The belief that a heat pump cannot warm a home in the dead of winter is widespread, particularly 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% felt that air source heat pumps were not up to the job.
The study, carried out by data firm Electrify Research, found that more than 40% of Brits agreed that heat pumps were not up to the challenge, compared to 36% in Germany, 35% in the US and 26% in France.
The research report described the significantly higher level of 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 households have heat pumps installed.
The claim
Critics of heat pumps often get straight to the point: they simply don’t work, especially when you need them most.
To understand whether they work in subfreezing temperatures, first consider how they work: Just as a refrigerator absorbs heat from its interior and releases it via coils on its back, a heat pump absorbs heat from the outside air to keep it warm Raising the temperature of the water can then be pumped through the radiators of a home’s heating system.
Critics claim that heat pumps do not work as well during frost periods because less heat energy needs to be absorbed from the air.
Willie Haughey, a Labor peer and refrigeration tycoon whose company installs heat pumps, 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 are not as efficient as in other countries. Water can only be heated to 54°C, which is lower than the 60°C recommended by the Health and Safety Executive.”
In an interview with BBC Radio 4, he reiterated his claim that heat pumps should not be viewed as an alternative heating source to gas.
There are also many sceptics in the heating industry. One boiler installer said: “They just don’t work. The water temperature is usually set at 40°C, so if it’s very cold outside, your house just won’t heat. My son is doing the accreditation to install heat pumps, but that’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 freezing cold days there is still some heat energy in the air that can be harnessed. It’s helpful to take a closer look at the science behind a heat pump.
First, they absorb heat energy from the air via an evaporator that contains a cold liquid refrigerant. The coolant boils at extremely low temperatures, often around -20°C, and becomes a gas. This means that even if the outside temperature is -10°C, the air is still 10°C warmer than the coolant.
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 flows through a home heating system into radiators and underfloor heating pipes.
Heat pumps have to work harder in cold temperatures but work well in all weather conditions, according to the Energy Systems Catapult (ESC), an independent research and technology organisation run by the British 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, when average daily temperatures fell as low as -6°C. Only a slight decrease in the efficiency of a heat pump heating system was found.
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In the first Mythbuster article in this series, we learned that the measure used to evaluate 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) 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 in cold weather – 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 by the Regulatory Assistance Project (RAP), which analyzed 550 households 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 reservations
In most climates, heat pumps will work in cold weather, but proper installation is critical. Ran Boydell, associate professor at Heriot-Watt University, said: “If a heat pump is not working properly in cold weather, the unit has probably been incorrectly specified for the location.”
However, assuming you have a properly installed heat pump that can heat your home even in sub-zero temperatures, you should be aware that your running costs will likely 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, particularly if you choose an electricity tariff tailored to heat pump users. At times when the temperature drops to -6°C, it is likely that the running costs will be just above those of a gas boiler – but only during these periods. Over the course of a normal year – including typical cold spells – the cost of heat pumps is still attractive compared to gas boilers.
Sometimes heat pumps actually need a backup. In many Nordic countries it is common for heat pumps to be supported by fossil fuels at temperatures of -20°C or below. But even at these temperatures, there are ways to reduce your dependence on gas or oil heating. Geothermal heat pumps, for example, can be more efficient than air source heat pumps because the ground can often store 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 judgment
“We can finally dispel the idea that heat pumps don’t work in cold weather and that their operation is inefficient. We observed exactly the opposite,” said Marc Brown, managing director at ESC.
Science shows that heat pumps can work even in winter temperatures. But poor advice and installation can undermine even the most encouraging scientific findings.
Richard Halsey, director of the ESC, said: “One of the key findings from our study is that correct design and installation is vital to delivering a heat pump that works for the home in which it is located.”
“For most homes – regardless of building type or age – a well-installed heat pump that produces a flow temperature of around 40 °C should still provide a comfortable home. But there is no one-size-fits-all solution for a heat pump.”