In the heat of the night

In this post I discuss how findings from a study I was involved in suggest a challenge for meeting morning peaks in heat demand in winter if we electrify heating in the UK.

I was interested to read a recent report from UKERC on gas use in Britain during our recent cold winter.

The main takeaway is that during really cold periods, the difference in gas demand between 5am and 8am on weekdays can be really big (up to 116 GW). The system can currently (just about) handle such quick and large fluctuations because of the inherent flexibility of the gas grid, which can be pressurised overnight so there is enough gas to meet the morning demand.

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The gas network can be pressurised overnight to meet morning heating demand. (image)

The challenge comes when we think about the need to decarbonise heating, which will probably involve substantial electrification. Electricity systems can be generally be operated more economically if changes in demand are gradual and not too large — meaning the kind of abrupt, big peaks seen in gas demand (mainly for heating) are likely to pose problems.

If heating is electrified, the main technology likely to be employed is the heat pump. By ‘pumping’ heat from the ground or air into a building, heat pumps can warm up a space much more energy efficiently than conventional resistance heaters (think your grandparents’ electric fire).

Heat pumps work most efficiently if they operate reasonably constantly at a reasonably low temperature — so far, so good for the electricity system (no rapid changes). However, on really cold mornings (especially in poorly insulated homes) this means they need to start heating up very early — basically during the night — to be warm enough when the people living there get out of bed.

In a future where we also have ‘time of use’ pricing, energy costs could go up during the morning peak, meaning heat pumps might need to heat up even earlier and to higher temperatures so that the home is warm enough for occupants in the morning.

A few years ago a trial I was involved (reported on here) in tested a new heat pump controller that set out to operate the technology in roughly the ways described above, while still ensuring the comfort of participating residents. It was my job to see what the participants thought of it — and what we found was really interesting.

The results were basically mixed — some people didn’t notice (or had no problem with) the extra night-time heating, while others really didn’t like it:

“… it was like being in the tropics for two nights … the butter melted in the butter dish on my kitchen table and there’s no heat out there!” [participant ‘Christopher’]

“… this thing’s been running on and off all night. The temperature in that bathroom this morning was something silly …” (participant ‘David’)

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“the butter melted in the butter dish on my kitchen table” (image)

When we dug deeper into why this could be, it seemed possible that there was some connection between other aspects of how people were managing the heat in their homes and the experience of night-time overheating. For example, participants who didn’t have a problem said things like:

“ I set those [radiator valves] when I came in, I set it at what temperature I wanted, the bedroom for instance, I don’t have that terribly warm. (participant ‘Georgina’)”

“I don’t worry overnight because I’m asleep and, my bedroom, I turn all the radiators off anyway and I always have the window open, even if it’s snowing” (participant ‘Alan’)

“We close the bedroom door, so this radiator’s not working extra hard to warm the bedroom” (participant ‘Francis’)

Participants who experienced problems, on the other hand, mentioned not touching radiator controls, or even that they were broken.

Sensor data backed this up. In the case of Georgina above, temperatures in her bedroom remained constant in the early mornings even while her living room heated up quite significantly. In other homes the living room and bedroom temperature tracked almost exactly together.

It seems that people who didn’t experience any problems were naturally containing the pre-heating to areas of the house that didn’t bother them overnight, such as the living room (when they were in the bedroom). Those that did have problems had no such separation from the pre-heating.

Given that this part of the work was based on a small sample of just 12 people in 8 homes, what can we say about what this means for tackling the challenge or abrupt/high demand for heat in the morning?

It certainly isn’t possible to make any inference about the proportion of homes that might be affected by such issues — but we do now know that such problems can occur, and the sorts of circumstances that might be associated with them. While subsequent trials have been looking at possible responses, here are a couple of observations we made.

Firstly, the homes in this trial didn’t have any dedicated heat storage (such as a hot water tank), which is why the pre-heating had to take place directly into the home. Ensuring that all heat pumps are installed with storage would go a long way to resolving the night-time overheating issue. But this would, of course, add cost.

There are also space issues. Over half of UK homes now have a combi boiler which doesn’t need a hot water tank (see this slightly old pdf, p50). Which means a lot of homes — like mine — have used the airing cupboard for other things:

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No space for heat storage.

This is especially the case in small homes.

Secondly, this trial made it really clear that while smart controls can permit a high degree of automation of the heating system itself, so much of the overall effectiveness of the systems depends on things that are unlikely to be automated any time soon. Doors and windows in domestic properties, for example. It is a fallacy to think we can automate our way out of energy challenges.

The smart controllers we tested landed in a home whose occupants had (of course) already developed their own approaches to managing warmth. In some cases — such as for those who closed bedroom doors and turned down bedroom radiators — this happened to make pre-heating work for them. Others weren’t so lucky.

Changing these management strategies is no doubt possible, but is unlikely to be straightforward or necessarily desirable for many.

Finally, of course, it is worth mentioning that if the buildings we live in could just be better insulated, many of these problems would be solved. But that is probably a lot to hope for given the current policy ambition in this area. We shall see!

This post draws on my co-authored article ‘Domestic demand-side response with heat pumps: controls and tariffs’, as well as my PhD thesis. Comments and feedback very welcome: @mikefsway or

Written by

Researcher on energy flexibility at @UCL_Energy by day. Also interested in art and open science.

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