Boiler thermostat#

combi boiler facade has dials for controlling boiler and hot water temperature

Your boiler thermostat determines the flow temperature - the temperature of the hot water supplied to your radiators. If you have a condensing boiler, then it can recover heat that would otherwise be lost, but this will only work if you run the boiler flow temperature low enough. Condensing boilers can’t recover heat unless the flow temperature is around 70C or below. The lower the flow temperature (down to 45C), the better the heat recovery works.

Lowering the temperatures is more likely work with generously sized radiators, buildings with lower heat loss, and buildings that are heated most of the day rather than for individual events. If you have an older building in a reasonable state, you might get away with 60C. If you have a newer building that’s insulated, 50-55C might be more appropriate. Reducing the boiler temperature will increase the building’s warmup time, which will matter more if you don’t use your building that much. It’s hard to judge what to do in this case without careful experimentation, but small increases in warmup time are probably worth it and large ones probably aren’t.

This advice is sometimes unsuitable

You should ingore this page if:

  • You have a non-condensing boilers. These wear out faster if you run them at a low temperature. They are designed to be run at 70C or above. You can tell whether your boiler is condensing by looking for a small white plastic condensate pipe coming out of it, usually at the bottom. Almost all boilers installed since 2005 are condensing. Non-condensing boilers can only be installed in places where it’s not economically viable to put a condensing boiler because of difficulties arranging a flue or drainage.

  • you use a hot water storage tank (cylinder). Then you need to make sure Legionella bacteria can’t grow in your cylinder. The usual way to do this is by keeping the tank at 60C, which means you need a flow temperature of around 70C.

  • About hot water cylinders and Legionella

  • Fan convectors usually require higher flow temperatures. This will be specified in the manual.

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