Do heat pumps work in sub-zero temperatures? | Heat pumps

The belief that a heat pump is unable to heat a home in the depths of winter is widespread, especially in the UK.

A recent survey of more than 4,000 adults in the UK, Germany, France and the US last autumn found that 35% thought air source heat pumps were not suitable for work.

The survey, carried out by data company Electrify Research, found that more than 40% of Britons agree that heat pumps are not up to the challenge, compared with 36% in Germany, 35% in the US and 26% in France.

The research report described the markedly higher skepticism in the UK as “ironic”, given that winter temperatures are much milder than those in Norway, where the average winter temperature is -7C and heat pumps are installed in two-thirds of homes .

The claim

Critics of heat pumps are often quite succinct: they just don’t work, but especially not when you need them most.

To find out if they can work in freezing temperatures, first consider how they work: just as a refrigerator absorbs heat from its interior and dissipates it through coils on its back, a heat pump absorbs heat from the outside air to raise the temperature of the water, which then this can be pumped through the radiators of the home’s heating system.

Heat pumps absorb heat from the outside air and transfer it to the water. Photo: KBImages/Alamy

Critics argue that heat pumps don’t work as well during periods of freezing temperatures because there is less heat energy in the air to absorb.

Willie Howie, a Labor and refrigeration tycoon whose business makes heat pump installations, has repeatedly warned that they are not suitable for the Scottish climate.

He told the Times last year: “The truth is heat pumps don’t work as effectively in Scotland as they do in other countries. The water can only be heated to 54C, which is lower than the Health and Safety Executive’s recommended figure of 60C.’

Heat pump temperature chart

He reiterated his claim that heat pumps should not be considered an alternative source of heating to gas in an interview with BBC Radio 4.

There are many skeptics working in the heating industry as well. 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 will not touch them.” His views are typical of some in the industry, but are they supported by evidence?

The science

It may not be obvious, but even on frosty days there is still some heat energy in the air that can be harnessed. It helps to take a closer look at the science behind the heat pump.

First, they absorb heat energy from the air through an evaporator that contains a cold liquid refrigerant. Refrigerant boils to become 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 pushes the gas molecules together to raise the temperature of the gas. The pipes containing the hot gas are used to heat cold water, which is then passed through a home heating system into radiators and underfloor heating pipes.

Heat pumps have to work harder in cold temperatures, but still perform well regardless of the weather, according to the Energy Systems Catapult (ESC), an independent research and technology organization set up by the UK government.

In a study spanning nearly two years, the government-funded demonstration project analyzed the performance of 750 heat pumps on some of the country’s coldest days, where average daily temperatures fell to -6C. He found only a slight drop in the efficiency of the heat pump heating system.

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Heat pumps have to work harder to achieve the same indoor temperature when it’s cold than when the weather is warm. Photo: michal-rojek/Getty Images/iStockphoto

In the first myth-busting feature of this series, we learned that the measure used to rate a heat pump’s performance 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 average coefficient of performance (CoP) dropped to 2.44 on the coldest days of the year. This means the heat pump will have to work harder to achieve the same indoor temperature when it’s cold – but the difference will be negligible and is already factored into the SCoP averaged over the year, including the winter months.

The findings are supported by a separate study undertaken by the Regulatory Assistance Project (RAP), which analyzed 550 homes across Europe. It was found that even at temperatures of -20C the efficiency of the heat pump would drop to an average CoP of 2.

The warnings

For most climates, heat pumps will work in cold weather, but proper installation is vital. Ran Boydell, associate professor at Heriot-Watt University, said: “If the heat pump is not working effectively in cold weather, it is likely that the device is mis-located.”

Assuming you have a properly installed heat pump that is capable of heating your home in sub-zero temperatures, one caveat to note is that your running costs over this period are likely to be higher as the efficiency of the heat pump it falls

Correct installation is vital for heat pumps. Photo: Tomas Zaida/Alamy

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 slightly above that of a gas boiler – but only for those periods. Over the course of a normal year – including the typical cold spells – the costs of heat pumps are still attractive compared to gas boilers.

Sometimes heat pumps need a backup. In many Nordic countries it is common for heat pumps to have some sort of fossil fuel boost when temperatures drop to -20C or below. But there are ways to reduce reliance on gas or oil heating even at these temperatures. Ground source heat pumps, for example, can be more efficient than air source heat pumps because the ground is often able to retain heat better during cold weather. A new generation air source heat pump designed for cold climates can also help reduce the need for backup heating.

The verdict

“We can finally let go of the idea that heat pumps don’t work in cold weather and that they are inefficient in operation. We observed the exact opposite,” said Mark Brown, business leader at ESC.

Science shows that heat pumps can work even in winter temperatures. But bad advice and installation can reverse even the most encouraging scientific findings.

Richard Halsey, director at ESC, said: “One of the key takeaways from our research is that proper design and installation are key to delivering a heat pump that works for the home it’s in.

“For most homes – regardless of property type or age – a well-installed heat pump producing a flow temperature of around 40C should still result in a comfortable home most of the time. But there is no one-size-fits-all setting for a heat pump.”

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