP is a ratio without units. It can also be expressed as COP × 100 when a stated convention uses a percentage: COP 3.0 corresponds to 300% in the 2021 England Approved Document L definition. Output exceeding electrical input does not create energy; the additional contribution is heat taken from the source.

The cycle in more detail

• At the evaporator, refrigerant at low pressure boils while absorbing heat. For an air-source unit in heating mode, it is colder than the outdoor air, so heat can enter it even when that air is below 0°C.
• The compressor takes in vapour and raises its pressure and temperature. It adds work to the refrigerant; it is not the only place where energy enters, because the evaporator also receives heat.
• Hot refrigerant gives up heat in the condenser and condenses. In an air-to-water unit, that heat crosses into a separate water circuit. The expansion device then reduces refrigerant pressure and, for a thermostatic or electronic expansion valve, meters its flow to the evaporator.
• An outdoor monobloc contains the refrigerant circuit in one enclosure. Heating-water pipes connect it to the building. A split arrangement places refrigerant components on both sides of the wall, linked by refrigerant pipes. Check the actual equipment layout.
• Air-to-air describes space heat delivered to room air through indoor units. Air-to-water supplies a wet heating circuit. Some air-to-air systems can also serve a compatible hot-water tank; the space-heating label alone does not establish the model’s hot-water capability.

Temperature and defrost

Raising the required heating-water temperature increases the temperature lift and usually reduces COP. Cold outdoor air can also reduce performance; use the model’s capacity data rather than assuming every unit loses output at the same rate. Improved fabric and suitable emitters may allow lower water temperatures while meeting the room loads.

In cool, damp weather frost can restrict the outdoor coil. Reverse-cycle defrost directs hot refrigerant to that coil, temporarily drawing heat from the indoor side to clear it. An air-to-water system can use heat stored in its water circuit; an air-to-air system can use indoor air. A short interruption can be normal. Persistent poor defrost needs investigation. Melting frost does not remove leaves or dirt, so the air path and coil still need the specified inspection and maintenance.

Heating duties and supplementary sources

Start by naming the duty: space heating and domestic hot water can have different arrangements. Monovalent space heating relies on heat-pump output across the design conditions; it does not specify how many heat-pump units are fitted. A separate cylinder heater does not automatically change that space-heating description.

With bivalent operation, another source supports the heat pump or takes over a duty under defined conditions. Direct-electric assistance is commonly called monoenergetic when both devices use electricity. For bivalent-parallel control, the selected outdoor-temperature point permits assistance as demand requires while the heat pump continues. An alternative point can switch the heat pump off. Hybrid controls may instead select operation using tariffs and demand, and can permit the sources to run together. Check the actual design and controller terminology.

Seasonal performance and limits

SCOP uses reference seasonal conditions, including part-load operation, to relate heating energy to electrical energy. It is not a simple arithmetic average of COP readings or a guarantee of a particular household bill. Actual demand, controls, weather, hot-water duty and tariffs matter. For monitored operation, report the seasonal performance factor with its period and the heat and electricity included by the meters.

Electricity-related operating emissions can be estimated from electricity use and the relevant carbon factor. That calculation does not include every possible contribution, such as refrigerant leakage, equipment manufacture or fuel used by a separate backup heater. Understanding these principles does not establish competence to work on a refrigerant circuit.