For an air source heat pump, the most efficient flow temperature is typically between 35°C and 45°C, while your room thermostat should be set to your usual comfort level, around 18°C to 21°C. Unlike a gas boiler, which often runs at 60°C to 70°C, a heat pump works best at lower temperatures to maintain its Coefficient of Performance (CoP). The Energy Saving Trust states that lowering the flow temperature by 5°C can improve efficiency by roughly 2-3%, saving about £30-£50 a year on a typical bill of £1,200.
- Flow temperature: The water temperature sent to radiators or underfloor heating. Set this to 35°C for underfloor heating or 40-45°C for radiators.
- Room thermostat: Controls your indoor air temperature. Set this to 18°C for bedrooms and 20-21°C for living areas, as recommended by the NHS for a healthy home.
- Weather compensation: Many heat pumps automatically adjust the flow temperature based on outdoor temperature, which can save up to 10% on running costs, per MCS data.
| Setting | Typical Value | Annual Cost Impact |
|---|---|---|
| Flow temperature (radiators) | 40-45°C | Baseline |
| Flow temperature (underfloor) | 35°C | ~£30-£50 lower than 45°C |
| Room thermostat | 18-21°C | Each 1°C reduction saves ~£80-£100 |
This matters more if your home has poor insulation or oversized radiators, as a lower flow temperature may struggle to keep the house warm. It matters less in a well-insulated new build with underfloor heating, where 35°C is standard and efficiency is already high.
Bottom line: Set your heat pump flow temperature to 35-45°C and your room thermostat to 18-21°C for the best balance of comfort and running costs. Adjust the flow temperature down if your home stays warm enough, and up only if rooms feel cold.
Sources
A worked example
For a typical 1930s semi-detached home in Manchester with a 9kW air source heat pump and standard radiators, setting the flow temperature to 42°C instead of 50°C saves £85 annually on a £1,200 heating bill. This home has cavity wall insulation and 270mm loft insulation, meeting Energy Saving Trust recommendations for efficient heat pump operation. The household runs the room thermostat at 20°C in living areas and 18°C in bedrooms, with weather compensation enabled to automatically adjust the flow temperature on milder days. With the BUS grant covering £7,500 of the installation cost and 0% VAT on heat pumps until March 2027, the total upfront cost after grants is approximately £3,500. The £85 yearly saving from the lower flow temperature setting, combined with a £100 annual saving from reducing the living room thermostat from 21°C to 20°C, gives total yearly savings of £185 and a payback period of roughly 19 years on the net installation cost.
| Item | Figure |
|---|---|
| Upfront cost after grants | £3,500 |
| Yearly savings | £185 |
| Payback period | 19 years |
| 25-year lifetime savings | £4,625 |
What homeowners often get wrong
The most common mistake is setting the heat pump flow temperature as high as a gas boiler, around 60°C to 70°C, which destroys efficiency and doubles running costs. Here are three frequent errors and how to avoid them.
- Setting flow temperature too high Many homeowners believe a higher flow temperature heats the home faster, but a heat pump running at 55°C instead of 40°C can cut its CoP from 3.5 to 2.0, wasting roughly £300 per year on a typical £1,200 bill.
- Turning the room thermostat up and down daily Unlike a gas boiler, a heat pump works best maintaining a steady temperature. Frequent adjustments force the system to work harder and can increase running costs by up to 20%, voiding the efficiency gains from lower flow temperatures.
- Ignoring weather compensation settings Many installers set weather compensation to default curves that are too aggressive. Not adjusting this for your home can waste up to 10% of annual running costs, as the system may overshoot on milder days.
Quick reference
- Set your heat pump flow temperature to 35°C for underfloor heating and 40°C to 45°C for standard radiators to maintain a CoP above 3.0.
- Lowering the flow temperature by 5°C improves efficiency by 2-3% and saves roughly £30 to £50 per year on a typical £1,200 heating bill.
- The BUS grant covers up to £7,500 of the installation cost for air source heat pumps, and 0% VAT applies until March 2027.
- Each 1°C reduction in your room thermostat saves around £80 to £100 annually on a typical £1,200 bill, according to the Energy Saving Trust.
- Enable weather compensation to automatically adjust flow temperature based on outdoor conditions, this can save up to 10% on running costs.
Frequently Asked Questions
For radiators, set your heat pump flow temperature between 40°C and 45°C. This is lower than a gas boiler's typical 60-70°C, but it maintains the heat pump's efficiency. Lowering it by 5°C can save around £30-£50 per year on your energy bill.
Set your room thermostat to 18-21°C for optimal comfort and efficiency. The NHS recommends 18°C for bedrooms and 20-21°C for living areas. Each 1°C reduction can save about £80-£100 annually on your heating costs.
Yes, lowering the flow temperature by 5°C can improve efficiency by 2-3% and save £30-£50 per year on a typical £1,200 bill. However, ensure your home stays warm enough; if rooms feel cold, increase it slightly. Weather compensation can also save up to 10% on running costs.
Yes, under the Boiler Upgrade Scheme, homeowners in England and Wales can get £7,500 off the cost of installing an air source heat pump. This grant is available until 2028. Check eligibility on gov.uk as it replaces previous schemes like the Renewable Heat Incentive.
For underfloor heating, set the flow temperature to around 35°C. This is highly efficient and works well with heat pumps, especially in well-insulated homes. At this setting, you can save £30-£50 annually compared to running at 45°C for radiators.
