Upated 14th April 2022 including ‘Summary’ and new section ‘Cost-effectiveness of fabric/ renovation measures to deal with peak heat demand’
- Consider all retrofit options
- insulation [both simple (loft) and deeper (e.g. external wall)], draught-proofing, moisture management, and last but not least, a heat pump.
- a householder will in most cases need to make choices: don’t let the perfect be the enemy of the good
- two identical homes may come to different decisions – there is no single ‘right’ answer
- ignore anyone who says “you need deep retrofit before considering a heat pump” (the essay includes fully referenced debunking of this assertion, but it is widely believed and repeated ad nauseum)
- Be clear about your priorities (comfort, costs, climate)
- comfort is important, but it is subjective. MCS (Microgeneration Certification Scheme) has standards for target temperature in homes (21°C in living spaces, 18°C for halls and bedrooms). The term ‘comfort’ does not necessarily justify exceeding this standard.
- capital costs and running costs both need to be considered – fluid and escalating gas prices are a major issue, whereas electricity can come from many sources (wind, solar, nuclear, tidal, etc.), so is future proofed.
- if climate is your priority, be aware that timing is key, and the UK and other countries need to decarbonise heating, transport, etc. by 2050
- in terms of domestic heating, getting off gas is the single best thing you can do, and because heat pumps are so efficient, heat pumps deliver the greatest carbon savings per capital investment by a very large margin of all retrofit measures, without necessarily an increase in running costs!
- Maybe don’t rip out a NEW kitchen or NEW gas boiler
- so if you spent £20,000 on a new kitchen 5 years ago, and are now told that the back wall needs insulating, and can’t be done externally for various reasons, maybe this option is not in play.
- if you have a new gas boiler, check it is operating at optimal efficiency (that it is condensing and is running at lowest possible flow temperature to meet heat demand), thus reducing bills while maintaining comfort; and maybe deferring decision to switch to a heat pump. The Heating Hub offer ideas and support on optimising existing gas boilers, along with many other topics.
- Decide on budgets/timescales
- even with grants, household expenditure may be highly constrained
- consider the disruption as well as costs of different measures, and a realistic plan
- fabric measures can take several (or many) years to complete (when living with the work).
- decide on maximum budget and timescale for all measures
- Do as much fabric as budget allows
- be aware that deep (fabric) retrofit could exceed cost of heat pump by factor of 3 ot 4
- prioritise the “must do” ‘bangs for bucks’ measures such as draught proofing and loft insulation that are relatively cheap and with very high payback
- going deeper is where the householder must make a balanced (dare I say “pragmatic”) decision.
- Leave some money in budget for an air-source heat pump (ASHP), if you want one
- ignore myths like “heat pumps can’t heat old buildings” or “they don’t work when its cold” (see here)
- since an ASHP is much more affordable than alternatives (ground or water), it will be the default heat pump option (for those that are not in flats that may alternatively be connected to a district heating system, which itself can be ‘powered’ using a commercial-scale water-source heat pump).
- if you are not planning ‘deep retrofit’ there are limited risks from modest ‘oversizing’ of an ASHP if installed before all insulation measures are complete (as a modern ASHP can already handle seasonal variations in demand); but discuss with expert installer.
- you can get an ASHP early in your retrofit journey, if climate is your priority (and increasingly, running costs also); with no regrets!
If you are confused about what to do about retrofit, you are probably not alone. There is so much mixed and conflicting messaging. Often statements are made in the media that are untrue and go unchallenged.
Some experts say we need to insulate our homes so well they will hardly need any heating! Others say we need to get off gas as fast as possible by installing heat pumps.
Who is right?
Part of the confusion is that commentators can have different objectives in mind when expressing their opinions:
- To reduce household bills;
- To improve comfort;
- To reduce reliance on gas;
- To lower risks to future bills, from volatile gas markets;
- To reduce the carbon footprint of heating.
Or some combination of these. But these assumptions are often not made clear, and homeowners can be led down different paths depending on who they talk to.
Now, in the face of the climate emergency, everyone is saying that the last of these is something they care deeply about, but the pathway to getting to net zero in heating is something that is hotly debated.
We don’t have much time to get this right, and as Voltaire once noted, the best should not be the enemy of the good. We need a pragmatic way forward.
Energy Performance Certificates
Householders will often be further confused when they look at the Energy Performance Certificate (EPC) of their home or one they want to buy. EPCs are increasingly seen as unfit for purpose in the effort to decarbonise heating. The Country Land and Business Association (CLA) stated (as quoted in an Historic England report from 2018).:
“The EPC confounds cost-effectiveness, energy efficiency and environmental performance, giving an inadequate estimate of all three. … it must focus solely on one of .. [to] be an effective baseline for policy interventions”
An EPC in its current form has never recommended a heat pump as a primary measure, because of in-built biases against heat pumps. If we really want to encourage ‘whole house’ retrofit that includes a sufficiency of insulation work and displacing gas (or oil or LPG) boilers with heat pumps, we will need instruments that are fit for purpose (see Updates A.)
So what to do?
Householders will naturally ask: How much will it cost? How fast can it be done? Who can I get to advise me? What is the carbon reduction? Who can do the work to a good standard?
Is ‘deep retrofit’ required?
The Committee on Climate Change (CCC) in their 6th Carbon Budget stated (based on very detailed modelling of scenarios, costs and risks):
‘By 2030 37% of public and commercial heat demand is met by low-carbon sources. Of this low-carbon heat demand 65% is met by heat pumps, 32% district heating and 3% biomass. By 2050 all heat demand is met by low-carbon sources of which 52% is heat pumps, 42% is district heat, 5% is hydrogen boilers and around 1% is new direct electric heating.’
for their ‘balanced pathway’, and they did not assume deep levels of retrofit (p.113):
‘Energy efficiency and behavioural measures in our Balanced Pathway deliver a 12% reduction in heat demand to 2050’,
which implies quite modest fabric retrofit. This, on average, requires an estimated budget (see p. 297) of just £10,000 per household. This is far below what is the estimated ‘deep retrofit’ budget of nearly £40,000 .
The CCC are clearly working on the basis of pragmatic or sufficient levels of insulation and other fabric measures, not ‘deep’ retrofit.
The Retrofit Academy is devoted to training to improve the quality of assessments and implementation of ‘fabric’ measures (insulation, air quality, etc.), which is to be applauded. It is however concerning that they essentially marginalise heat pumps :
“Deep extensive retrofit and fabric first approach needs to be the main focus of reducing carbon emissions before we will be able to move to low carbon heating technologies 100%”.
There is clearly a problem here, as this is not an isolated opinion.
The ‘retrofit community’ generally have established an article of faith that ‘deep retrofit’ is essential. This is a belief that has very deep roots and predates concerns about the climate emergency. Key organisations in the public and private sector promote this belief.
Their motivation is to create greater comfort in homes and to lower heating bills, and who can argue with this?
The problem is that it isn’t a realistic strategy for reaching net zero in the fastest time possible .
The benefits in financial terms for householders do not favour a deep retrofit approach , but suggests that buyers do value heat pumps .
The Retrofit Academy justify their position on heat pumps based on the belief that that the grid cannot cope.
This is the same kind of argument that is often used for why we can’t adopt Electric Vehicles (EVs): because there aren’t enough charging points. On that basis we’d never have replaced horse-drawn carriages with petrol cars, or indeed any technology that displaces an old technology. In all such cases, the infrastructure is developed in parallel with the adoption of the technology in use. You don’t wait till you have a fully developed charging network and beefed-up electricity grid (particularly at its periphery) before you start selling EVs.
The electrification of much of our energy use is an inevitable strategic transformation of the energy system for many reasons, not least of which is the end-use efficiency improvements that technologies like EVs and heat pumps deliver. The other strategic game changer is that the end-use of energy does not care where the electricity comes from: a wind farm in the North Sea; the solar PV on a householder’s roof; a community solar scheme; a nuclear power station; or even, fusion energy (if it ever becomes a commercial reality). Electrification completely future proofs our energy system (even those parts of the economy like Aviation that need ‘chemistry’ to decarbonise, can get synthetic fuels from renewable electricity).
As for the grid, the issue has been overstated. There will be some strengthening of the grid required but a whole host of measures mitigate peaks in demand, including energy storage (at multiple scales), demand shifting, smart metering, etc. These will ensure that the grid can readily cope with future demand. No one is expecting that we have a 100% switch to heat pumps overnight, any more than petrol cars replaced horse-drawn carriages overnight. It is a multi-track transformation of energy generation, distribution and use. Local generation can have a remearkable impact on the scaling up of renewables as discussed here.
A Net Zero Toolkit for Retrofit
Retrofit assessors need to take an holistic and pragmatic view of the problem of decarbonising heating.
The ‘Net Zero Toolkit’  is an encouraging document because it takes an approach which is very much along these lines. This document reiterates what PAS2035 is trying to achieve:
‘PAS 2035 follows two core principles:
- A ‘fabric first’ approach to reduce the heat demand of a building as much as possible and to ensure newly airtight homes are well ventilated and avoid issues with damp and humidity.
- A ‘whole house approach to retrofit’ to ensure retrofit plans for homes consider improvements to the fabric, services and renewable energy generation in a coherent way to minimise both risks and carbon emissions.‘
In other words, we need to consider fabric measures and getting off gas (or other fossil fuels) in parallel.
It also takes a ‘risk’ based approach, recommending that assessors consider the possible hurdles not only the benefits of different courses of action.
For a 90m² home (the average floor area for UK houses) the ‘Net Zero Toolkit’ provides costing for a both ‘shallow’ and ‘deep’ retrofit. Including all the potential measures it comes to a total cost of £14,770 for ‘shallow’ and £54,220 for ‘deep’ retrofit. But a heat pump is only included in the ‘deep’ retrofit case, so this is still pursuing the view that deep fabric measures are required before including a heat pump.
Leaving heat pumps till later, after the retrofit budget has potentially been blown on fabric measures, is not the answer. So while the ‘Net Zero Toolkit’ is a great improvement on the apparent Retrofit Academy position, it could go further.
In terms of actual measures recommended, I feel it still falls short of recognising that heat pumps need to be included much earlier in the conversation.
If we include only those measures related to ‘fabric’ (i.e. exclude heating systems and solar energy) the costs are reduced to £10,970 and £38,720, respectively.
How many 90m² floor area home owners have £38,000 to spend, and still have money and appetite left over to do the heat pump project?
‘Fabric first’ can easily become ‘Fabric only’ on this path.
We still have a lack of recognition of the urgency of getting off gas.
What does the Government say?
The Department of Business, Energy and Industrial Strategy (BEIS) in a recent study have findings that completely contradict the position of the Retrofit Academy. BEIS conclude:
‘This project shows that Great Britain’s homes can convert to electric heating at a cost far lower than the accepted wisdom. This can be achieved with no threat to comfort, and greenhouse gas emissions will fall very dramatically as a result.’
In answer to the question on what should be ‘the balance of heating technologies to insulation measures’ they conclude:
‘The work focused on total costs of ownership over 15 years. For most house types and most electric heating systems, the cost-optimal packages of measures have very limited fabric improvements – most commonly just draught-sealing and top-up loft insulation. High-cost improvements, like internal or external wall insulation, hardly ever repay the capital costs over 15 years.’
This is in part why the Government and Climate Change Committee are following a pragmatic approach and see a combination of heat pumps and district heating as cornerstones of heating decarbonisation.
Cost-effectiveness of fabric/ renovation measures to deal with peak heat demand
This essay is focused on decision making at the householder level, not at national system level, but since some concerns has been raised regarding peak (electricity) demand for space heating in winter, I have added this section to look at the cost dimension. There is research published since I first wrote this essay, that analyses the relative cost effectiveness of fabric measures in dealing with peak heat demand at a national level . The paper says:
“Geographically, the amount of saved space heat differs strongly between countries (see figure 8). The strength of building renovation depends on the interplay between the costs of refurbishment and those for energy supply during the heating season. … Countries with a large share of wind generation, such as Great Britain, Denmark or Portugal, have cheaper electricity in winter and therefore a lower [requirement for] renovation as a result.”
In the UK, and assuming the distribution is allowed to strengthen (why wouldn’t it be, but conservatively with transmission grid is as it is today), then only a 10% reduction in heat demand using renovation/ fabric measures is cost effective. This is a suprising result, but arisies from the UK’s very significant wind assets and future potential, which correlates well with peak heat demand. It is similar to the number projected by the UK’s Climate Change Committee of 12% cited earlier.
I intend to write a separate essay ‘Peak Anxiety’, exploring the national system issue of peak electricity demand. Now I’ll return to the householder perspective.
Why heat pumps must be considered at the start of a retrofit conversation
If we focus on avoiding dangerous global warming, the single biggest thing a householder can do to reduce their carbon foot print is to install a heat pump.
Yes, it must be a fair transition and poorer families need help with grants or other measures to switch away from fossil fuels, but the direction of travel is clear.
I previously illustrated this (see here ), using data from the Energy Saving Trust, plotting the capital cost of different measures versus the carbon saving of those measures per year. I am including this graphic below.
Air-Source Heat Pumps (ASHPs) are now so efficient they compete very favourable with Ground-Source Heat Pumps (GSHPs), and at half the project cost, so we focus on ASHPs, which are likely to dominate the market .
An ASHP is the single best way for a householder to reduce their carbon footprint, by a long way.
A retrofit assessor may say,
‘Yes, but we have to consider comfort too. That bay window is poorly insulated so, whether it is a gas boiler or heat pump heating the home, sitting by the window will feel cool and only fabric measures can fix that’.
This is true and a householder needs to express their requirements clearly, and be presented with the options and costs. They can then judge which measures they ‘value’, in terms of the different criteria – comfort, capital costs, running costs and carbon reduction.
Different people with exactly the same situation may arrive at different conclusions.
But if they say that carbon saving is their number one priority, and secondly, they’d like to keep running costs similar, then a heat pump and modest fabric measures is an option that will score extremely well (or should do, if the assessment tools are fit for purpose).
There is another reason why we shouldn’t put all our eggs in the basket of insulation and other fabric measures. Recent research suggests that following such measures, there is a rebound effect and householders make behavioural changes (such as less clothing and higher thermostat settings) that can cancel out the carbon savings (see Update D).
Maybe, instead of the mantra ‘Fabric first’, we need ‘Efficiency first’, because it is that which delivers lower carbon emissions.
How do we deal with hard to treat homes?
The conversation often centres on old leaky homes, of which the UK famously has many. The Buildings Research Eastablishment (BRE) estimated a while ago that the UK had over 10 million ‘hard to treat’ homes (and there are nearly 30 million homes with gas boilers in the UK). About half of these buildings (about 5 million) were built before 1900.
These 10 million are often but not exclusively larger homes with high gas heating bills. So addressing the needs of this 1/3rd of the retrofit challenge would make a disproportionately large contribution to decarbonising heating in the UK.
But whether it is Roger Harrabin reporting on the BBC, or many others who count themselves as ‘green’, we hear it stated repeatedly (without reference to evidence) that householders must have high levels of retrofit before even considering a heat pump.
Some will even repeat the myth that you cannot heat old ‘leaky’ buildings with a heat pump. This is one of the myths that is addressed here.
Heat pump scepticism is wrong for several reasons:
- you can heat any building with a heat pump that can be heated with a gas boiler (you just need to size the heat pump and the emitters/ radiators correctly);
- with the efficiency of modern heat pumps and quite modest insulation, a heat pump can match or even reduce the running costs of the boiler it is replacing, as shown here and here;
- because the electricity grid is getting greener and greener every year, once a heat pump is installed the heating gets greener and greener with every year that follows (as illustrated in the graphic earlier).
But the questions remain: how do we deal with hard to treat homes? How much insulation do we do before we get rid of the old gas boiler?
The heat demand of a building is an important measure of its efficiency, but how do you compare the thermal efficiency of a large 6-bedroom detached house with a 3-bedroom semi? The fair way to do it is to divide the heat demand by the floor area of the house, which gives a measure – the heat demand per unit area – that is a universal measure of the ‘efficiency’ of the building’s fabric.
In the UK, the average home has an annual heat demand, using this measure, of about 130 kilowatthours (thermal energy) per square metre per annum (or 130 kWh/m².a for short). A new build, highly efficient ‘PassivHaus’ requires only 15 kWh/m².a. The Association of Environmentally Conscious Builders (AECB) have a target of 50 kWh/m².a when carrying out a (fabric) retrofit project, but they will relax this (e.g. for a Listed Building) to 100 kWh/m².a in some case, because some measures (like wall insulation) may prove impractical or impossible to include.
The implementation of retrofit on old buildings needs to be done with considerable experience and care, as a report by the Sustainable Traditional Buildings Alliance (STBA), in part sponsored by Historic England, explored.
Let’s start with a 90-100m² home with solid walls that is poorly insulated and ‘hard to treat’, and requires nearly 200 kWh/m².a to heat it currently with its gas boiler.
The following sequence considers a sequence of options (A-E) for when to install an Air-Source Heat Pump (ASHP), alongside increasing levels of ‘fabric’ retrofit measures. As we move from left to right on the bottom axis, fabric measures are added that reduce the heat demand of the building. That in turn will reduce the cost of the heat pump project.
Because we still need hot water and some heating, the drop in the cost of the heat pump project is less dramatic than the rise in the cost of the fabric measures, and there will be a cross-over point where the cumulative cost of the fabric measures is equal to the cost of installing a heat pump (at that level of building efficiency). Let’s run through the options.
A. Doing nothing on fabric or gas means bills will escalate
This is the start – the ‘do nothing’ option.
There is a serious risk that such a home will have lower resale value in the future, and will of course not contribute to lowering the carbon footprint of the home.
By starting to think about retrofit (including getting off gas), home owners might find themselves doing things they have put off for years, like clearing the loft (ready for insulation), and fixing that leaky front door.
B. Getting off gas early prioritises planet, without bills needing to rise
In this case, the householder installs an ASHP early in their retrofit journey, alongside limited fabric measures, such as loft insulation to modern standard, and seals / brushes for doors and sash windows to deal with drafts.
It may be a surprise to people that getting off gas early prioritises planet, without bills needing to rise. The reasons for this are:
- A 25 year old, 70% efficient gas boiler wastes energy, so the net cost of a unit of ‘heat energy’ delivered is greater than 3p (the nominal unit price for a kWh of gas in July 2021), so 3p/0.7 = 4.3p per kWh of heat delivered/ required.
- The nominal cost of electricity to run the heat pump (at July 2021 rates) is 15p per kWh. Taking a performance of 300% for a modern properly installed heat pump over the seasons, the householder would be paying 15p/3.0 = 5p per kWh of heat delivered.
- Assuming that the limited measures taken mean that heat demand reduces by 20% less then we would paying effectively 4p instead, which is lower more than old unit cost (4.3p)
- As levies on electricity move over to gas in coming years (as the Government has indicated), the running costs of the ASHP will lower further (and will rise for the gas).
- As the electricity grid gets greener and greener, so does the heat pump, without the householder having to do anything, so the carbon reductions delivered improve year on year.
- It is crucial that the house has a proper heat loss assessment done, and the heat pump is sized correctly, and that radiators are also assessed and upgraded where necessary on a room by room basis.
This refutes the belief that early adoption of a heat pump is a no-go area for hard to treat homes.
C. Further pragmatic fabric measures lower heat demand and bills
In this case a householder installs an ASHP, and in addition to limited fabric measures – loft insulation to modern standard, and seals / brushes for doors and sash windows to deal with drafts – installs:
- pragmatic window measures (replacing some windows with double or triple glazing but prioritising lower cost secondary glazing, particularly in conservation area), and;
- for one or two rooms, additional measures for cold walls or floors where possible, for comfort reasons if nothing else, and;
- might add localised mechanical ventilation and heat recovery (MVHR) for a specific room or two (kitchen and shower) to deal with condensation issues.
Alongside reducing bills, these fabric measures can deliver improved comfort (such as in key problem areas like bay windows).
D. More fabric measures reduce bills, but can delay getting off gas
The householder installs an ASHP late after extensive and often disruptive retrofit measures to many rooms, including double or triple glazed new windows throughout and insulation for some floors and walls, and extensive MVHR (Mechanical Ventilation and Heat Recovery) recommended to deal with moisture that would otherwise be trapped.
Older buildings are used to ‘breathing’ and that prevents the build up of moisture. As we greatly reduce leaks in these buildings, and add insulation, there are significant risks of harm to the traditional underlying fabric of the building due to moisture. Historic England has documented many cases where harm has been done in old buildings, and they recommend the use of breathable insulation materials to minimise such risks. Moisture can give rise to health issues if mould results.
That is why PAS2035/ PAS2030 aims to deliver improved skills in doing more extensive fabric retrofit. I am concerned that the skills required to effectively assess and implement these more extensive measures, and the costs, will deplete a house owners ‘retrofit budget’ to the extent that there is no money left to switch off gas and install a heat pump.
This is also problematic because a householder will rarely implement fabric measures in a single short-term project. In practice it can take many years to implement a wide range of measures; especially where householders are living with the work.
Often, debates on retrofit fail to take account of these real-world issues of limited budgets, extended timelines, and risks of poor delivery of deeper retrofit. Conversely, the challenges of fitting heat pumps are overstated by comparison. We need a much better balance in these debates.
E. Further fabric measures very difficult to justify
A householder installs an ASHP very late after an extensive and disruptive building project:
- Removing problematic fabric and replacing with energy efficient materials for walls (internal or external), floors and windows;
- Possibly going below ground floor level at walls to eliminate thermal bridging issues with floor insulation, and;
- Full external cladding of building, or internal wall insulation;
- Installs MVHR throughout the house.
These measures would greatly increase comfort and minimise bills. Heating requirements theoretically become minimal (although hot water would still be required, and specialised heat pumps dedicated to hot water are available).
However, in practice, such levels of fabric retrofit are not achievable for hard-to-treat homes at reasonable levels of cost and disruption. And for Britain’s housing stock, this is not achievable on a timescale commensurate with the climate emergency. This point seems to be lost on advocates for deep retrofit.
People talk about the lack of heat pump engineers, but I would argue that training these up is a relatively simple task when compared with the breadth of knowledge required to deal with a large range of historic and current building materials and how to use them in a way that avoids creating problems.
Pragmatic ‘save the planet’ Retrofit
So these are the householder options:
A) Doing nothing on fabric or gas means bills will escalate;
B) Getting off gas early prioritises planet, without bills needing to rise;
C) Further pragmatic fabric measures lower heat demand and bills;
D) More fabric measures reduce bills, but can delay getting off gas;
E) Further fabric measures very difficult to justify.
And for me, concerned about the urgency to limit dangerous global warming, options B or C are the pragmatic way forward in many cases.
‘Insulate Britain! Yes, but by how much?‘ House owners are asking.
‘By enough’ is the answer, and far less than is the received wisdom of those calling for ‘deep retrofit’.
It certainly needs to be at a level that leaves enough in the budget to get off burning fossil fuels. For many or most householders, that means installing an Air-Source Heat Pump .
Anything less is not treating the climate emergency with the urgency it requires.
(c) Richard W. Erskine, 2021 (updates added below, 14th January 2022)
 ‘Net Zero Carbon Toolkit’ by Levitt Bernstein, Elementa, Passivhaus Trust and Etude, www.cotswold.gov.uk/media/05couqdd/net-zero-carbon-toolkit.pdf
This toolkit was commissioned by West Oxfordshire, Cotswold and Forest of Dean District Councils, funded by the LGA Housing Advisers Programme. It is licensed under Creative Commons Licence 4.0 International (CC BY-NC-SA 4.0). Licence Deed: https://creativecommons.org/licenses/by-nc-sa/4.0/legalcode
 Guide to Heat Pumps, https://www.retrofitacademy.org/coe/wp-content/uploads/2021/04/Guide-to-Heat-Pumps.pdf
 Consider a householder who spent £25,000 on a new kitchen 7 years ago and is advised that they need to insulate the back wall of the kitchen and the floor. This would require the kitchen to be removed and expensive and disruptive work must be done to accomplish the work, even assuming the kitchen can be refitted. In practice, many of those who do attempt ‘deep’ retrofit do so only over an extended period rather than as a ‘big bang’ project.
 Lucien Cook of Savills was on BBC Radio 4’s ‘Your and Yours’ (8-11-21), quoting from research done by Savills, said that to get from EPC D to C, a householder would need to spend £6,500 but would only reduce energy bills by £180 per year (which would take 36 years to break even).
 Lawrence Bowles of Savills, commenting on research on valuations of homes:
‘By analysing average values of homes transacted between 2018 and 2020 we found that homes with newer, cleaner, methods of energy demand a much higher price tag. Across England and Wales, buyers purchasing a home with a heat pump fitted are paying on average 68 per cent more for the offer of cleaner energy.’
 As the Renewable Heat Incentive (RHI) ceases at the end of March 2022, with a much higher grant for GSHPs than ASHPs, to be replaced (it has been signalled) by an upfront grant with an expected marginal uplift for GSHPs, the likelihood is that the great majority of heat pump installations will be air source (even for the minority of home homeowners that have the land area for laying the slinkies required; and bore holes are even more costly and risky for a single householder to attempt).
 For those who live in flats or dense dwellings in towns and cities an ASHP may be problematic because of lack of space for a cylinder, for example (although small systems are being developed). For such case, and for office buildings, low carbon District Heating will often be the preferred alternative, as the Climate Change Committee recognises. But remember that District Heating refers to a heat distribution network, which still needs a heat source. The heat source may itself be a large scale heat pump, such as the water-source heat pump planned for Stroud District Council. Since towns and cities are typically close to rivers or the sea – carrying huge quantities of thermal energy – this is likely to be a popular approach that is already being implemented, to decarbonise heating in many urban settings.
 “Mitigating heat demand peaks in buildings in a highly renewable European energy system”, Elisabeth Zeyen, Veit Hagenmeyer, Tom Brown, https://doi.org/10.48550/arXiv.2012.01831
Supplementary material https://www.sciencedirect.com/science/article/pii/S036054422101032X#appsec1
Thanks also to Lisa Zeyen for private communications regarding these results, although I naturally suggest readers access the original work to get a full and complete understanding of the results. I hope I have not misrepresented them!
A. Published 8th November 2021: A Progress report on improving Energy Performance Certificates (EPCs) in the UK. This is positive news. The devil will be in the detail, of course, but encouraging.
B. The latest Government (BEIS) research concludes:
“Decarbonised electricity offers the promise of very low or even zero-carbon heating for homes – without necessarily carrying out extensive deep retrofit work. This project shows that Great Britain’s homes can convert to electric heating at a cost far lower than the accepted wisdom. This can be achieved with no threat to comfort, and greenhouse gas emissions will fall very dramatically as a result.”
and tellingly also concludes:
“The work focused on total costs of ownership over 15 years. For most house types and most electric heating systems, the cost-optimal packages of measures have very limited fabric improvements – most commonly just draught-sealing and top-up loft insulation. High-cost improvements, like internal or external wall insulation, hardly ever repay the capital costs over 15 years.”
Although some might argue with the 15 year time horizon, this is hardly a slam dunk for deep retrofit; quite the opposite.
C. On the interesting question of continuous versus intermittment heating when using a heat pump Nicola Terry (in “Will heating your house constantly use more energy?”, 12th January 2022) clearly comes down in favour of continuous heating (the main reason being the relative inefficiency when a heat pump has to heat a house from cold/ colder state).
D. ‘Insulation only provides short-term reduction in household gas consumption’, Cristina Penasco and Laura Diaz Anadon, 1st January 2023, https://www.cam.ac.uk/research/news/insulation-only-provides-short-term-reduction-in-household-gas-consumption-study-of-uk-housing
. . . o o O o o . . .