What is a heat pump, and how can installing one reduce bills and carbon emissions? This is my experience of upgrading our draughty, cold Victorian house with an air source heat pump, and the effect it’s had on our family’s energy usage.
In 1859, just outside of Edinburgh, the Bursar of Kinleith Paper Mill built himself a brand new house. With open fireplaces in every room and thick solid sandstone walls, it was built to last. Over generations, it’s been modernised with an oil boiler, gas fires, and then gas central heating.
When we moved in 157 years after it was first built, it was time to modernise it again. We wanted to make it more efficient, cheaper to run, and most importantly, reduce its impact on the environment. So one of the things we decided to do was install an air source heat pump.
What is a heat pump?
A heat pump takes ambient heat (even when it’s below freezing) from the air, transfers it to ‘refrigerant’ gas, and compresses it. The gas gets significantly hotter as it’s compressed and the heat is then transferred into the house. This is exactly the same process that happens in a fridge or air conditioning unit, but in reverse.
If you have a bike and a pump, you can test this out yourself by pumping up the tyre really quickly - the valve can get quite hot.
Heat pumps are powered by electricity, but the actual heat created is from the ambient temperature, so they can put out 3+ units of heat from just 1 unit of electricity. This is in contrast to a traditional electric heater, which – at best – can output 1 unit of heat per unit of electricity. A good gas boiler may be 90% efficient, but a heat pump could be 300% efficient or more – this is how it reduces your energy usage!
Types of heat pump
One of the most common types of heat pump in the UK is ‘air-to-water’ (also known as an air source heat pump). This takes the ambient heat from the air and transfers it into water, where it flows through a standard central heating system.
Ground source heat pumps use long horizontal pipes or vertical boreholes to take heat from the ground. As the ground is about the same temperature all year round (in contrast to the air which fluctuates), ground source heat pumps can be more efficient but tend to be more expensive to install, and not all houses have the land area or suitable access for machinery. A horizontal system can require around 700 square meters.
Air-to-air heat pumps are very common in some countries such as New Zealand. They’re cheaper and easy to install, and can be used for heating or cooling. However, unless you install pumped air ducts, they only heat one room, so work best in open plan houses.
Water source heat pumps, as you can imagine, take heat from water, but they're not as common as most people don’t have a lake in their garden!