What flow temperature do heat pumps use for space heating?

The number you will see most often is 55°C. The Product Characteristics Database, which underpins SAP calculations, uses 55°C as its default flow temperature for heat pumps, with 35°C and 45°C as the other recorded values². In practice, air source heat pumps usually run at lower flow temperatures of around 45°C, according to Northern Ireland's official advice³, and one council-led energy strategy describes an optimal supply temperature of 45°C with a 40°C return⁴.

Understanding flow temperature and its typical values

Flow temperature describes the water leaving the heat pump and flowing into the heat distribution system⁸. Because a heat pump moves heat rather than burning fuel to create it, every degree cooler the water is, the less electricity the compressor has to use. That is why the whole design of a heat pump system, from the emitters to the insulation, is built around running as cool as the house allows.

The range across UK guidance is consistent. A flow temperature of 45°C is described as usual in heat pumps⁹, and well-insulated buildings tend to achieve higher efficiency using a flow temperature of 35 to 55°C¹⁰. The SAP methodology's own default for heat pumps with a wet distribution system is 58°C, a conservative figure used for calculations rather than a recommendation for your living room¹¹. Where the house is well insulated and the heat load is low, lower flow temperature operation becomes possible, which improves efficiency further¹².

Adjusting flow temperature in line with outdoor conditions

If your heat pump's flow temperature changes through the day, that is usually weather compensation at work. Modern heat pump systems use weather-compensated control, which adjusts the flow temperature based on the outdoor temperature: mild weather means a lower flow temperature, cold weather a higher one⁸. The control adjusts the target flow temperature against the outdoor temperature using what is called a heating curve¹³, and the system measures local outdoor temperature automatically¹⁴.

Weather compensation kits and load compensators that change the flow temperature for you can also be fitted to existing systems¹⁵. The control strategy shapes how quickly a room responds, because a system set to a lower flow temperature takes longer to lift room temperatures than one running hotter.

How lower running temperatures improve COP performance

The relationship is simple and consistent across every source: lower flow temperatures improve COP, while higher flow temperatures reduce it¹⁶. If the same heat pump were tested at 55°C flow temperature instead of 35°C, the COP would be significantly lower⁸. Higher flow temperatures generally reduce efficiency and increase electricity consumption for the same heat output¹⁷. The key factor is the temperature difference between the heat source entering the heat pump and the distribution temperature leaving it: the smaller that difference, the more efficient the system¹.

Official methodology puts the same point in formal terms: heat pumps operate more efficiently when the source temperature is higher and the sink temperature is lower¹⁸. In practical terms, the higher the flow temperature, the harder the heat pump has to work¹⁹. There is a trade-off to understand: lowering the flow temperature reduces the reaction time of the heating system, meaning it takes longer to warm the house, so the heat pump needs to run for longer hours at a lower output⁹. Cold weather compounds this, as winter overnight air temperature can drop by 5°C or more, reducing efficiency when heating water to a high temperature⁹.

Radiators, underfloor heating and fan coils at different flow temperatures

The emitter decides how low you can go. Heat pumps typically send water to radiators at 35 to 45°C, lower than an equivalent fossil-fuel system²⁰. Guidance for radiator-based systems suggests sizing the radiators for a maximum flow temperature of around 45°C²¹. Larger radiators release more heat at these cooler temperatures, which is why radiator and emitter choice matters so much to a heat pump's running cost.

Underfloor heating allows the lowest temperatures of all. A flow temperature of 35 to 40°C is typical for underfloor heating⁵, and with underfloor heating the flow temperature could be lower compared with radiators, which should result in the heat pump running much more efficiently²². Fan coil units sit in between: supply water to fan coil units should be in the range 35 to 45°C⁶, and in practice the heat pump sends 35°C water to the coil while the fan blows 20°C room air across the fins to absorb the heat²³. For more on how the whole system is matched to the house, see heat pump sizing.

Heat pumps designed for high temperatures and their limits

Not every home can run at 45°C, and the industry has answered with high-temperature units. Many high- and mid-temperature heat pumps now deliver a flow temperature above 60°C²⁴, and high temperature heat pumps can deliver water up to approximately 75°C, even at external temperatures down to −10°C⁷. Energy Systems Catapult notes that some supply heat at a similar temperature to your current boiler²⁵.

One caution applies to advertised maximums. An air-source heat pump may be advertised with a maximum flow temperature of 70°C, which alone does not prove it can deliver that continuously at −12°C outdoor temperature²⁶. The full picture of these units is on our high-temperature heat pumps page.

Part L rules across England, Scotland, Wales and Northern Ireland

Building regulations are pushing new systems towards the cooler end of the range. In Wales, Approved Document L Volume 1 requires that where a wet heating system is newly installed or fully replaced, all parts of the system, including pipework and emitters, should be sized to allow the space heating system to operate at a maximum flow temperature of 45°C or lower²⁷. Independent UK guidance points the same way, citing 45°C for heat pump installations in new-build properties²⁸.

For existing systems, the standard benchmark is 55°C. Heat pumps typically operate at a lower temperature than gas boilers, for example 55°C, in line with Building Regulations²⁹, and low-temperature heating systems are described as those with radiators and pipework suitable for 55°C flow temperature or below³⁰. The rules matter most at the point of a new installation or full replacement, which is covered in detail in heat pump installation.

Realistic expectations for households

In a real home, the flow temperature is a design decision made at installation, not a fixed property of the heat pump. Many installers set the heat pump at 55°C by default to guarantee heat, but reducing the flow temperature to 45°C can still keep a house warm and improve efficiency³¹. What that temperature can be depends on the heat load of the building: better insulation lowers the heat load and makes lower flow temperature operation possible¹².

A household should expect the flow temperature to vary rather than sit still. With weather compensation, the controller will show a lower figure on a mild afternoon and a higher one on a freezing night⁸. The practical signs of a well-set system are steady room temperatures, longer running hours and lower electricity use, rather than a single number on the display. For the wider picture of how these systems perform in real UK homes, see heat pump performance in UK homes and our main heat pumps guide.