The belief that a heat pump is unable to heat a home in the dead of winter is widely held, particularly in the UK.
One recent survey of over 4,000 adults in the UK, Germany, France and the US from last autumn found that 35% believed that air source heat pumps were not suitable for this purpose.
The study, carried out by data company Electrify Research, found that more than 40% of Britons agreed that heat pumps do not solve the challenge, compared to 36% in Germany, 35% in the US and 26% in France.
The research report described the UK’s significantly higher skepticism as “ironic”, given that winter temperatures there are much milder than in Norway, where the average winter temperature is -7°C and heat pumps are installed in two-thirds of homes.
Claim
Critics of heat pumps are often very succinct: they just don’t work, but especially not when you need them most.
To understand whether they can work in sub-freezing temperatures, first consider how they work: just as a refrigerator absorbs heat from its interior and dissipates it through coils on the back, a heat pump absorbs heat from the outside air to raise the temperature of the water . can then be pumped through the radiators of the domestic heating system.
Critics argue that heat pumps don’t work as well during freezing weather because there is less heat energy in the air to absorb.
Willie Haughey, a Labor tycoon and refrigeration tycoon whose firm installs heat pumps, has repeatedly warned they are unsuitable for Scotland’s climate.
He told the Times last year: “The truth is that heat pumps in Scotland do not work as efficiently as in other countries. The water can only be heated to 54C, which is less than the Health and Safety Executive’s recommended 60C.”
He repeated his claim that heat pumps should not be considered an alternative heating source to gas in an interview with BBC Radio 4.
Many skeptics also work in the heating industry. One boiler fitter said: “They just don’t work. The water temperature is usually set to 40C, so it just can’t heat your house when it’s really cold outside. My son is doing heat pump installation accreditation but it’s not for me. I won’t touch them.” His views are typical of some in the industry, but are they backed by evidence?
Science
It may not be obvious, but even on freezing days there is still some thermal energy in the air that can be used. It helps to take a closer look at the science behind a heat pump.
First, they absorb heat energy from the air through an evaporator that contains a cold liquid refrigerant. Refrigerant boils and becomes a gas at extremely low temperatures, often around -20C, meaning that even if the outside air temperature is as low as -10C, it is still 10C warmer than the refrigerant.
The heat pump compressor then compresses the gas molecules together to raise the temperature of the gas. Pipes containing hot gas are used to heat cold water, which is then passed through the home’s heating system to radiators and underfloor heating pipes.
According to the Energy Systems Catapult (ESC), an independent research and technology organization set up by the UK government, heat pumps have to work harder in cold temperatures, but still perform well regardless of the weather.
In a nearly two-year study, the government-funded demonstration project analyzed the performance of 750 heat pumps on some of the country’s coldest days, when average daily temperatures fell as low as -6C. He found only a marginal decrease in the efficiency of the heat pump heating system.
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In the first mythbuster feature of this series, we learned that the metric used to rate a heat pump’s efficiency over a year is known as the Seasonal Coefficient of Performance (SCoP), which is typically 2.9 for an air source heat pump.
The ESC study found that the mean coefficient of performance (CoP) dropped to 2.44 on the coldest days of the year. This means that the heat pump would have to work harder to achieve the same indoor temperature when it is cold – but the difference would be marginal and is already included in the SCoP averaged over the year including the winter months.
The findings are supported by a separate study carried out by the Regulatory Assistance Project (RAP), which analyzed 550 households across Europe. It was found that even at temperatures of -20 C, the efficiency of the heat pump would drop to an average CoP of 2.
Notice
For most climates, heat pumps will work even in cold weather, but proper installation is essential. Ran Boydell, Associate Professor at Heriot-Watt University, said: “If a heat pump is not working effectively in cold weather, it is likely that the unit has been incorrectly specified for the location.”
Assuming you have a properly installed heat pump that is capable of heating your home until freezing, you should be aware that your running costs are likely to be higher during this period as the efficiency of the heat pump decreases.
Heat pumps can be cheaper to run than gas boilers in the UK, especially if you choose an electricity tariff designed for heat pump users. During periods when the temperature drops to -6C, running costs are likely to rise just above those of a gas boiler – but only for those periods. Over the course of the year – including typical cold periods – heat pump costs are still attractive compared to gas boilers.
Sometimes heat pumps need backup. In many Nordic countries it is common for heat pumps to have some sort of fossil fuel boost in case temperatures drop to -20°C or below. However, there are ways to reduce reliance on gas or oil heating even at these temperatures. For example, ground-to-water heat pumps can be more efficient than air-to-water heat pumps because the ground is often better able to retain heat during cold periods. A new generation air source heat pump, designed for cold climates, can also help reduce the need for backup heating.
Verdict
“Finally, we can put to rest the idea that heat pumps don’t work in cold weather and that their operation is inefficient. We observed the exact opposite,” said Marc Brown, Commercial Manager at ESC.
Science shows that heat pumps can work in winter temperatures. But bad advice and installations can overturn even the most encouraging scientific findings.
Richard Halsey, director of the ESC, said: “One of the key findings of our study is that proper design and installation are at the heart of delivering a heat pump that works for the household in which it is located.
“For most homes – regardless of property type or age – a well-installed heat pump with an output temperature of around 40C should result in a comfortable home most of the time. But there is no one-size-fits-all setting for a heat pump.”