Heat pump mythbusters commentary - part 2: Do heat pumps work at freezing temperatures?

Domestic heat pump in back garden, with pots and fence in the background

In parallel with The Guardian’s mini-series, Heat Pump Mythbusters, we’ll be sharing a running commentary on each article and our additional thoughts on the opportunities and challenges of electrifying heat in the UK.

Article 2: do heat pumps work at freezing temperatures?

Article summary

1. There is a widely held belief that heat pumps cannot adequately heat a home in the winter temperatures experienced in the UK. However, other countries such as Norway, experience much more extreme and prolonged cold periods than the UK and heat pumps are prevalent in these parts of the world.

2. Heat pumps utilise fluids known as refrigerants to extract heat from a source (typically air) and deliver it to where it is required. It is the physical properties of these refrigerants, specifically their low boiling point, which allows them to do this even in extremely cold temperatures. The Guardian have produced a very neat interactive infographic, explaining this in further detail.

3. Heat pumps produce heat more efficiently than fossil fuel boilers, even at -20°C external temperature, and become increasingly efficient with increasing external temperature.

4. The quality of an installation is vital to the success and efficiency of a heat pump heating system.

Read the full article here.


Additional commentary 

1. Not mentioned in the article but relevant to the efficiency of all heating systems (not just heat pumps) is adjusting the temperature of the heating water produced in line with the weather – this is known as weather compensation. This means that the heat source works less hard on warmer days, when the heating demand is lower and therefore the water temperature produced can be lower. With heat pumps, this is typically controlled automatically based on the external temperature, however it is vital that this is set up correctly by an installer and/or user, otherwise the energy performance across the spread of a year will be lost.

2. There is a common misconception that radiators need to feel hot to the touch to be providing warmth to a room – this is not the case. So long as the radiator is warmer than the room it is in, it will be emitting heat energy to that room. There is a balance to strike between heating water temperature and heat emitter size. With smaller heat emitters, higher water temperatures are required to deliver the same energy into a room as larger heat emitters running at a lower water temperature. Higher water temperatures will require more energy to produce. This is why it is important when installing a heat pump system to also calculate the heat losses for each room accurately and then select your heat emitters accordingly. Previously these may been sized for a flow temperature of 60-80°C from a gas boiler. Heat pump systems will work most efficiently at flow temperatures of 25-35°C, however they are still efficient at higher flow temperatures. In my poorly insulated 1900s terraced house, the typical weather-compensated flow temperature of my heat pump over winter has been around 30°C.

3. There is a claim in the article that heat pumps can only heat water to 54°C. This may have been true of a previous generation of heat pumps, but new heat pumps using refrigerants such as propane (R290) or CO2 (R744) can easily generate water above 60°C. Additionally, these 'natural' refrigerants are considerably less harmful to the environment than refrigerants such as R410A and R32, which are progressively being phased out in Europe and are also likely to be phased out in the UK.

4. There is a further claim in the article that the Health and Safety Executive recommends a hot water storage temperature of 60°C. This is true in commercial buildings, with extensive distribution systems, but is not necessarily required in all domestic settings. Heat Geek – an organisation set up to improve knowledge and installation quality in the domestic heating industry - has produced extensive advice on this topic. Personally, I store my hot water at 48°C, above legionella grow temperatures, which allows the heat pump to run more efficiently than if stored at higher temperatures, however, individuals should consider what would be suitable in their specific situation.

5. Backup heating systems, such as fossil fuel boilers, are referenced as sometimes being needed in Norway. However, it should be stated that these should not generally be required in domestic properties in the UK, and designing a 'hybrid' system which combines heat pumps and boilers is likely to cost the consumer more to install, maintain and run.

6. There is a passing reference to ground source heat pumps which can be more efficient than air source heat pumps. However, ground source heat pumps are likely to be more expensive to install due to the requirement for pipework to collect heat from the ground and so are less suitable than air source heat pumps for most domestic properties in the UK.


Simple diagram showing heat pump freezing temperatures.
Photo of control screen for heat pump controller.

Heat pump controller at -6.5°C