Which? has said it began encouraging people to lower the flow temperature of their combi boiler before energy bills increased sharply in October 2022, with government guidance on the same measure following in December 20221. The consumer body set out the position in an article on boiler controls published on 16 September 20251.
The article states that lowering the flow temperature, the temperature of the water a boiler sends to radiators, reduces energy use while increasing the amount of heat the boiler recovers that would otherwise leave through the flue1. Which? said savings from setting a boiler up efficiently could be "as much as 10% or more" on a monthly gas bill1.
Which? also described the Boiler Plus legislation, introduced in 2018, which requires energy-saving controls to be fitted when a new boiler is installed1. It named three control types under the standard1:
| Control type | What it does |
|---|---|
| Smart heating controls | Automate the system, using features such as geofencing and open-window detection |
| Load compensation controls | Lower the flow temperature to the level needed to reach the thermostat setting |
| Weather compensation controls | Adjust boiler output using an outside sensor |
Which? said there is still leeway within the regulation to allow inefficient set-ups1. It added that some smart thermostats, including some Hive thermostats installed by British Gas, operate boilers on on/off controls that do not modulate and so do not save energy1. It also said there is no universal standard governing boilers and thermostats, so compatibility depends on the boiler model1.
On limits, Which? said compensation controls are one part of an interdependent heating system and will not make everything perfect1. It said homes with small, fairly old radiators will probably only be able to run at low flow temperatures up to a point before the boiler struggles to meet the temperature the thermostat demands, and that heat loss needs to be mitigated for slow, low-temperature heating to work1. It also said a boiler must be the right size for the home, because larger boilers have higher minimum outputs1.
Why it matters for households
Flow temperature is the setting that decides how hot a boiler runs the water in a central heating system, and it is separate from the room thermostat setting1. A condensing boiler recovers heat from its flue gases when it runs below maximum temperature, so a lower flow temperature means less gas is used for the same room temperature1. For a household, that is a change to how much energy the home consumes rather than to how much it buys, and it sits alongside the wider question of what a home's energy bills and energy independence look like over time.
Which? said the setting can be adjusted by hand, but that load or weather compensation does it automatically and that an engineer should set it up because the work is fairly precise1. It also said the right flow temperature depends on the home: large radiators and good insulation allow lower temperatures, while small or old radiators limit how far it can be reduced1. Which? said it is best to avoid blanket rules on how long heating should be on, because what suits an energy-efficient home may not suit one that is not1.
What happens next
No further steps are dated in the article. Which? said its guidance on adjusting flow temperature can be followed manually, and that an installer can say which controls are compatible with a given boiler and fit them1. It said a weather compensation sensor may need to be placed outside the home1.