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Do heat pumps work at freezing temperatures?
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Do heat pumps work at freezing temperatures?

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

One recent survey of more than 4,000 adults across the UK, Germany, France and the US last autumn found that 35% believed that air-source heat pumps are not up to the job.

The study, undertaken by the data company Electrify Research, found that more than 40% of Britons agreed 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 UK’s markedly higher scepticism as “ironic” given that its winter temperatures are far milder than 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 do not work, but especially not when you need them most.

To understand whether they can perform in freezing temperatures, first consider how they work: just like a fridge 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 water which can then be pumped through the radiators of a home’s heating system.

Air source heat pumpView image in fullscreen

Critics claim heat pumps do not work as well during freezing periods because there is less heat energy in the air to absorb.

Willie Haughey, a Labour peer and refrigeration tycoon, whose business carries out heat pump installations, has repeatedly warned that they are not suitable for the Scottish climate.

He told the Times last year: “The truth of the matter is that heat pumps don’t work as efficiently 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.”

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

There are plenty of sceptics working in the heating industry, too. One boiler installer 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 the accreditation to install heat pumps, 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?

The science

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

First, they absorb heat energy from the air via an evaporator which contains a cold liquid refrigerant. The refrigerant boils to become a gas at extremely low temperatures, often about -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’s compressor then pushes the gas molecules together to raise the temperature of the gas. Pipes containing the hot gas are used to heat cold water, which then runs through a home heating system into radiators and underfloor heating pipes.

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

In a study spanning almost two years, the government-funded demonstration project analysed the performance of 750 heat pumps on some of the country’s coldest days, where mean daily temperatures fell to as low as -6C. It found only a marginal decline in the efficiency of a heat pump heating system.

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Heat pump installed at the wall of a house

In the first mythbuster feature of this series we learned that the measure used to rate the efficiency of a heat pump 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 median coefficient of performance (CoP) fell to 2.44 on the coldest days of the year. This means that a heat pump would need 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 wintry months.

The findings are backed up by a separate study undertaken by the Regulatory Assistance Project (RAP) which analysed 550 homes across Europe. It found that even at temperatures of -20C a heat pump’s efficiency would fall to an average CoP of 2.

The caveats

For most climates heat pumps will work in cold weather, but correct installation is vital. Ran Boydell, an associate professor at Heriot-Watt University, said: “If a heat pump doesn’t perform effectively in cold weather, chances are the unit was incorrectly specified for the location.”

Assuming you have a properly installed heat pump which is able to heat your home through freezing temperatures, one caveat to note is that your running costs for this period are likely to be higher as the efficiency of the heat pump falls.

Heat pump technicianView image in fullscreen

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 dips to -6C it is likely that the running costs would rise to just above those of a gas boiler – but only for these periods. Over the course of a normal year – including typical cold spells – heat pump costs are still attractive when compared with gas boilers.

Sometimes heat pumps do need backup. In many Nordic countries it is common for heat pumps to have some kind of fossil fuel reinforcement for when temperatures plunge 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 a cold spell. A new generation of air-source heat pump, designed for cold climates, can also help to reduce the need for backup heating.

The verdict

“We can finally put to bed the notion that heat pumps do not work in cold weather conditions and that they are inefficient to run. We’ve observed the exact opposite,” said Marc Brown, a business leader at ESC.

The science shows that heat pumps can work at winter temperatures. But poor advice and installation can upend even the most encouraging scientific findings.

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

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

Source: theguardian.com