My sleepover in a subzero climate lab with a heat pump

I will also find out whether the heat pump can supply me with a hot bath and a hot shower when it’s cold outside — and what my energy bill would be overnight. Would it cost a (potentially frostbitten) arm and a leg?

The research facility is the biggest of its kind in the world. The study will shape not only the technology fitted in new homes but also how we retrofit existing homes for the climate crisis.

• Read more expert advice on property, interiors and home improvement

With gas or oil boilers still heating nine in ten UK homes, the government believes a switch to heat pumps is key to its push for “net zero” carbon emissions by 2050. Despite their widespread use in icy Scandinavia, public scepticism remains about heat pumps in Britain. Increasingly, this banal piece of home heating tech has become a flashpoint of the culture wars.

Over the past two years, Professor Richard Fitton and his team have been testing heat pumps among a range of heating, insulation and ventilation systems in two prototype properties constructed by two of Britain’s biggest housebuilders. One home is by Barratt Redrow, the other by Bellway.

In separate weather chambers, the team are also testing energy upgrades of a replica Victorian terrace — and they are about to build a 1930s semi and a 1980s developer house to study how best to retrofit those.

The Sunday Times is the first newspaper invited to stay the night, in Barratt Redrow’s detached three-bedroom concept house. Icicles hang off its exterior when I arrive for the experiment. Thick frost covers the triple-glazed windows.

A snow cannon, the same type you’d find on a ski slope, has partially covered the house — and its heat pump — in an inch of snow. “If you hear footsteps crunching in the night, it won’t be a polar bear,” Fitton says, joking. “It’s our security guard patrolling.”

Presumably, they would not have hosted a journalist if they thought I might freeze to death. I take some comfort in this notion, but have packed thermal underwear nonetheless.

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Throughout my visit, a giant system of ducts and chillers will keep the laboratory’s weather below minus 5C, which according to Fitton is typical of a cold British winter’s day.

“Two weeks ago, we were at 40C,” the professor says. “Yesterday we were down to 20C below freezing. This house has taken a fair beating, I think.”

An external air source heat pump, Vaillant’s aroTherm Plus 5kW, heats the house via innovative ThermaSkirt skirting board radiators. These take up less space than standard radiators, and are simpler and cheaper to install than underfloor heating.

“Now it’s minus 5C, that air source heat pump has to work very hard,” says Oliver Novakovic, technical and innovation director at Barratt. The heat pump takes heat out of external air, amplifies it and moves it inside the house, he explains. “Obviously now there’s not a lot of heat in the [outside] air.”

When I touch the skirting board radiators inside, they feel lukewarm. “They’re running at 34C,” Novakovic says. This is not nearly as hot as radiator surfaces heated by a gas boiler, which typically reach 60C. Can this heating system keep me cosy overnight?

Last year tests found this heat-pump system “severely underachieved” in minus 5C weather because the skirting board radiators were too small to distribute enough heat inside the house. When the heat pump came on in the mornings and evenings only, the living room barely reached 11C.

Engineers have since fitted bigger ThermaSkirt radiators, with three pipes instead of two inside the skirting boards, to distribute more heat. They also insulated all the heating pipes and added an extra hot-water tank, because heat pumps run most efficiently when there is plenty of water in the circuit.

Today Fitton is re-running the tests to see if it worked. The tweaks seem to have done the trick.

The home feels surprisingly toasty. In fact, by the time I go to bed, it is so warm inside that I struggle to go to sleep. In my upstairs bedroom it is 23.2C. No need for thermal underwear then.

I nudge the house’s Hive thermostat down to get more comfortable. My super-insulated bedroom cools only slightly while I sleep, reaching a low of 20.5C by 7am.

The heat pump stays on overnight. A key finding has been that heat pumps work best when left on constantly, unlike gas boilers, which are typically turned on and off. Fitton says: “We’ve got pretty concrete evidence to say that air source heat pump systems have to be designed right and installed right and used right. If you keep them on all day, every day in cold weather, you will get the [comfortable] temperatures we want to see inside. But if you don’t, they will run inefficiently and you will not get the temperatures. It’s going to be a seismic shift in terms of how people use heating.”

What about hot water? In the en suite bathroom, an electronic blind closes automatically when I turn on the water-saving Kelda shower. Despite the freezing weather, my morning shower is pleasingly hot — as was my evening bath. Both are heated by the same heat pump, which remains blanketed in ice and snow throughout.

The biggest question is: what would my energy bill be when using a heat pump in subzero conditions?

Fitton’s team calculate that I used 1.75kWh of electricity per hour on average, including heating, hot water, lighting, appliances and ventilation. Over 24 hours, that would cost £11.34 (at the present electricity price cap of 26.35p per kWh, plus a 53.68p daily standing charge). That’s not cheap.

Would it cost less with a gas boiler? I can’t tell you. The study has not tested this new-build house with a gas boiler because the government’s imminent new Future Homes Standard will require almost all new homes to have heat pumps.

As I eat a croissant for breakfast, I am grateful for a cosy night’s sleep — and that I don’t have to pay this sort of electricity bill just yet.

What about cooling houses down when temperatures soar?

While Britain sweltered in its hottest summer on record this year, the scientists also tested how the house would cope in a heatwave.

When they held temperatures in the laboratory at 35C for three days (with 12C nights), the house’s high levels of insulation and airtightness kept it cooler inside while windows remained closed. However, heat still built up inside to reach 31C, even though there was no sun shining in through the windows.

Opening the windows in the day made the house even hotter. Keeping windows shut blocked out 85 per cent of the external heat, compared with 40 per cent when windows were open.

This backs up the Mediterranean approach of closing windows and shutters in the day, the scientists found.

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It also shows recent UK building regulations designed to prevent overheating “wouldn’t have the required effect” in heatwaves above 32C, says Professor Lubo Jankovic, who led the heatwave research. “We recommend that the regulations are reviewed.”

The rules assume that blocking out the sun and opening windows to cross-ventilate will limit overheating. That’s not enough, the researchers say.

They recommend fitting heat pumps that can both heat and cool — which is more sustainable than air conditioning — and burglar-proof downstairs windows that can securely open to cool the house at night.

Homeowners and tenants should be advised to close windows when indoor temperatures rise above 22C, they add.

“We also found that increasing thermal insulation helps to reduce risk of overheating,” Jankovic adds. “This is counterintuitive because you wouldn’t be putting your winter coat on in summer outside. But for houses, that works.”