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What is flow temperature?
It's the temperature of the water your boiler sends round the radiators. Many boilers are set to 70 to 80°C out of the box. Modern condensing boilers work most efficiently when the water coming back is below about 55°C, which happens far more often with a lower flow temperature.
How much can it save?
Nesta's tests at the University of Salford's Energy House found that lowering a combi boiler's flow temperature from 80°C to 60°C cut gas for heating by 12%, which is about 9% of a home's total gas. For a typical home using 9,500 kWh a year, that's about £68.14 a year at today's gas price.
Your saving depends on your home and your starting setting. The test house was a solid-walled terrace, and Nesta notes rooms may take up to 15 minutes longer to warm up.
How to change it on a combi boiler
- Find the radiator control on your boiler, usually marked with a radiator symbol. Don't change the tap (hot water) symbol yet.
- Turn it down to around 60°C. Some boilers show numbers or a scale rather than degrees, so check the manual or the manufacturer's website.
- Live with it for a week. If rooms don't get warm enough on the coldest days, turn it up a few degrees.
When to be careful
- System and regular boilers with a hot water cylinder: the cylinder must still be heated to at least 60°C to keep bacteria such as Legionella away. Get advice before changing these.
- Very cold weather: you may need to turn it up on the coldest days.
- If you're unsure, ask your boiler engineer to set it at your next service.
Combine this with a sensible heating timer for bigger savings. See what your boiler costs to run.
Sources: Nesta, Ofgem typical consumption values, Octopus Energy tariff data.