When does it make sense to switch to a heat pump?

Chris Mason on BBC News at Ten (tonight, on 9th August 2021, the day that the IPCC published the science part of their 6th Assessment Report) stated that (in relation to heat pumps):

“you need lots of insulation to make them work”.

This is completely false: you can heat any building with a heat pump that you can with a gas boiler. Why do BBC reporters keep repeating these myths?

In fact, and in the context of the IPCC report, if one’s main concern was the household carbon footprint, a heat pump would be the first thing anyone would do, as I showed here (given the diminishing carbon intensity of the UK’s electricity grid).

The counter argument commonly used is that the costs of running a heat pump (given today’s unit price for electricity) is unaffordable unless there is huge levels of insulation. Of course if we had a fair fight between gas and renewables, electricity prices would come down relative to gas.

But simple maths shows that even at current gas and electricity prices, if one were to replace an old 70% efficient gas boiler with a modern air-source heat pump (ASHP), then the running costs would not be any greater, with only modest insulation measures, as shown here.

A heat pump can heat any home that a gas boiler can. But it makes no sense to try to heat a barn – with a heat pump or a gas boiler! Insulation and draught reduction make sense – and help improve the comfort of a building – and so ‘fabric first’ is an important message. This leaves open the question: how much insulation a householder considers before they invest in a heat pump?

Depending on how an individual wants to spend their retrofit budget, there will be a cross-over point where acquiring an ASHP will trump any further increment in fabric spending. Adding fabric measures will reduce the size of the heat pump required, but only to a point, as there are some base costs for the system, and we need hot water whatever the state of the fabric.

There are small and large houses, well insulated ones and leaky ones. How do we make sense of the numbers?

A useful metric is the heat energy required to heat one square metre of a home per year (this is measured in kilowatthours per metre square per annum, or kWh/m².a). The average UK house – because our historic housing stock is quite leaky – requires about 130 kWh/m².a. The Association of Environmentally Conscious Builders (AECB) aims, when insulating homes, to reduce this measure to 50 kWh/m².a, although might accept as much as 100 kWh/m².a in the case of (say) a Listed Building. A new build Passivhaus aims to achieve just 15 kWh/m².a.

It is easy to work out what figure currently applies to your house.

Look at your annual energy bills. If heated by gas look at the kWh total for the year. Make a guesstimate for how much of this is space heating, say 80% in the current case. Now divide this figure by the floor area of the home. The question then to think – with the help of a retrofit assessor – how far you can reduce this number.

If you currently have poor loft insulation, then fixing this is relatively cheap and has great pay back. Similarly for cavity wall insulation, and for reducing draughts from doors and windows. You don’t need to rip out your sash windows and replace with double glazing; window brushes, and secondary glazing can make a great contribution with a modest investment. Pragmatism is often required, when assessing where you can get the ‘biggest bang for your buck’.

The other key idea is to think in terms not of ripping out things, but taking opportunities when they arise. So, if a new kitchen is being fitted, then why not use the opportunity to insulate that cold back wall, and maybe even consider underfloor insulation and heating? This is why retrofit can often best be seen as a journey to be followed over a number of years.

What follows is an illustrative schematic showing the balance between the money spent on fabric measures (solid line) and what would need to be spent on an ASHP system at a given level of heat demand. As the heat demand reduces (as a result of fabric spend), so does the cost of the ASHP system (including the heat pump and radiators). The schematic envisages a 4-bedroom semi-detached house with solid walls and poor insulation that is hard to treat, and starts (at the left hand side of schematic) with a terrible figure of 200 kWh/m².a [the numbers are illustrative only – each house is different]:

We then start to move from the left towards the right. Spending even modest money on fabric will mean that the size (and cost) of the heat pump system you might buy progressively reduces (e.g. there is a big drop if one moves from a cascade heat pump system to a single heat pump).

At some point, the marginal cost of incremental insulation will rise above the cost of an ASHP (when the solid line cross the dashed line). For example, replacing all the windows with double or triple glazing is a non-trivial expenditure.

And of course, to try to turn a leaky Victorian house into a Passivhaus makes little sense, so there are natural constraints in how far one goes, depending very much on house and site specific factors.

Some people may decide to adopt the ASHP early for a number of reasons (they need to replace an old gas boiler and care about the climate future, or they live in a house in a conservation area where measures like external wall insulation will not be accepted). They may live on a terrace and external wall insulation for one house without the whole terrace joining in, would meet a lot of resistance (including from the planning department). For whatever reason they make their choice, I call these, ASHP ‘eary adopters’.

On the other hand, they may live in a house that can absorb a lot of retrofit insulation measures – perhaps as new owners wanting to start with a ‘blankish’ canvas – and with the help of a retrofit assessor/ expert, strive to get to the AECB 50 kWh/m².a figure. Let’s suppose they don’t have constraints such as conservation issues to deal with. We might call them ASHP ‘late adopters’.

In practice, householders will be somewhere on a spectrum between these two example – in a decision spread zone. A whole set of factors may come into play in their decision making: wish to improve comfort, or reduce carbon emissions, or concern over gas prices in the future, to name just a few.

It therefore makes no sense to say “you need lots of insulation to make them work”.

No, you need “a sufficiency of insulation” to make the running costs “fit your expectations”, and everyone may arrive at different expectations.

But don’t try to heat a barn, with a gas boiler or a heat pump.

(c) Richard W. Erskine, 2021.


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2 responses to “When does it make sense to switch to a heat pump?

  1. Hi Richard

    Have you tried writing to the BBC – they are constantly getting everything wrong in relation to heat pumps and hydrogen.



    • I do from time to time, such as in 2007 complaining about Terry Wogan’s incessant garbage on climate and in that case did get a substantive response. But no change in behaviour, or giving in to Lord Lawson and Peter Lilley. They made it a personal mission to harangue senior people in the BBC (this done out of sight of any public scrutiny, just as the same malign forces are trying to stop action on climate such as decarbonising heating – same people, same MO, same lack of any democratic accountability for their actions).

      Then, the BBC claimed to give time to alternative voices on the science in the name of balance, and their guidelines. It took until 2011 for the BBC to publish a report by Professor Steve Jones which criticised the BBC for allowing ‘false balance’ http://downloads.bbc.co.uk/bbctrust/assets/files/pdf/our_work/science_impartiality/science_impartiality.pdf
      (skip to page 14 to read his report; the first 13 are the BBC trying to put a positive spin on things). Prof. Jones wrote:

      ‘I recommend that the BBC takes a less rigid view of “due impartiality” as it applies to science (in practice and not just in its guidelines) and takes into account the non‐contentious nature of some material and the need to avoid giving undue attention to marginal opinion.’

      On the climate science, the BBC is now in much better shape; excellent even. On thermodynamics, not so much. It feels they are now dependent on stretched resources, and senior reporters who are struggling to keep up (frankly), dealing with powerful lobby groups. On heating homes the BBC never misses an opportunity to mention hydrogen (even though the climate change committe says that in 2050, they’d expect only 11% of homes to use hydrogen, with the rest using heat pumps or on heat networks). They also cite long repeated myths on heat pumps, or conduct handwringing pieces about how hard it is to retrofit a house. B%^&cks frankly.

      It feels like groundhog day on how they deal with evidence versus opinion. And freebies to visit homes heated by hydrogen (all funded by fossil fuel interests) does not look like impartiality to me.

      So maybe I have just drafted my next letter to them. Thanks for the prod!


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