Can a heat pump give you a hot bath on a cold day?

Times

22-04-2025

If you ditched your gas boiler for an air source heat pump, could you still get a steaming hot bath on a chilly day? The world's biggest climate change laboratory, near Manchester, has put it to the test for the first time.
Inside a giant chamber that can mimic everything from minus 20C snow storms to 40C heatwaves, scientists at the University of Salford have tested whether heat pumps can meet the daily hot water needs of the average family.
It is part of a series of tests at Energy House 2.0, the university's £16 million laboratory, to see how typical new-build homes will perform in every weather the climate emergency is set to bring.
With gas boilers to be phased out in new homes, the team has already tested which electrical heating systems will keep you warm without big energy bills. An air source heat pump worked best, paired with underfloor heating downstairs and radiators upstairs. It was up to three times cheaper to run than infrared heaters — electric panels that heat you directly, rather than the air around you.
In the latest tests, the researchers compared two rival heat pump systems in a three-bedroom house built inside the laboratory by Barratt Redrow, Britain's biggest housebuilder. They simulated a range of cooler days — with temperatures of 5C, 7C and 14C — to check whether the taps could maintain a steady flow of hot water for washing up, cleaning and a 'worst-case scenario' (think teenagers) of multiple long baths and showers, says Professor Lubo Jankovic, who led the work.
The first system tested is typical of what could be fitted in a flat or terraced house, which lacks outdoor space for a bulky external air source heat pump. It consists of a standalone 195-litre Cürv HP200M3 cylinder with a built-in air source heat pump, which supplies hot water only — not heating. It is designed to work alongside infrared heaters.
About the size of a fridge-freezer, the Cürv cylinder can fit inside a tall 60cm-wide kitchen cabinet. It sucks in air from outside through a vented duct, then amplifies the ambient heat in the air to heat water for all taps.
It used less energy and was more efficient than the rival system in supplying hot water. In typical British winter weather of 5C, the Cürv cylinder consumed 5.61kWh of electricity per day. That would cost £1.52 at the current electricity price cap. It meant the hot water delivery efficiency was 204 per cent, with 1kWh of electricity producing 2.04kWh of heat. (A gas boiler, by comparison, has about 90 per cent efficiency.)
At 14C, the Cürv system needed slightly less power to run (4.15kWh per day), costing £1.12. Its hot water delivery efficiency increased to 256 per cent.
The second system is typical for a three-bedroom family house. It supplies both hot water and heating through an Vaillant Arotherm air source heat pump outside, linked to a 236-litre Vaillant Unistor storage cylinder inside the home.
This Vaillant system passed the hot water test in all weathers. However, the colder it was outside, the less efficient the heat pump became at supplying hot water. At 5C, it used an average of 8.58kWh per day for hot water, costing £2.32. Its hot water delivery efficiency was 184 per cent.
When the temperature rose to 14C, its electricity consumption dropped to 6.62kWh per day, costing £1.79. This raised the Vaillaint system's hot water delivery efficiency to 238 per cent.
Both systems can comfortably fill a hot bath in normal 5C winter weather, the study concluded. 'We are delighted that our research can provide reassuring information to consumers about the forthcoming transition from gas boilers to heat pumps,' Jankovic said.
But which system is best? 'The Vaillant system is more stable and reliable, while the Cürv system provides a more compact, energy-efficient alternative,' the scientists wrote.
At 5C, hot water would be 80p per day cheaper coming from the Cürv cylinder than from its Vaillant rival. However, the Cürv cylinder is designed to be paired with infrared heaters, which would cost around £8.10 per day to run at 5C, the researchers found last year. That would bring your daily Cürv bill to almost £10 per day for hot water and heating.
By comparison, the Vaillant air source heat pump provided heating for £2.83 per day in 5C weather. Adding its £2.32 hot water cost would bring your total Vaillant heat pump bill to just over £5 a day. That's half the running cost of its rival system.
Yet both systems have their place. Developers are fitting air source hot water cylinders paired with infrared heaters — like the Cürv system — in flats that are so well insulated that they need little heating, says Oliver Novakovic, technical and innovation director at Barratt Redrow.
'We're going to be putting in air source heat pumps in homes. The goal is that our customers would pay similar bills than they would on gas, to ensure that they wouldn't really have change in how they use the home,' Novakovic says. 'We are close to equalising on bills with gas boilers.'
But he adds: 'It's very important to install air source heat pumps and the services correctly to ensure they perform as it says on the tin.'