Condensing combi boilers often have their flow temperatures set between 70°C-80°C by default (flow temperatures are the temperature that a boiler heats water to before sending it through the radiators). A high flow temperature prevents boilers from reaching their maximum efficiencies because they do not operate in condensing mode. Encouraging households to lower boiler flow temperatures to 60°C, or even lower in warmer months, is a promising approach to increasing boiler efficiency and reducing gas consumption in homes. If the water returns to the boiler after passing through radiators below 54°C, the boiler starts condensing more, and there is more condensing and more efficiency if the return temperature is lower still.

Lab experiments and modelling have shown that lowering boiler flow temperatures can reduce gas consumption. However, there is currently a lack of real-world evidence from randomised controlled trials to support these findings.

We ran a field randomised controlled trial with 61,000 users of Loop, a smart meter energy advice app, to evaluate the impact of giving flow temperature advice between January and April 2023. We tested the impact of season-specific flow temperature advice on gas use and thermal comfort.

• Giving participants month-specific boiler flow temperature advice via email resulted in a small, but statistically significant, decrease in daily gas consumption compared to the control group. These participants received three emails over the trial period recommending different flow temperatures for each month (for customers with combi boilers): 60°C for January and February, 55°C for March and 50°C for April. We did not find a statistically significant difference in daily gas consumption for participants who received a one-off email advising a 60°C flow temperature compared to the control group.
• The month-specific advice also resulted in a statistically significant increase in the proportion of participants that reported turning down their boiler flow temperature in the last three months. However, we didn’t find any statistically significant differences for those who received a one-off email advising a 60°C flow temperature.
• We didn’t find any differences in the proportion of self-reported thermal discomfort between the treatment groups and the control group, nor any differences in daily electricity consumption.

Our findings indicate that giving month-specific combi boiler flow temperature advice could result in greater gas savings than a one-off recommendation of lowering flow temperatures to 60°C (which is the current prevalent advice). Importantly, this research didn’t find evidence that doing so increases electricity use, or has a negative impact on household thermal comfort

We recommend that energy companies and advice-giving organisations who have regular contact with households give monthly flow temperature recommendations based on the seasons to customers with combi boilers, to ensure optimal efficiency, savings and comfort for households. We would recommend that companies and organisations with less regular contact with households include information about the option of changing flow temperatures with the seasons on any resources about lowering flow temperatures to 60°C, or more generally.

We would broadly recommend the following for condensing combi-boilers:

• Winter: 60°C
• Early spring and late autumn: 55°C
• Mid spring and mid autumn: 50°C

Late spring and early autumn: 50°C or below if comfortable