Jonathan Shutt

What is a heat pump and how much does it cost?

4 min Read
House lit up from the inside at night

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?

Our 16kW pump - many are much smaller than this

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!

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How much does it cost to run?

In our experience so far, our heat pump has cost us slightly less than heating our home with gas. Though a heat pump is several times more efficient than a gas boiler, electricity usually costs 3-4 times more than gas. So while we use less energy to heat our home, our bills usually work out to about the same over the course of the year. 

Of course, energy bills will be much higher in winter. The chart below shows how January can account for nearly 20% of the annual costs of running the heat pump, but July and August were less than 1%. So your bills could be £200 in January, but less than £10 in summer. 

With permission from Grant.

Tips for bringing down energy bills

Heat pumps are most efficient when outputting lower temperatures than gas boilers. This doesn’t mean the house is colder; it just means the radiators are warm all day instead of pumping out high heat for a few hours at a time. 

Radiators with bigger surface areas, or even better - underfloor heating - are great if you have a heat pump. The added benefit of this is the house stays warm all day, and the radiators are much safer to touch.

Our one-year-old inspecting the safe 40°C towel rail

Generating and storing your own electricity is another great way to bring costs and carbon down. Even in February in Scotland, our solar panels are covering more than 20% of our electricity on some days. As we move into spring, this will climb steadily higher. 

Combining solar panels with home battery storage will reduce the amount of energy you pull from the grid, and can reduce electricity bills to almost nothing during the summer months. You can also sign up to an electricity tariff with a cheap night rate and charge these batteries when the price is lower.

Doing so also helps the grid by reducing the amount of power used at peak times (usually 4-7pm), when the dirtiest power stations get used to fulfil demand.

Our house getting its own rooftop power station installed

Does it work?

'It’s been proven that heat pumps don’t work in the UK'

This is a comment I saw written under an article about heat pumps recently. I can now ‘prove’ this commentator was completely wrong! Unless I’m mistaken, I’m writing this on a stormy February day, in Scotland, in an old house, heated purely by an air source heat pump, and I’m feeling toasty warm and happy our house no longer needs to be run off fossil fuels!

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