The £120 trick: why your boiler's flow temperature is probably wrong

Here's an uncomfortable fact about British home heating: the majority of combi boilers installed in the last decade are running inefficiently, and the installer knows it.

That's because almost every new boiler is commissioned with a flow temperature of 70–80°C — the old default — when the engineering sweet spot for a modern condensing boiler is closer to 55°C.

The physics, quickly

Condensing boilers earn their efficiency rating by pulling latent heat out of the flue gases as water vapour condenses back to liquid. That only happens when the return water coming back from your radiators is below about 54°C.

If your flow is set at 75°C, the return is too hot. The boiler acts like a 1990s non-condensing unit — with the efficiency rating to match. Independent lab tests show real-world gains of 6–12% of gas consumption from lowering flow temperature alone.

How to change it (it takes 30 seconds)

1. Look at the control panel on the boiler itself — not the thermostat on the wall.
2. Find the radiator / CH / flow temperature dial (distinct from the hot-water dial).
3. Turn it down to 55°C or the first mark below the middle.
4. Give the house 48 hours to adjust. Rooms may take 20 minutes longer to reach temperature but will be equally warm.

When it won't work

• Older radiators in a poorly insulated home may be undersized for low-flow operation. If the house stops reaching temperature after two days, bump back up to 60°C.
• Hot water is a separate setting — keep that at 60°C to prevent legionella risk.
• Heat pumps already run at 35–45°C flow. This advice doesn't apply.

Nest and Hive won't do this for you. A screwdriver and two minutes will. The Energy Savings Trust estimates the average household saves £110–£130 a year.

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