Flow temperature

1 What it does

Flow temperature is not room temperature. Vaillant's 2023 manual gives 65 °C as an example of a flow temperature and 21 °C as an example of a desired room temperature¹. It also affects how well a condensing boiler condenses. Ideal says 60°C will make a modern one run in condensing mode⁷. Worcester Bosch's manual says approximately 65 °C on systems set around a 20 °C heat loss, to keep the return below 52 °C⁸. These are separate makers' statements, and neither is a guarantee for your system.

2 Two numbers that get confused

The 60°C figure is government running advice, in the Clean Energy campaign, for combis. It says most boilers are set too high and that lowering it can improve efficiency and cut bills². The 55°C figure is a design rule. Under England's Approved Document L, wet systems that are new or wholly replaced should be sized for 55°C or lower where feasible⁹. A boiler swap on existing radiators does not fall within that wording.

3 How to change it

Each boiler has its own method, so use your manual. Vaillant's 2023 ecoTEC plus sets it from the basic display, unless a room control is connected¹⁰. Glow-worm's Energy7 sets it on the boiler, or sets a maximum when a control does the rest¹¹. Ideal's Logic+ uses the central heating knob, with an e position for economy⁵. Ideal's blog says it may take some experimenting, and to turn it up slightly if rooms are slow to warm⁷.

Installer levels are not for homeowners; Glow-worm and Vaillant reserve them for competent people¹⁰¹¹.

4 When to take more care

The campaign tip names combis only, so if you have a hot water cylinder ask a heating engineer before lowering anything. Viessmann also says only to change the set values on its Vitodens 050-W if heat supply is insufficient⁶, and its FAQ says there is no generally valid ideal figure³. Uneven rooms may point to balancing instead. For automatic adjustment, see weather compensation.