What is heat pump flow temperature?
Flow temperature is the temperature of the water your heat pump sends through your radiators or underfloor heating to warm your home. It is one of the most important settings affecting heat pump efficiency, electricity consumption and running costs. Generally, the lower the temperature, the more efficiently the system can operate, provided your home still reaches a comfortable temperature.
Why it matters so much
- 35°C water temperature → CoP approximately 4.0: Around 4 units of heat for every 1 unit of electricity used.
- 45°C water temperature → CoP approximately 3.0: Efficiency falls as the system has to produce hotter water.
- 55°C water temperature → CoP approximately 2.2: Higher water temperatures generally mean higher electricity use and running costs.
- 65°C water temperature → CoP approximately 1.6: Efficiency can fall significantly at high temperatures, making the system much more expensive to run.
These CoP figures are illustrative rather than universal. Actual heat pump performance depends on the model, outdoor temperature, system design and other operating conditions, but the underlying principle is consistent: lower water temperatures generally improve heat pump efficiency.
What your heat pump flow temperature should be
For many UK homes, a well-designed system can operate efficiently at relatively low water temperatures. Around 35–40°C may be suitable for a well-insulated home with appropriately sized radiators, while 40–45°C can be appropriate where heat demand or radiator output is higher. Energy Saving Trust notes that heat pumps commonly operate with radiator temperatures around 35–45°C. A temperature above 50°C is not automatically wrong, but it is worth checking whether insulation, radiator sizing, system design or controls could be improved before simply running the heat pump hotter.
How to check your heat pump temperature
Your heat pump controller, thermostat or manufacturer app should display the setting. Look for “flow temperature,” “leaving water temperature (LWT)” or a similar term. If the temperature appears higher than necessary, do not simply reduce it sharply yourself: check your manufacturer’s guidance or ask your installer to review the heating curve, radiator sizing and heat pump settings. Lowering the temperature gradually can improve efficiency and reduce running costs while maintaining comfort.
