Definition: Flow temperature is the temperature of the water your boiler or heat pump sends out to your radiators. Running a lower flow temperature makes condensing boilers and heat pumps meaningfully more efficient — and most combi boilers leave the factory set higher than they need to be.

Why lower = cheaper

Condensing boilers only recover extra heat from their flue gases when the return water is below roughly 55°C. Nesta’s research found that turning a combi’s flow temperature down from 80°C to 60°C cuts total gas use by around 9% — about 1,080 kWh, or roughly £79 a year for a home using 12,000 kWh of gas at the July–September 2026 cap rate of 7.33p/kWh. Radiators feel cooler and run for longer, but rooms reach the same temperature. Our guide to flow temperature and radiators explains how to check and change yours safely.

On most combis you can turn it down yourself using the dial or menu marked with a radiator symbol — aim for 60°C. One caution: if you have a system or regular boiler feeding a hot water cylinder, the stored water still needs to reach 60°C, so ask a heating engineer before making big changes.

Flow temperature and heat pumps

Heat pumps are designed to run at 35–50°C, and every degree lower improves their SCOP — which is why installers often fit larger radiators. Weather compensation adjusts the flow temperature automatically as the weather changes, so the system never runs hotter than it needs to.

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