What SCOP can I expect from an air source heat pump?

The range across the evidence is wide. Independent guidance puts typical UK performance at around 3.0¹, while a maker's own figures suggest an average air-to-water heat pump reaches between 3.5 and 4.5, with top models above 6². The difference is not a contradiction so much as a difference in what is being measured: a laboratory or manufacturer's rating under standard conditions, against what a real house with real radiators and real weather actually delivers.

What matters for a household is that SCOP is the number that determines running costs, and it is the number the Boiler Upgrade Scheme uses to decide whether a heat pump qualifies for public money. Air-to-water systems must achieve an in situ SCOP of at least 2.8³. Below that, the grant is not available.

What SCOP means and how it is measured

SCOP is a way to rate a heat pump's performance in a given climate⁶. It is the average COP across the whole year⁷. Where COP is a snapshot taken at one moment and one set of conditions, SCOP rolls those moments together into a single seasonal figure that reflects how the machine actually behaves through autumn, winter and spring.

The measurement standard is BS EN 14825:2013, which covers air conditioners, liquid chilling packages and heat pumps with electrically driven compressors for space heating and cooling, testing and rating at part load conditions and calculation of seasonal performance⁸. In Europe, SCOP is calculated according to EN 14825, which defines how seasonal efficiency must be determined under standard conditions⁹. The calculation is not a simple average of a few test points. It includes performance at multiple outdoor temperatures, part-load operation, standby and auxiliary electricity consumption, and defrost cycles for air-source systems⁹.

For heat pumps providing space heating, EN 14825 test data is used in combination with calculations from BS EN 15316-4-2:2017 to calculate the performance of the system¹⁰. That matters because it means the SCOP figure on a product sheet is not just a measure of the compressor: it is a modelled estimate of how the whole system performs across a year of varying weather.

The practical consequence is that SCOP is a better guide to running costs than COP, but it is still a modelled figure. It assumes a particular climate, a particular flow temperature and a particular pattern of use. Change any of those and the real number moves.

What SCOP to expect: typically 3.0 to 4.0 in UK homes

In the UK, air source heat pumps generally run at a SCOP of around 3.0, or 300% efficient, but this can be higher or lower depending on the set-up¹. That figure is the anchor for most household calculations. It is also the figure that determines whether a heat pump saves money against a gas boiler: a household with typical medium gas use would need a heat pump with a SCOP of at least 2.9 before it started to make annual savings at standard electricity rates¹.

The upper end of the realistic range is around 4.0. Heat pumps can have a SCOP of 4.0 or more, indicating 400% efficiency across the year¹¹. That is achievable, but it depends on the property, the emitters and the flow temperature, not on the badge on the outdoor unit.

Official consultations have taken a different view of what should be expected. For heat pumps in existing dwellings, one consultation proposed a minimum SCOP of 3.0 for heating and 2.0 for domestic hot water for all systems other than direct expansion units¹². That is a proposed regulatory floor, not a description of current performance, but it shows where policy is heading.

For high-temperature air source heat pumps, the picture is lower. SCOPs range from 2.55 to 3.25 when reported at 65C¹³. That is the trade-off: a heat pump designed to run hot enough to work with existing radiators without upgrading them will generally be less efficient than one running at a lower temperature.

Flow temperature: low temperature units above 4.7, high temperature units 2 to 2.5

The flow temperature is the temperature of the water the heat pump sends around the heating circuit. It is the single most powerful lever on SCOP. Lower heating water temperatures improve efficiency significantly, and floor heating systems typically enable higher SCOP than high-temperature radiator systems². Low temperature heat pumps, operating at flow temperatures of around 35°C to 55°C, reach a SCOP above 4.7, while high temperature heat pumps, typically operating at flow temperatures between 60°C to 80°C, range from 2 to 2.5².

That is why the guidance on flow temperature is consistent across official and independent sources. An air source heat pump utilises an optimal supply/flow temperature of 45°C and a return temperature of 40°C¹⁴. Air source heat pumps usually run at lower flow temperatures of around 45 degrees centigrade¹⁵. Ground source heat pumps benefit from operating at flow temperatures below 55°C¹⁶.

Lowering the flow temperature does have a cost in responsiveness. Lowering the heat pump's flow temperature reduces the reaction time of the heating system, which means it takes longer to warm up but costs less to run¹⁷. The system runs for longer at a lower temperature rather than blasting hot water for a short period. That is why weather compensation matters: it lets the heat pump vary its flow temperature with the outdoor conditions rather than running at a fixed high temperature all winter.

Minimum SCOP rules and the Boiler Upgrade Scheme's 2.8 threshold

The Boiler Upgrade Scheme is the main grant for heat pumps in England and Wales. To qualify, air-to-water heat pumps and ground source heat pumps must achieve an in situ seasonal coefficient of performance (SCOP) of at least 2.8³. The same threshold appears in the scheme's installer guidance, which states that eligible heat pumps must have a minimum SCOP rating of 2.8¹⁸.

The scope of that requirement is narrower than it first appears. The SCOP 2.8 requirement applies only to air-to-water systems, not all heat pumps¹⁹. That distinction matters for anyone considering an air-to-air heat pump, which is not covered by the same rule. The property owner guidance repeats the point: the SCOP 2.8 requirement applies only to air-to-water systems²⁰.

The grant itself is £7,500 towards the cost of an air source or ground source heat pump installation⁴. The scheme also requires that the heat pump meets the property's heating and hot-water demands, with a capacity of no more than 45 kWth and a SCOP of at least 2.8²⁰. Older versions of the guidance set the grant at £5,000 for air source heat pumps and £6,000 for ground source heat pumps, but those figures applied until 22 October 2023 and have since been superseded²¹.

For the Domestic Renewable Heat Incentive, a separate and now-closed scheme, eligible air source heat pumps had to have a minimum Seasonal Performance Factor (SPF) of 2.5²². That is a different metric from SCOP, and a lower threshold, which is one reason the two schemes are not directly comparable.

Weather compensation and why real-world SCOP changes through the year

SCOP is a seasonal average, and the reason it is not a single fixed number is that a heat pump's efficiency changes with the outdoor temperature. Most modern heat pumps feature weather compensation that automatically adjusts indoor temperature based on outdoor conditions²³. The system varies the flow temperature according to the season, which is called weather compensation²⁴.

The effect is that the heat pump runs at a lower flow temperature in mild weather, when it needs less heat and can afford to be more efficient, and at a higher flow temperature in cold weather, when it needs to work harder. An air source heat pump can get heat from the air even when the temperature is as low as -15°C²⁵, but the efficiency at that point is lower than on a mild day.

The SCOP calculation captures this. It includes performance at multiple outdoor temperatures, part-load operation, standby and auxiliary electricity consumption, and defrost cycles for air-source systems⁹. Defrost cycles matter in the UK because damp cold weather causes frost to build on the outdoor coil, and the heat pump has to periodically reverse to melt it, which uses energy without heating the house.

The way performance is estimated has also changed. The previous method for heat pumps used Seasonal Coefficient of Performance (SCoP), based on a presumed flow temperature, to estimate the Seasonal Performance Factor (SPF) and calculate the amount of electricity required to deliver the annual heat demand²⁶. That presumed flow temperature is the weak point: if the installed system runs hotter than assumed, the estimate is optimistic.

For a household, the practical message is that SCOP is a guide, not a guarantee. A system designed and installed to run at a low flow temperature, with emitters sized to match, will get closer to the top of the range. A system bolted onto existing undersized radiators and run hot will land nearer the bottom.

Which heat pumps have the highest SCOP

Among the models with published figures, the Vaillant aroTHERM Split air source heat pump is listed with a SCOP of up to 4.88⁵. That is a maker's figure for a specific model and should be read as the top of that model's range rather than a promise for every installation.

More broadly, top model air-to-water heat pumps can go above 6, while an average air-to-water heat pump reaches between 3.5 and 4.5². Those figures come from a maker's guidance and sit above the UK average of around 3.0¹, which reflects the difference between a rated performance under standard conditions and a real installation.

Air source heat pumps generally have a COP between 2.0 and 5.0²⁷. COP is the instantaneous figure, so it will always look better than SCOP on a mild day and worse on a cold one. The SCOP is the number that matters for annual running costs.

For anyone comparing models, the useful approach is to look at the SCOP at the flow temperature the house will actually run at, not the headline figure. A model rated at 4.88 at a low flow temperature may perform no better than a cheaper model if the house needs 55°C water. The heat pump efficiency page explains how COP, SCOP and SPF relate to each other, and the flow temperature page covers what temperature a system should run at.

What this means for a household's energy independence

A higher SCOP means less electricity bought to deliver the same heat. That is the direct link between efficiency and independence: every unit of heat delivered at a SCOP of 4.0 rather than 3.0 is a quarter less electricity drawn from the grid. For a household with solar panels and a battery, a high SCOP also means more of the year's heat can be covered by self-generated electricity rather than imported.

The dependence that remains is on the electricity grid and a supplier. A heat pump does not store heat in the way a gas boiler's fuel tank does, and it cannot run without electricity. The efficiency gain reduces the size of that dependence but does not remove it. A household that wants to go further can look at heat pumps and energy independence and at how solar and a battery work with a heat pump.

The other dependence is on the installation itself. SCOP is not a property of the box alone: it is a property of the box, the emitters, the controls and the commissioning. A well-designed system with weather compensation and correctly sized radiators will deliver a higher SCOP than the same heat pump installed badly. That is why the sizing and heat loss calculation and the radiator and emitter design matter as much as the model chosen.