Heat pump flex trials cause half of customers ‘discomfort’

Published 26 March 2024

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Heat pump flex trials cause half of customers ‘discomfort’

Early trials to determine the potential of domestic heat pumps for providing flexibility have revealed a mixed bag of results. Utility Week unpacks the main findings.

More than half of participants in a heat pump flexibility trial held by electricity network operator National Grid reported feeling “discomfort” at least some of the time while their heat pumps were turned off to reduce the strain on the electricity grid.

The Equinox project, which is in the process of completing its second winter of trials, asked a pool of 350 customers to turn off their heat pump for two hours between 5-7pm from December 2022 to March 2023 to help determine the amount of aggregated flexibility which could be procured from domestic heat pumps without compromising customer comfort and safety.

This will ultimately help assess whether heat pump flexibility is a viable option for distribution network operators (DNOs).

The first-year results showed a mixed bag of results for comfort levels. At least 57% of participants, depending on how they were segregated, reported feeling discomfort at least some of the time during the trial, while between 3-5% of customers felt uncomfortable most of the time.

Ryan Huxtable, innovation engineer at National Grid Electricity Distribution, said: “We got mixed views on comfort from within the same household. For some of them the thought of the heat pump being turned off was what caused them to feel uncomfortable.”

These findings differ from that of another small-scale pilot heat pump trial held between February and April 2023 by Octopus Energy’s Centre for Net Zero and Nesta, called HeatFlex, which found that 76% of customers were either ‘comfortable’ or ‘very comfortable’ during the flex windows.

“The take home message here is that out of our participant pool people were quite happy with our intervention in terms of comfort,” says Daniel Lopez Garcia, director of trials and analysis at the Centre for Net Zero.

However, the trial does acknowledge that its small participant base of just 12 households is not representative of the general customer base as most were more engaged in their energy use than the average household.

Some had quite sophisticated bespoke systems consisting of different combinations of batteries, solar panels and thermal storage, and four out of ten respondents to the trial survey said they had made adjustments to their heating systems to optimise cost and comfort in response to the HeatFlex events.

This suggests that discomfort levels may have been higher in a less engaged pool of customers. In both trials some customers have turned to alternative sources of heating, such as underfloor or log burners, again suggesting that comfort levels may have been lower had these not been options.

While more than 50% of households in the HeatFlex trial had an alternative form of heating, the use of log burners was less prevalent in the Equinox trial, says Huxtable.

“We asked all customers and got a range of answers, some saying they had underfloor heating or other electric standalone heaters or woodburners. It’s a minority in the trial set but it was definitely a factor in how they were able to reduce their heat demand with a heat pump, that small subset do often use those alternative sources.

“Ultimately for trial two the way we work out payments for this links to their reduction in electricity demand at the smart meter, so for those customers that are using alternative electric sources we will explain how the baselining works and the fact that they won’t receive as much incentive. But predominantly we found that people were only using a heat pump for heating.”

Customer fatigue

Measuring comfort levels has been a key feature of heat pump flexibility trials to date as it is thought that customer discomfort could limit long-term participation in any flexibility market in the future. However, studies have shown that comfort is difficult to measure and can vary wildly from person to person.

The range of temperatures participants in the HeatFlex trial would be comfortable at spanned from 13 to 28 degrees Celsius, with individual ranges being between two and ten degrees.

Despite more than half of participants feeling uncomfortable at points, the Equinox trial saw minimal drop-off in participation levels over time, dropping from around 80-90% in the first trial to 70-80% in the last.

However, the trial did see a significant drop-off in participation rates when events were held on consecutive days, dropping by around 20% in the second trail in one instance.

The Equinox trial also found that paying participants after each event for taking part prevented this drop-off and kept participation levels consistently higher than those that were paid monthly. It has switched to a model of paying per kWh of turndown for this year’s trial.

Equinox’s 22 events provided 9.25MWh of measurable turndown, with each participant’s average turndown for each event being 1.53kWh. Of the 386 households that took part, 21 allowed their heat pumps to be controlled remotely by an aggregator, which is the option that offers the greatest returns for consumers.

However, for these customers the ‘snapback’, where heat pumps increase their energy consumption following the trial, was more significant.

“We had quite an overwhelming snapback effect after the event for the Sero customers, which is something we have looked to avoid this year. That related to the control method we were using to turn the heat pumps off and on because it was the first trial and quite a quick process to put it together,” says Huxtable.

The HeatFlex trial avoided the need for any snapback effect by instead trialling a period of pre-heating before the flexibility window. During the pre-heating phase the heat pumps were remotely turned up to achieve the maximum temperature pre-set by each household, before being turned off during the flexibility window.

Pre-heating forms an important part of models for decarbonisation pathways from both the Climate Change Committee and the Department for Energy Security and Net Zero.

While the customer base in the trial was too small to provide a statistically significant set of results, they do show that while consumption increased on average by 7% during the pre-heating window, it decreased by 18% during the flex window.

Meanwhile, the majority of participants did not report a drop in temperature between the start of the flex period and the end, with the average drop being just 0.3 degrees Celsius.

“That would mean that we are not only shifting the electricity consumption to other times of the day, but we are actually killing or destroying some demand. We are effectively reducing the demand of this household,” says Lopez Garcia.

While pre-heating worked well for maintaining comfort levels while providing flexibility in the afternoon, the same was not found to be true for trials run in the morning by HeatFlex.

The trial ran three events where pre-heating started at 5am for two hours, before being switched off between 7-9am. Participants reported being woken up by their heat pump and said they did not like having the heating on before getting up, or heating rooms that would remain largely unoccupied at that time of the morning.

Nine out of ten participants in the HeatFlex trial reported automation as being acceptable, although added that the usability of the R&D app they were required to use could be improved.

However, the trial was biased towards those customers who would be more likely to be in favour of automation, given they were all customers of Octopus Energy and many had more complex heating systems.

Huxtable says that the first year of the Equinox trial showed that while automation is a more valuable option for customers, many would not accept remote control.

“There’s definitely a mix, there are some customers who are automated and wouldn’t look to do it any other way because the returns you get are more valuable. But there are some customers that really wouldn’t like that and were worried about the implications of more control in the home and more data going back to different places, so in the future, I think there needs to be an option for both.

“I don’t think not wanting automation is a reason for being left behind.”

This article first appeared in Utility Week’s Digital Weekly edition. To real the full issue, click here.