What temperature is a heat pump not effective?

Many homeowners in the UK may be wondering how a heat pump can warm their home using heat from outside when the outdoor temperature is cold. But heat pumps work in such a way that they can use the smallest amount of heat and compress it to produce more heat. This is the crucially important thing about how a heat pump works.

These appliances can contribute towards heating your home even when it’s down to  -20 ℃ outside, making them perfect for cold weather in countries such as the UK, Germany and other areas of Europe. However, there will be a temperature at which your heat pump begins to struggle and requires more electricity in order to heat your home. As the temperature drops, the heat pump will need to work harder, making it less efficient.

Generally, heat pumps are extremely efficient. Heat pump efficiency is measured in Coefficient of Performance (CoP). This value demonstrates the amount of input compared to the output. For example, if a ground source heat pump uses one kW of electrical energy to produce four kWs of heat, the CoP would be four. The higher the CoP rating, the more efficient your heat pump.

Your heat pump may not work as efficiently when the outdoor temperature falls to below  -5 ℃. When the temperature outside is around 7 ℃, the average heat pump should have a CoP of around 4.5, however this figure can drop to 2.3 when the temperature outside decreases to  -7 ℃. This shows how much the cold weather can affect the efficiency of the appliance.

However, in the UK, these very cold temperatures aren’t that common and, when it does reach those minus temperatures, it’s unlikely that it will remain that cold all day. For instance, the temperature may be  -6 ℃ in the early morning, but by 10 or 11 o’clock, this figure will likely have risen by a few degrees and may even be above 0 ℃. This means that you could wake up feeling a little chilly but as the day gets warmer, so will your home.

Some homeowners choose to install a ground source heat pump instead of an air source model. While this may be more expensive to install, the ground is generally warmer than the air and so your heat pump may run more efficiently and cost less per year than an air source heat pump. Not only does the ground temperature remain fairly constant but at around 15 metres deep, the temperature can be between eight and 11 ℃ at any time of the year.

Different heat pumps made by different manufacturers can vary in performance efficiency. Although the heat pump efficiency will drop as the air temperature drops to below zero, it’s worth noting that the heat pump won’t completely stop working at these temperatures, but can continue operating down to around  -20 ℃. Instead, it may not be able to bring your home up to the desired temperature and the building might feel a little colder than you would wish. However, it will still be able to heat your home, so you won’t be left with a freezing cold house. For example, the heat pump might only be able to heat your home to 18 ℃ instead of 21 ℃ on a very cold day. The shortfall, on extremely cold days, could, however, be covered by an auxiliary electrical heater or combi boiler as an example.

You need to be cautious when using emergency heat or a backup system as your heating costs can increase quickly. These settings should only really be used when the heat pump has reached its efficient design limit or stopped working altogether through a fault.

If you’re worried that a heat pump won’t be able to cope with very cold temperatures, you can install one alongside another heat supply, such as a boiler. The heat pump can be used throughout summer to cool your home and through most of the autumn and winter to heat it. However, on those days when it isn’t bringing your home quite up to the temperature you want, a boiler could do this job for you.