By Paul Homewood

With the pressure on for the public to replace gas boilers with heat pumps, there has been a ramping up of puff pieces in the media. One in the Telegraph, for instance, suggested they were the best thing since sliced bread, with one chap saying they are cheaper to run and capable of keeping the house nice and warm.

Time then to review the costs, given the rise in energy costs this year.

To recap, this is how a heat pump works:

According to sites like Greenmatch, who are heavily promoting them, air source heat pumps (ASHP) will typically cost between £8000 and £18000 for a 2 to 3 bedroom house.  This excludes the cost of replacing radiators, extra insulation etc.

Because heat pumps use refrigerants to transfer heat, they can supply more heat energy than the input electricity. The measure of efficiency is known as COP:

We commonly hear the claim that ASHPs typically work to a COP of 3.0, but this is variable, dependent on the outside temperature. When it is mild, COPs of 4.0 can often be achieved; however you probably won’t even have the system switched on when it is so warm.

On the other hand, when it is freezing outside, the pump has to work much harder, and COP can easily drop well below 3.0.

Hence the need for the Seasonal Performance Factor (SPF), which measures the total useful heat that has been generated annually from the heat pump and then divided by the annual electricity consumption.

A survey by the Energy Saving Trust found an SPF average of 2.45, based on 44 homes spread geographically around Britain.

No extra insulation was fitted, other than a bit of pipe lagging and so on. The study was carried out in 2013, so more recent installations may be slightly more efficient. However, the installations  monitored in 2013 were deliberately engineered for best practice, including, for example, visits by manufacturers to check and advise on best practice.

It therefore seems unlikely that new installations now will be significantly more efficient.

GreenMatch, another site promoting heat pumps reckons that SPF would be 2.8, but this assumed a “well insulated house plus new radiators”.

So it seems reasonable to work on a figure of 2.6, without extra insulation.

So we can now start to work out running costs.

  • According to OFGEM, the latest price cap is set at 19.1p/KWh for electricity, and 3.8p /KWh for gas.
  • A typical house would use 15,000 kwh of gas a year, split 11,000 for heating and 4,000 for hot water.
  • Assuming an energy efficiency of 85%, a gas boiler would produce 9350 KWh of usable heat [11000 x 85%], at a gas cost of £418 pa [11000 x 3.8p].
  • In comparison, to produce the same 9350 KWh, a heat pump would use 3596 KWh of electricity, at a cost of £687.

Hot water poses an extra problem for the heat pump scenario, as a typical ASHP can only heat water to about 50C, well below the threshold of 60C, below which Legionella thrives.

There are various solutions, such as a separate immersion tank, or a top up system to superheat the ASHP water flow. But these and other alternatives will both cost money to install, and cost more to run than a gas boiler, as they would need electricity.

Even if all of the Environmental Levies were switched from electricity to gas, heat pumps would still be about £90 a year dearer to run.


October 21, 2021