The belief that a heat pump is unable to heat a home in the dead of winter is becoming widespread, especially in Britain.
A recent survey of more than 4,000 adults in Britain, Germany, France and the US last autumn found that 35% believed that air source heat pumps were not suitable for their work.
The survey, carried out by data company Electrify Research, found that more than 40% of Brits agreed that heat pumps are not up to the challenge, compared to 36% in Germany, 35% in the US and 26% in France.
The research report describes the significantly higher skepticism in Britain as ‘ironic’ as winter temperatures there are much milder than in Norway, where the average winter temperature is -7 degrees Celsius and two-thirds of homes have heat pumps installed.
The claim
Critics of heat pumps are often short and sweet: they simply don’t work, but certainly not when you need them most.
To understand whether they can perform in freezing temperatures, first consider how they work: Just as a refrigerator absorbs heat from the inside and expels it through coils in the back, a heat pump absorbs heat from the outside air to maintain the temperature of the water. to increase. can then be pumped through the radiators of a home’s heating system.
Critics claim that heat pumps don’t work as well during frost spells because there is less heat energy in the air to absorb.
Willie Haughey, a Labor peer and refrigeration magnate whose company carries out heat pump installations, has repeatedly warned that they are not suitable for Scotland’s climate.
Last year he told the Times: “The truth is that heat pumps in Scotland don’t work as efficiently as in other countries. The water can only be heated to 54 degrees Celsius, which is lower than the Health and Safety Executive’s recommended figure of 60 degrees Celsius.”
In an interview with BBC Radio 4, he reiterated his claim that heat pumps should not be considered an alternative heating source to gas.
There are also many skeptics in the heating industry. One boiler installer said: “They just don’t work. The water temperature is usually set at 40 degrees Celsius, so it cannot heat your house when it is very 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 supported by evidence?
The science
It may not be obvious, but even on frigid 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 a heat pump.
First, they absorb heat energy from the air through an evaporator containing a cold liquid refrigerant. The refrigerant boils and becomes a gas at extremely low temperatures, often around -20°C, meaning that even if the outside air temperature is as low as -10°C, it is still 10°C warmer than the refrigerant.
The heat pump’s compressor then pushes the gas molecules together to increase the temperature of the gas. Pipes containing the hot gas are used to heat cold water, which then runs through a home heating system to radiators and underfloor heating pipes.
Heat pumps have to work harder in low temperatures but still perform well regardless of weather conditions, according to the Energy Systems Catapult (ESC), an independent research and technology organization set up by the UK government.
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, where average daily temperatures dropped as low as -6ºC. Only a marginal decline in the efficiency of a heat pump heating system was found.
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In the first mythbuster part of this series we learned that the metric used to assess 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 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 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 conducted by the Regulatory Assistance Project (RAP), which analyzed 550 homes across Europe. It turned out that even at temperatures of -20°C the efficiency of a heat pump would drop to an average CoP of 2.
The notes
In most climates, heat pumps operate in cold weather, but proper installation is critical. Ran Boydell, associate professor at Heriot-Watt University, said: “If a heat pump does not perform 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 can heat your home to sub-zero temperatures, keep in mind that your running costs for this period will likely be higher as the heat pump’s efficiency 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 temperatures drop to -6ºC it is likely that running costs will rise just above that of a gas boiler – but only during these periods. Over the course of a normal year – including typical cold spells – the costs of heat pumps are still attractive compared to gas boilers.
Sometimes heat pumps need a backup. In many Scandinavian countries it is common for heat pumps to have some form of fossil fuel boost for when temperatures drop to -20 degrees Celsius or lower. But there are ways to reduce dependence on gas or oil heating even at these temperatures. For example, ground heat pumps can be more efficient than air heat pumps, because the ground can often retain heat better during cold periods. A new generation of air source heat pumps, designed for cold climates, can also help reduce the need for backup heating.
The verdict
“We can finally put behind us the idea that heat pumps don’t work in cold weather and that they are inefficient to use. We have observed exactly the opposite,” says Marc Brown, business manager at ESC.
Science shows that heat pumps can work in winter temperatures. But bad advice and poor installation can undermine even the most encouraging scientific findings.
Richard Halsey, director at the ESC, said: “One of the key findings from our research is that good design and installation are at the heart of delivering a heat pump that works for the home in which it is located.
“For most homes – regardless of home type or age – a properly installed heat pump that produces a supply temperature of around 40 degrees Celsius should usually provide a comfortable home. But there is no one-size-fits-all setup for a heat pump.”