Are Air Source Heat Pumps (ASHPs) a Silver Bullet?

According to the Committee on Climate Change heating our homes makes up 40% of our energy use and 20% of our carbon footprint. While there have been dramatic improvements in building standards since 1970 there remains a legacy of poorly insulated homes.

Retrofitting our often old housing stock to reduce heat loss is crucial, but we also need to stop using natural gas as the source of heating if we are to have any chance of meeting our goal of halting global heating. 

It got me thinking about this question – if someone asked about retrofitting their house, and was motivated by the desire to reduce the carbon footprint of heating their home: 

what is the first thing they should do?

It may seem a somewhat artificial question, because in any real world situation, several measures are likely to be advisable, but bear with me.

Many retrofit professionals repeat the mantra “fabric first”, which means, focusing on insulating the building, dealing with leaks, and so forth. This sounds like good advice, given that the cost of some measures, like insulating a loft, are relatively cheap and deliver big savings in carbon emissions.

However, in many cases this is expressed in stronger terms, like “deep retrofit”, which can mean doing everything possible to reduce the heat loss of a building. This could include external wall insulation to homes with solid walls (which cannot benefit from cavity wall insulation), new windows, and dealing with associated issues related to moisture, for example. This school of thought suggests that we should only consider using a heat pump after deep retrofit is complete [Note 1]. 

The mantra “fabric first” then effectively turns into fabric only, because it is not difficult to exhaust a householder’s retrofit budget with changes to the fabric of a building. 

So why should we be considering heat pumps alongside changes to the fabric of a building? 

A heat pump harvests the ambient energy outside a house – either from the air, the ground or water. This ambient energy comes from the sun (when the ground is used as a source it is never deep enough to harvest energy from the core of the Earth, even with a bore hole, and is simply extracting energy from the ground that has been warmed by the sun and stored there). 

For every unit of electrical energy put in to drive the heat pump, it is able to deliver at least 3 units of heat energy into the home. A nice simple explanation of this process is provided here.

Professor David Mackay wrote in his seminal 2010 book Sustainable Energy without the hot air (p.151):

“Let me spell this out. Heat pumps are superior in efficiency to condensing boilers, even if the heat pumps are powered by electricity from a power station burning natural gas. … It’s not necessary to dig big holes in the garden and install underfloor heating to get the benefits of heat pumps”

He was calling for the adoption of Air Source Heat Pumps (ASHPs). He didn’t use the words ‘silver bullet’ but it is clear he was a big fan and frustrated at the low level of take-up. As he wrote 

“heat pumps are already widely used in continental Europe, but strangely rare in Britain”.  

I thought about how to present some information to help explore the question I have posed, and compare fabric related measures to an ASHP. I took data from the Energy Saving Trust website for a typical semi-detached house and plotted the capital cost of different interventions against the annual carbon saving that would result.

The capital costs are indicative and include the parts and labour required.

The only change I made to the Energy Saving Trust data was I reduced the savings for an Air Source Heat Pump (ASHP) from about 4.5 tonnes of CO₂ to about 3, because that better reflects the carbon intensity of the national grid in the UK in 2020.

I also indicate the level of disruption involved with colour coding, because this can be a factor in a householder’s decision making. The graph is based on the data tabulated in the Table at the end of this essay [Note 4] (click on image to see higher resolution):

This householder is aiming for the highest carbon saving, and there is only one answer glaring out at you from this graph: the ASHP.

If I change the question slightly we get a more nuanced answer:

what is the first thing they should do, based on carbon reduction ‘bangs for your buck’?

The ratio of the annual carbon saving to the capital cost is a measure of ‘bangs for your buck’.  On this criterion, it makes sense to do the low hanging fruit of loft insulation and fixing drafts, but then once again the ASHP scores very well (this allows for the fact that typically more than half the radiators will need to be upgraded [Note 2], and are included in the cost estimate).

Whereas external wall insulation would typically be similar in cost to an ASHP, but deliver only a third of the annual carbon saving and be a much more disruptive intervention; and in many cases not one that is practical to implement.

A counter argument would be that if we did manage (for a solid walled home) to do all the fabric related measures, we would achieve about 2,400 kgCO₂/yr carbon saving (over half the current emissions of 4,540 kgCO₂/yr) for an outlay of about £18,500; and then the ASHP could be added and would then ‘only’ need to deal with the remaining half, and that could mean that a lower capacity heat pump could be installed, reducing its cost somewhat.

Each situation will be different and depend on what interventions are possible. If a building is listed and external installation is prohibited (and the alternative of internal insulation dismissed), then the fabric related measures would then total 1,500 kgCO₂/yr, leaving two thirds of emissions to be dealt with. 

In either case, in order to maximise the emissions reduction one would require a heat pump.

My argument is not that you must fit a heat pump first, but that you should consider all the available options and to think about a plan (possibly over a number of years). 

I went on to plot another graph where I included the following additional features:

• To show the ‘best’ reasonable case fabric interventions (which would raise the cost of say, wall insulation, but at the same time increase the carbon saving). 
• To include a Ground Source Heat Pump (GSHP) option, either with horizontally laid slinkies, or using vertical bore hole(s).
• To show what happens as the UK electricity grid moves from 2020 carbon intensity levels to being 100% green.

The following illustrative graph was the result:

There are a number of interesting observations based on this graphic.

• Firstly, as the arrows show, we can increase the carbon savings for each fabric related measure, but these improvements come at extra cost. There will be some trade off for each householder situation as to how far they can go.

• Secondly, once you have a heat pump, the annual carbon saving will increase every year as the grid gets greener (as it has been in the UK), without the householder having to lift a finger.

• Thirdly, while a GSHP may have a better performance than an ASHP, it is likely to be quite limited [Note 3], and as Paul Kenny said in his talk to Carbon Coop ‘Heat Pumps – Learning and experiences from Ireland’, if you have extra money spare, why not do the easy thing and spend it on further upgrades to radiators, when you can achieve a target COP without the major disruption and risks associated with a GSHP project (assuming that is even an option).

• Fourthly, a borehole GSHP is even more costly, and more risky. There are significant risks associated with, for example, drilling into water tables, but the real killer is the cost. For a single householder it makes little sense. Of course, one can imagine scenarios where several houses could share the costs, but these are likely to be exceptional projects; not the basis for mass roll-out of heat pumps.

Some will argue that an ASHP requires supplementary heating during very cold spells in winter. However, in the same talk referred to above, Paul Kenny used data from a significant number of retrofits in Ireland that had ASHPs, using a design parameter of -3°C for cold winters. When the beast from the east came and these houses experienced -6°C, they were all fine and did not require supplementary heating. He wrote a piece on LinkedIn about this experience, which flies in the face of much of the ‘received wisdom’ in the retrofit community.

And in the UK, without the much larger grant that GSHPs enjoy, as compared to ASHPs, it is doubtful there would be anything other than a marginal role for GSHPs. It will be no surprise if ASHPs dominate the heat pump market in coming years and for some installers, this is already the case.

So, are Air Source Heat Pumps a silver bullet to decarbonising the heating of homes? 

One has to say in many ways they are!

But of course, in reality, it makes sense to consider them in the mix of other retrofit measures, and to carry out some improvements to the fabric of a building as part of a ‘whole house’ plan.

We just shouldn’t let the ‘deep retrofit’ mantra put people off considering an ASHP; maybe even as one of the first things you do.

(c) Richard W. Erskine, 2021

Notes

1. While I am a huge fan of PassivHaus and similar standards, we must remember that these standards cannot easily be applied to existing stock, and would be hugely expensive. 80% of the homes in 2050 already exist, so 80% of the problem of decarbonising heat in homes is already there; and BRE estimates there are 9 million ‘hard to treat’ homes in the UK.

2. Upgrading radiators is usually required to increase the effective surface area. This is needed because heat pumps operate at a lower flow temperature, and the heat delivered is a function of the temperature of the radiator and its surface area. The surface area can be increased by using 2 or more panels with fins sandwiched between them. This can also help reduce height and width of the radiator that would otherwise be necessary, while making the radiator somewhat deeper / fatter.

3. A GSHP has a better Coefficient of Performance (COP) in winter, an ASHP could do better in Spring and Autumn. The overall Seasonal COP for a GSHP will probably be higher but unlikely to be higher than 15%. We need real world studies to get a good figure here. But the cost of a GSHP using slinkies in 1.2m trenches (for those unusual cases where householders have sufficient land to achieve the area necessary) is something like double that for the ASHP. 

4. Table of Typical three bedroom, solid walled, semi-detached house