What’s the problem with heat pumps?

by | 2 Aug 2023

Air Source Heat Pumps are presented as the solution to decarbonise heating, but the debate so far has failed properly to take account of problems and limitations - and there are plenty

(I wrote yesterday that I’d do a piece today with a positive vision of how to sell environmental change to people. I’ll do that in my Common Weal newsletter piece on Friday. But there is one more technical thing I want to explain before I do that…)

Are Air Source Heat Pumps (ASHP) the magic bullet solution they’re being made out to be? And if not, what is the alternative? This question isn’t a simple one.

First of all, let’s look at the benefits. Heat pumps can be fitted to a single building, one of the very few clean heating sources which can be installed building-at-a-time. They are cheaper than direct electric heating (electric radiators) because they have a technology which means that for each kW of electricity you use, this will produce up to 4 kW of heat.

This means that it is significantly cheaper to heat your house with ASHP than electric radiators, but not necessarily than gas or even oil. And there is a trade-off – because ASHPs draw heat from the air, they are more efficient when air is warmer. Except that’s precisely the time when we need extra heat least.

The cost of an ASHP is significant – with installation it is going go come out between £10,000 and £20,000. A replacement gas boiler can be supplied and fitted for a tenth of that cost, meaning that any ongoing cost savings from lower heating bills are dwarfed by the capital cost.

There are other issues – most houses are designed around the fast, high heat levels provided by gas. ASHPs generally produce heat at a lower level for a longer period. That may work fine with your existing house set-up, but it may also mean that you need to spend on upgrading radiators.

It is more efficient if you install a thermal store (a large water tank that stores the heat), but that is an additional cost and takes up significant space. It isn’t practical in many houses. And the feasibility of ASHP also varies significantly by property type.

Basically, if you don’t take a ‘fabric first’ approach and ensure that your house is insulated and draughtproofed to the standard you expect to live with, you will need to install a much bigger and more expensive ASHP than you otherwise would have. ASHPs do not play nice with heat-leaky buildings which can leak heat faster than an ASHP can provide it (unless there is a thermal store).

So ASHP can work very well indeed in new-build housing and can be effectively retrofitted to more modern houses, but can struggle badly in older houses if they haven’t been brought up to modern heat loss standards. There are places where an ASHP is going to be nigh-on the only option so they will undoubtedly play a part in energy transition.

However, they aren’t just very capital-intensive once, they have limited lifespans and are prone to breaking down. Unlike a (much more efficient and reliable) Ground Source Heat Pump or a standard gas boiler, there are very significant large, fast-moving parts. It breaks down, you need an engineer, and it has a limited lifespan of around 15 years. It will then need to be replaced.

If we invest our limited resources in an unplanned system, a planned system becomes increasingly unviable

This is an important factor; while fitting a replacement may not have quite the same cost as the first install, it won’t be far away. If the replacement costs you £15k to buy and install, you are effectively paying a capital price of £1k every single year to maintain a ASHP. You may get a grant to install, but will you get a grant to replace?

And the install grant will still be likely to leave you with an additional bill of at least £5k and potentially as much as £10,000. Cost savings from lower energy bills become somewhat illusory when you look through the lifecycle. This is potentially very expensive heat indeed.

Nor is the implication of moving universally to ASHPs properly discussed. They are large and intrusive. This is fine if you have a back garden, but on a block of flats you would have a large ASHP stuck out of the back on every flat. These are metal; like the aging air conditioning units which clutter alleys and backstreets in hot climates, they deteriorate aesthetically. The jumble of large boxes sticking out of the back of a tenement will not age well as they compete with each other to suck the same heat out of the same air.

The problem with the current roll-out is that the install of ASHP is being done in a piecemeal fashion – the duty has been placed on the householder individually and they are then engaged directly with a commercial provider. A proper energy audit might well conclude not to install an ASHP until insulation is complete, but a commercial supplier might simply specify an unnecessarily large heat pump.

The problem is that this massive expenditure (the cost of replacing gas heating with ASHPs Scotland-wide will cost in the order of £35 billion) is not being coordinated to achieve the best effect. A properly planned approach might use a different technology altogether – Common Weal strongly favours district heating systems which are more expensive to install at first but are then reliable for hundreds of years and produce much cheaper heat.

Unfortunately, if we invest our limited resources in an unplanned system, a planned system becomes increasingly unviable. We are at the very real risk of spending much more than we have to and getting much less for it.

This could easily result in a profligate waste of public resources that would make Fergusson Marine look like a minor issue, and it could land Scotland with 100 years of bad heating instead of taking a step back and assessing whether this is really the best option or not.

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