What COP should a heat pump have?

There is no single mandatory COP for a domestic heat pump in the way there is a minimum boiler efficiency. What exists instead is a patchwork: a scheme threshold for the Boiler Upgrade Scheme, a consultation proposal under Part L, and reference figures published by manufacturers for their own products. The seasonal figure, SCOP, matters more than any single COP reading because it averages performance across the year.

What COP means: units of heat per unit of electricity

COP is calculated as heat output divided by power input, a formula set out in Approved Document L, the building regulations guidance for England⁵. A heat pump with a COP of 3.0 delivers three units of heat for every unit of electricity it uses⁶. The measure indicates how much heat you get from one unit of electricity, and it is the standard way the industry expresses heat pump efficiency⁷.

The critical qualification is that COP is published under specific test conditions. In Europe, COP performance testing is typically done using EN 14511, which defines rating terms and test conditions for heat pumps and similar equipment⁸. That means a quoted COP is tied to a stated outdoor temperature and flow temperature. A figure quoted at one set of conditions is not directly comparable with a figure quoted at another, which is why manufacturer reference figures always carry a label such as A2W35 (air at 2°C, water flow at 35°C).

There is no set limit for COP ratings, but the higher the score, the greater the efficiency of the heat pump⁹. In practice, the COP a household sees depends on the heat source temperature, the flow temperature the system runs at, and how well the building retains heat. A well-insulated home allows a lower flow temperature and therefore a higher COP; a poorly insulated one forces the system to work harder.

What COP should a heat pump have?

There is no single answer, because the benchmark depends on the purpose. For building regulations compliance, the current Part L 2021 standard for other types of heat pump sets a minimum space heating COP of 3.0 and a domestic hot water heating COP of 2.0, according to the 2023 Future Homes and Buildings Standards consultation¹⁰. That is a regulatory floor, not a performance target.

For the Boiler Upgrade Scheme, the requirement is expressed as SCOP rather than COP: an in situ seasonal coefficient of performance of at least 2.8 for air-to-water heat pumps². Ofgem guidance confirms the threshold, alongside a capacity limit of no more than 45 kWth¹¹. A later Ofgem update clarifies that the SCOP 2.8 requirement applies only to air-to-water systems, not all heat pumps¹².

Manufacturers publish their own reference figures. WOLF states that A2W35 air-to-water heat pumps should have a COP of at least 4, B0W35 brine-to-water heat pumps at least 4.5, and W10W35 water-to-water heat pumps at least 5.5¹³. These are maker benchmarks for its own product categories, not regulatory requirements.

COP by heat pump type: air, ground and water source compared

Heat pump types include air source, ground source, and air-to-air¹⁴. Air source heat pumps draw heat from the outside air, while ground source heat pumps extract it from the ground¹⁵. Water source heat pumps take heat from a body of water. The type of source determines the typical COP, because a warmer, more stable source requires less work to raise to a usable temperature.

Ground source heat pumps achieve the highest typical figures. A typical ground source heat pump can generate 3.5 to 4.5 units of heat for each unit of electricity it uses³. Approved Document L modelling references ground-to-water heat pumps with a COP of up to 4.5⁵. Ground source systems cost a lot more to install than air source heat pumps¹⁶, and the heat they produce is at a lower temperature than other forms of heating, working best with underfloor heating, which requires lower flow temperatures than radiators³.

Air source heat pumps are the most common type in UK homes. In the UK, SCOP is generally around 3.0, or 300% efficient, but can be higher or lower depending on the set-up⁷. Air or ground source heat pumps are particularly effective when used with wet underfloor heating, because heat pumps are designed to be left on for longer periods and operate at lower temperatures¹⁷.

Water source heat pumps sit between the two in principle, drawing from a more stable source than air but requiring access to a suitable water body. The available types across the market include open and closed source heat pumps underground, air source and water source¹⁸.

SCOP: the seasonal figure that matters more than a single COP

SCOP, or Seasonal Coefficient of Performance, is the average COP across the whole year⁶. You will also see Seasonal Coefficient of Performance or Seasonal Performance Factor, which shows the heat pump's efficiency averaged across the whole year¹. It is a way to rate a heat pump's performance in a given climate¹⁹.

The distinction matters because a single COP is measured at one set of conditions, while a home experiences a range of outdoor temperatures across a heating season. SCOP integrates that range into one figure, which is why scheme rules and installer calculations use it. Installers need to calculate SCOP separately, because the predicted energy label does not consider the SCOP of heat pumps²⁰.

For the Boiler Upgrade Scheme, heat pumps must have a seasonal coefficient of performance of at least 2.8¹¹. That threshold is the practical benchmark for a UK installation seeking grant support. It is a minimum, not a target: a system installed and controlled well will typically exceed it.

What affects COP in practice: outdoor temperature and flow temperature

Two variables dominate real-world COP: the temperature of the heat source and the flow temperature the system delivers. A warmer heat source and a lower required flow temperature give a higher COP; colder outdoor air with a higher flow temperature gives a lower COP⁸. This is the single most useful principle for understanding why two identical heat pumps in two homes can perform very differently.

Flow temperature is the variable a household has most influence over, through emitter sizing and controls. Heat pumps run at considerably lower distribution temperatures than traditional boilers⁴, and a maximum flow temperature of 55°C is typically recommended²¹. The drivers are straightforward: a warmer heat source and a lower required flow temperature give a higher COP, while colder outdoor air combined with a higher flow temperature gives a lower one²². One maker reports that running at 35°C instead of 55°C on a mild day can give a 30 to 50% COP increase, depending on system and heat pump type, and that a unit held at a fixed 55°C flow temperature on a 10°C day performs well below what the weather would allow²³.

Outdoor temperature is the variable a household cannot control. Air source heat pumps are less efficient in winter when the air temperature is colder¹⁵. During winter, the air temperature can drop by 5°C or more overnight, which reduces the heat pump's efficiency because it is heating water to a high temperature¹⁴. Ground source systems are less exposed to this because ground temperature is stable year-round, which is part of why their typical COP range is higher.

COP and running costs: what a higher COP saves you

A higher COP reduces the electricity needed per unit of heat, but it does not automatically produce lower bills. A heat pump operating at a low COP can end up costing more to run than a gas boiler, particularly in poorly insulated homes without pricing reform²². That is the risk case, and it is why installation quality and emitter sizing matter as much as the headline COP figure.

The saving case is real but conditional. Dr Jan Rosenow notes that a household could save more than 50% on running costs with a heat pump compared to a gas boiler, but this requires high quality installation, high efficiency and a heat pump tariff⁷. Which? reports the same figure with the same conditions¹⁷. The conditions are doing a lot of work in that sentence: a high COP alone does not deliver the saving.

Topping up wall, floor and loft insulation before installing a heat pump will help reduce its running costs¹⁸. The official Check if a heat pump could be suitable for you service compares the cost and carbon emission savings available and what is needed to install a heat pump¹⁹. For a household weighing independence, the COP determines how much electricity the home must buy or generate, but the tariff and the building fabric determine what that electricity costs.