The government issued guidance in December 2022 on lowering the flow temperature of combi boilers, according to Which?, the consumer body. Which? states that it had been encouraging people to lower their combi boiler flow temperature since before energy bills increased sharply in October 2022, and that the government's guidance came later, in December 20221.
Flow temperature is the temperature of the water a boiler sends to the radiators. Which? explains that adjusting it down means less energy is used while increasing the amount of heat the boiler can recover, heat that would otherwise be pushed out of the flue1. The guidance followed the October 2022 rise in energy bills1.
The practice sits alongside wider boiler control rules. Which? states that Boiler Plus legislation was introduced in 2018 to tackle the problem of older, basic boilers by requiring energy-saving controls to be set up during new boiler installations, though it adds that there is still leeway within the regulation to allow for inefficient set-ups1. Boiler Plus sets out three control types: smart heating controls, load compensation controls, which lower the flow temperature to a level sufficient to reach the thermostat setting, and weather compensation controls, which adjust output based on an outside sensor1.
Which? says compensation controls do the flow temperature adjustment automatically, determining how much heat the boiler needs and setting the temperature accordingly, and that while it is possible to guess the right setting from the boiler's control panel, it is best if an engineer sets the boiler to work it out automatically as it is fairly precise work1. It notes there is no universal standard governing boilers and thermostats yet, and that a boiler must be able to communicate with compensation controls, which depends on the model1.
Which? also sets out limits. It states that efficient hydronic central heating systems use low flow temperatures in properties with large radiators and good insulation, and that homes with small and fairly old radiators will probably only be able to go so low before the boiler struggles to meet the temperature the thermostat demands1. It adds that for a system to be as efficient as possible the boiler must also be the right size for the home, since large boilers have higher minimum outputs1.
Why it matters for households
A combi boiler's flow temperature setting determines how hard the boiler works to heat the water going to the radiators. Lowering it reduces the energy used per unit of heat delivered, because a condensing boiler recovers more heat at lower operating temperatures1. For a household, that is a change to how the existing boiler runs rather than a change of fuel or equipment, so it bears directly on how much gas a home consumes for the same comfort level.
The guidance applies to combi boilers specifically. Households with other boiler types, or with small or old radiators, may find the achievable reduction limited, because the system may not reach the temperature the thermostat demands at low flow temperatures1. Which? states that compensation controls, whether load or weather based, adjust the flow temperature automatically rather than relying on a manual setting1.
What happens next
No further steps are set out in the material. Which? notes that Boiler Plus requires energy-saving controls to be set up during new boiler installations, and that a household getting a new gas boiler will usually need to consider fitting one of the three control types1. It also states that there is no universal standard governing boilers and thermostats yet, and that compatibility depends on the boiler model1.