Lab experiments and modelling commissioned by Nesta suggest that households could save around 8% on their annual gas use by lowering the temperature of their combi boiler. However, this measure has never been tested with real households at scale or over a significant portion of the season when people have their heating on. We’re conducting a randomised controlled trial with 61,000 users of an energy advice app to investigate how giving advice to lower boiler flow temperatures can impact gas use and household comfort. The results could help boiler installers provide impactful recommendations to support households with saving energy and money in the future.

Many condensing boilers in the UK aren’t operating to their maximum efficiency. Ideally, these boilers should be set to a flow temperature of 60°C or lower to ensure they’re running as close as possible to their maximum efficiency. However, many are currently installed to have a flow temperature of between 70°C-80°C.

Almost all households can easily and safely lower the flow temperature of their boiler themselves without the need of a gas engineer. Nesta’s lab experiment with Salford University’s Energy House verified that doing so could significantly reduce household gas use. This experiment, together with modelling from Cambridge Architectural Research, suggests that households could save about 8% on their annual gas use by lowering their flow temperature from 80°C to 60°C.

Although our evidence suggested that lower flow temperatures reduced gas consumption due to an increase in boiler efficiency, it also suggested that some of the gas saving was due to room temperatures being slightly lower than before.

Although we have evidence from the lab experiment and modelling that lowering boiler flow temperatures reduces household gas use, we have not tested with real households at scale. It is especially important to test this advice in the real world now that we know some of the gas saving from lower flow temperatures could be due to slightly lower room temperatures.

The current evidence does not tell us whether people will notice a slightly cooler room temperature and how they might act if they do. Will they simply accept the temperature change or will they compensate for the lower temperatures in suboptimal ways that could reduce, eliminate or even reverse the gas savings? For example, by turning the thermostat up, extending heating hours or using electric heaters.

Cambridge Architectural Research estimates that if households were to compensate for the lower room temperature, the initial gas saving would be reduced by over 50%. In other words, the gas saved from lowering the boiler flow temperature might fall from 8% to 3.6%. That’s why it’s vital to go beyond the current evidence and understand how advice to reduce boiler flow temperatures works with real households. We want to confirm that a significant reduction in gas use would remain and that households aren’t making other changes that could remove this saving.

This research is especially important given the broad variety of property types, heating patterns, comfort preferences and ways in which people react to temperature changes. Through analysing the data we collect, we can explore whether households report turning the flow temperature back up, changing the thermostat setting, running heating for longer or indicate a change in thermal comfort.

Given the prevalence of advice around lowering boiler flow temperatures in the winter of 2022-2023, it’s important we better understand the real-world and medium-term impacts of this efficiency measure and whether savings can be further increased. This will ensure households can be better supported to maximise energy and cost savings with this measure, whilst maintaining their usual comfort.

We’re partnering with an energy advice app to conduct a randomised controlled trial from January 2023 to the end of April 2023. This will allow us to understand the impacts of giving advice to lower flow temperatures with a large, real-world sample and over a large proportion of the heating season – something that has never been done before.

We will randomly allocate 61,000 users of an energy advice app to one of our three trial conditions:

• Control condition: participants receive no intervention
• 60°C condition: participants will receive an intervention email in January advising them to lower their flow temperature to 60°C.
• Month-dependent condition: participants will receive three intervention emails. The first, sent in January, will advise them to lower their flow temperature to 60°C. The second, sent at the end of February, will advise them to lower their flow temperature to 55°C. The final, sent at the end of March, will advise them to lower their flow temperature to 50°C.

All of the intervention emails will include a link to an online walkthrough tool to help households lower their flow temperature.

The 60°C condition was chosen as this is the temperature that the government currently advises, and other organisations and research suggests this temperature would be suitable for the majority of homes in the coldest months whilst still significantly decreasing gas consumption.

The month-dependent condition was created to better understand how giving flow temperature guidance tailored to the time of year impacts gas savings and thermal comfort. The lower the flow temperature, the more gas is saved. Although many households may not be able to go below 60°C in the depths of winter, the majority could do so in the milder spring and autumn seasons. This could further increase boiler efficiency and energy savings beyond current recommendations.

We have two main research questions for this study.

1. Does receiving boiler optimisation advice result in changes in daily gas consumption (when compared to the control group or between trial conditions)?
   
2. Does following boiler optimisation advice result in changes in self-reported thermal comfort (when compared to the control group or between trial conditions)?

We will collect gas data from participants’ smart meters to help us answer the first research question. We will also issue surveys to all conditions (including the control group) before, during and after the trial to gather information on household thermal comfort which will allow us to answer the second research question.

We will also collect electricity data from smart meters and information on how households are using their heating systems. This will enable us to conduct some exploratory research into differences in electricity use and how participants use different parts of their heating system after receiving advice.

This trial will not be able to quantify the average saving made by a household changing their boiler settings because not everyone who receives our advice will turn down their boiler flow temperatures. There’s also a chance that some people in the control group may have already taken this advice – already due to the extensive media campaigns from the UK Government, Nesta and other organisations. This means the difference in gas consumption between the treatment groups and the control group isn’t the direct effect of turning down boiler flow temperature. Instead, the difference is the impact of receiving our advice (what is known as a “intention-to-treat” design). We believe this is important, as our estimate of the effect will take into account instances where people have already turned down their flow temperature, as well as adjusting the impact for the number of people who follow the advice.

Although the trial won’t give us an exact figure for the average saving made by lowering boiler flow temperatures, it can still tell us whether receiving this advice leads to a statistically significant reduction in gas use.

You can read more about our trial on the Open Science Framework.