When you understand the basic principle of a heat pump, and especially that of the air-to-water heat pump (the most widely used), you realize that the colder it is, the fewer calories there are to be recovered from the outside air, and therefore the less the heat pump is able to deliver the right heating power without the help of an electric booster.
What is extreme cold in our latitudes?
First of all, we need to agree on what constitutes extreme cold in France, Switzerland, Belgium - in short, in Western Europe, which is more or less continental, and for some regions more oceanic. But where it matters most is in continental climates.
Indeed, in these regions, the climate is known for its extremes (although less and less so as the years of this century go by). In winter, temperatures can reach between -5°C and -15°C, or -20°C even lower, but rarely. These include Switzerland, Alsace, Haute Savoie, all the Alps, Nord Pas de Calais, etc.
Extreme cold not only means sub-zero peak temperatures, it's also a phenomenon that lasts for some time. A week or several weeks, or in some regions several months.
Extreme cold would be -10°C without interruption for 1 month, for example, or -15°C for 2 weeks. It's a subjective definition, but it's what we generally imagine when we think about it. And then we think that our heat pump is going to struggle, and that our electricity bill is going to soar because the auxiliary heating element is going to kick in to maintain the set temperature.
In reality, we start talking about extreme cold (which happens every 30 years) when the temperature stays at -10°C for two weeks (look at this example in Switzerland in 2011/2012)
However, this is the kind of phenomenon our brains fantasize about. In fact, while there may be occasional downward spikes in temperature, the data to observe is the average temperature over the winter months. If it's -20°C over two days, the auxiliary heater will be switched on for two days, but the temperature will often rise again very quickly.
In fact, the current record for extreme cold in France, according to available records, is the winter from 1962 to 1963. And what's striking is that the temperature didn't even drop below 0°C on average. It averaged 0.7°C over December + January + February. Normally, that's around 5°C. Source
But rest assured, we're heading upwards, since in 2015 2016 we averaged 8°C over the winter.
How does the heat pump work in cold weather?
Air-to-water heat pumps are designed to capture energy from the calories in the outside air. The colder it is, the less energy there is, and the less energy they can release into the home's water system or domestic hot water.
Heat pumps are equipped with modern control systems that adjust the setpoint temperature. If the set temperature cannot be reached when the units are at maximum output, there are several solutions:
A setpoint shift is programmed in case of extreme cold. For example, if it's very cold, the owner can allow the room temperature to fall by up to 3°C below the set temperature. It's an ecological gesture (you take out your sweaters) and an economic one (you avoid consuming electricity via the auxiliary heater).
If this temperature drop is exceeded, an auxiliary resistor is switched on to provide the necessary heating energy for the house. So yes, the heat pump momentarily turns into a purely electric heating system.
This is not the reason to panic. In fact, as mentioned above, you need to look at the average temperature over the winter months. And understand how many times your heater will turn on over the months.
Let's say we're in the extreme case of an unbelievably cold 1 month every day, and your 3kW heating element has to be switched on every day, 24 hours a day. We're going to consume 31*24*3 = 2232kWh over the month, i.e. at 21cts per kWh, a total expenditure of 468CHF, spread over the 12 months of the year at around 40CHF per month. And this more or less every 30 years, as the climate warms up. In other years, the resistance will never be used overall, or only on an anecdotal basis.
Unless you make a mistake, if the resistance is ON by default...you're going to pick up the biggest bill of your life, before you figure out what happened.
The whole resistance thing only applies to a so-called monovalent system, i.e. one in which the heat pump provides all the heating in the house. Sometimes there are bivalent systems, in which a boiler or stove is used as a back-up to make up for the heat pump's lack of power when it's really cold.
Generally speaking, heat pumps operate with correct coefficients of performance down to -7°C (for water heated to 35°, in the case of underfloor heating), below which they deteriorate.
Learn more about heat pumps with these articles:
About the author
Juliena mechanical engineering graduate and specialist in climate engineering since 2009, has become a writer specializing in renewable energies, with expertise in heat pumps and photovoltaic solar panels for individual housing.