Heatflex floorplans

What if households could support a lower-carbon power grid by shifting the energy demand of their heat pump away from peak times? That’s the question we’ve been looking to answer as part of a pilot project called HeatFlex, in partnership with the Centre for Net Zero – a research unit founded by Octopus Energy.

During the pilot, the heating of 12 households (each Octopus Energy customers) was remotely controlled during events using a smart thermostat.

We asked participants to provide information about how hot or cold they were willing to have their thermostat set to at different times of the day. We used the information they gave us to personalise the HeatFlex events.

We aligned our HeatFlex events with the times of high grid demand, as these periods often have the highest carbon intensity. Carbon intensity is how many grams of carbon dioxide (CO2) are released to produce a kilowatt hour (kWh) of electricity.

Before taking part in the pilot, we would expect that a participant's heat pump would have a generally uniform energy consumption across the whole day.

Our intervention started with a pre-heat phase. Taking remote control of the thermostat to raise the target temperature. A pre-heat phase is beneficial as it will improve thermal comfort during the flexibility window. During this time we would expect the heat pump to be using more energy.

During the flexibility window we would use the smart thermostat to reduce the target temperature to the participant's minimum temperature. We would then expect the heat pump to no longer provide space heating, reducing the energy consumption during the period of high demand and grid carbon intensity.

As demand on the grid and carbon intensity drops, the thermostat would return to the participants’ usual schedule. The heat pump would then return to its normal energy consumption.

The current understanding of the flexibility potential of heat pumps is limited and relies heavily on models and assumed consumer behaviours. HeatFlex UK uses a mixed methods approach, incorporating data science, behavioural science and human-centred design to understand what influences the flexibility potential of heat pumps in homes, during a pilot conducted in participants' homes. As part of this research we interviewed five of the participants and identified the following themes as a result of these discussions.

Differences in participants’ homes may have contributed to differing flexibility potential

Thermostatic radiator valves (TRVs) may have affected how the HeatFlex intervention worked. During the pilot, only the smart thermostat could ‘call for heat’ – by which we mean tell the heat pump to provide heat to radiators.

However, traditional TRVs, as well as smart TRVs set in the way we asked participants to set them, may have reduced our ability to pre-heat homes as much as we had aimed to.

In this example the thermostat was placed in a cooler part of the home resulting in a continuous call for heat, even when other parts of the home had already reached the maximum preheat temperature. This would result in overheating in other areas of the home.

We were also able to gather valuable contextual insights that could help to explain any erroneous data. In this example, we heard that direct sunlight was affecting the thermostat.

1. All doors in the hallway are kept closed.

2. “We had to move the (black) smart thermostat out of the sun as it was being heated.”

The section in red was noticeably warmer during preheat events. It is on the opposite side of the building to the smart thermostat and the adjoining doors are kept closed meaning that the flow of heat around the home is reduced. The participant had to adjust the TRV in this room, restricting this room’s ability to pre-heat.

3. “I’ve had to turn the TRV down to 3 here”

The location of the smart thermostat may change how well the intervention works

Before the interviews, our only gauge of the thermal performance of the participants’ properties was their Energy Performance Certificates (EPCs). However, several homes had areas with very different thermal properties. The placement of the smart thermostat within these areas potentially affected the overall performance of the house.

The front of this property tended to be cooler. This north-facing area is the oldest part of the property and is poorly insulated.

1. We identified a number of behaviours that consistently affect the amount of demand reduction achieved. Closing doors to limit airflow was one such behaviour.

Although the back room tended to be heated well, the door connecting them is mostly kept shut, restricting heat flow between the two spaces.

The rear of the property is a south-facing extension, insulated to Passivhouse standards with large glazing to the rear.

Locating the main thermostat in this area will impact the performance of the poorer insulated areas of the property. Pre-heat may be stopped prior to the rest of the home reaching the desired temperature.

Moving the smart thermostat

The smart thermostat that was used in this home was portable and so we can’t be confident in how often it was moved.

In interviews, we heard that the smart thermostat was sometimes moved within rooms to try to elicit a desired response from the system. The impact of moving the smart thermostat depends on whether it was an evening or morning event, the fabric of the property and direct exposure to sunlight.

The smart thermostat was mostly kept in the lounge. It may have been affected by smart TRVs also controlling the heat in this space, a log burner and heat from the sun.

We learnt that this participant moved the thermostat, especially if they were cold. Moving the thermostat may elicit a call for heat, turning the heat pump back on before the end of a heatflex event.

1. “Sometimes when it's cold I'd take it (the smart thermostat) to bed. I tend to take the thermostat to where we are.”

TRV’s may have also prevented pre-heating from working during morning events as we heard in interviews that:

“The beauty of TRVs not calling for heat is that the bedroom didn’t turn on. So the TRV in the bedroom meant that the bedroom didn’t overheat. ”

Having the thermostat in the same room as a TRV that is preventing overheating may have resulted in the thermostat continuing to call for heat until the end of the pre-heat period, even if the desired temperature had been met in other rooms.

Types of activity

We asked participants to list any activities they were doing during flexibility events. When coupled with the information from the floor plan this gave us an insight into how participants were spending their time and the effect it might have on the acceptability of events.

The smart thermostat is in a position which may be affected by the underfloor heating, which could impact the heating in the rest of the house. As a result we saw very little difference in self-reported temperature.

A large proportion of the downstairs of the property has underfloor heating which is controlled separately to the radiators. We know that the participant spent at least 11 of the HeatFlex events in this room. Due to the fact that we may not have been controlling this room, their experience may not be representative.

“(the) UFH is on a different loop to the radiators. Thermostats in the hall and kitchen control the manifold. It pretty much runs itself.”

During winter 2023 we plan to run a larger-scale trial, which will be influenced by the findings from these interviews. We will also be looking to run a similar exercise with participants of the large-scale trial, to build further insight into the behaviours and factors that impact flexibility.

You can read more about the HeatFlex pilot, the approach and findings from the surveys and data collected in our accompanying report.