🔴 The whole pamphlet, Remaking Home Heating in the UK, is published on People & Nature, and can be downloaded for free here.
The UK’s heating system needs to be remade with a twin-track programme: electrification, and deep retrofit. Both tracks can and should be run over the course of 25 years: 2025-2050.
There is a wide political consensus in the UK around the need to electrify sources of home heat. Heat pumps are recognised as the crucial component of this. Other possible sources are district heat networks fed by heat pumps, and electric resistive heating (the least good option). (Hydrogen boilers are a dead end.)[1]
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| A plumber at work installing a circulation pump. Photo from AdobeStock |
All homes should have low-carbon heating by 2050. This has long been the recommendation of the Climate Change Committee (CCC), and it appears to be accepted by the government, at least in principle.
All these electrical sources of heat additionally need to be powered by zero-emissions sources of electricity. In this article, I focus not on grid electrification but on in-home modifications to the home heating system.
In my view, district heat is the best upstream source of low-carbon heat—fed by heat pumps powered by green electricity. Next best is heat pumps in individual homes. However, most of the current political and economic focus is around in-home heat pumps.
Figure 1 shows the typical elements of a home heating system. I distinguish between different quantities of energy need (N), use (U), and consumption (C).
(There is a note about how the terms energy need (N), energy use (U) and energy consumption (C) are being used here, in the Introduction to Remaking Home Heating in the UK.)
Figure 1. Elements of the home heating system
In yesterday’s post on Fuel Poverty and Leaky Homes (based on chapter 3 of Remaking Home Heating in the UK), we covered building fabric efficiency, and how that determines thermal energy needs (N). We saw that, according to LETI’s housing stock model, most homes in the UK would need to use 110-160 kWh/m2/year of thermal energy for space heating to maintain thermal comfort. The modelled average (mean) thermal energy need (N) for the whole UK mainland housing stock is given to be about 128 kWh/m2/year. There is plenty of scope to reduce this figure by improving the fabric efficiency of homes.
In this post, I look at what comes next, once a home is thermally uncomfortable and supplemental heating or cooling is required.
I begin with a brief description of the typical home heating system in the UK, insofar as the in-home equipment is concerned: gas boiler + central heating.
With regard to electrification, I provide an overview of heat pumps, focusing on their efficiency benefits when compared to gas boilers; I sketch an argument in favour of widespread deployment of district heat networks fed by heat pumps; and I briefly describe the benefits of electric storage radiators.
After that, I look at some typical central heating inefficiencies in UK homes and how they can be mitigated, and I look at the significant efficiency benefits of so-called “low flow temperature” central heating systems, and what those are.
Finally, I outline why I think firewood combustion in homes should be phased out over the course of the next decade.
When assessing the overall efficiency of supplemental heating, there are different “boundaries” that can be drawn around a home heating system (b). For example, you might only consider the heat source (i); or you can include different and varying parts of the heat distribution system (ii)—for example, in a wet system, water pump, pipes, radiators.[2]
Ideally, however, you want to include the whole system all together—all of (i) and all of (ii): heat source, and heat distribution together. Indeed, the efficiencies of the heat source and the distribution system often work in parallel to one another and interact—as is the case with the influence of flow temperature on the efficiency of condensing gas boilers and heat pumps.
A typical home heating system
In the UK, the dominant source of home heat is a gas boiler. The dominant heat distribution system is a “wet” (water-based) central heating system. In a wet central heating system, hot water is fed via pipes to a series of radiators (or through underfloor heating), which then emit heat into the home. Radiators, underfloor heating, and various forms of air-blown heating are referred to as heat “emitters”.[3]
Ninety-five per cent of dwellings in the UK now have central heating, and about 95% of those use wet systems; about 4% of homes have underfloor heating. As of 2020, 84% of homes were connected to the gas grid.
Very few homes in the UK use air-blown heat or cool air distribution systems, although these are common elsewhere. A minority of homes in the UK depend on electric resistive heating, fuel oil, or wood combustion—though wood burning and electrical heating are common “top up” sources of heat.
A gas boiler works by burning natural gas to produce temperatures well in excess of 100°C. In a wet central heating system, the thermal energy from a gas boiler or other heat source is used to heat water, generally to temperatures above 60°C.[4] “Flow temperature” is the temperature of the water as it leaves the heat source.
The average domestic gas boiler in the UK has an energy efficiency of about 85%. That means that 85% of the thermal energy released by combustion is transferred to the heated water—the rest of it is mostly vented out of the flue into the world outside, with some radiated into the home. Modern (condensing) gas boilers can have efficiencies above 90%.[5]
However, this higher efficiency requires other conditions to be met—which they rarely are. Subsequent efficiency losses are also common in the heat distribution system.
Since wet central heating is by far the most prevalent variety of home heat distribution system in the UK, electrifying and decarbonising home heating mostly means connecting a heat pump or district heat to a wet central heating system.
Heat pumps
Heat pumps come in various forms, but their consistent and most striking feature is very high energy efficiency. They allow enormous efficiency gains to be won simply through electrification.
Unlike combustion boilers, heat pumps work by transferring thermal energy in the environment from one place to another, via a liquid refrigerant – the reverse of a refrigerator, which uses a heat pump to move thermal energy from the inside of a fridge to the outside, to cool its interior.
Since a heat pump simply moves existing thermal energy, it can be far more efficient in delivering heat (or cooling) than heat sources based on combustion of fuel. Heat pumps can have an effective efficiency of well over 100%.
The heat transferred by a heat pump can come from different sources: from the air, from the ground, from a body of water, or from some other source—such as industrial waste heat, or sewage. In a domestic setting, it is most common to use an air-source heat pump (ASHP) or a ground source heat pump (GSHP).[6]
In the UK, heat pumps generally provide heat for use indoors. However (as in a fridge), they can also provide cooling by transferring thermal energy from the air indoors to outside.
Heat pumps can provide decent heat even when it is very cold outside, since even cold air, ground or water contains thermal energy. Heat pumps can provide heat for space heating and for hot water, although these systems usually run in parallel. While a heat pump provides space heat, a hot water tank can be heated solely via an electric immersion heater, or with an additional heat pump running in a hybrid set-up.
Heat pumps can also relay thermal energy for space heating in different ways. A heat pump that provides heat by heating water for a wet central heating system is called a “hydronic” heat pump. Hydronic ASHPs are currently the most common sort in the UK, and are likely to remain so.[7]
Air-blown systems, such as air-to-air (A2A) heat pumps, are an alternative common elsewhere in the world. They are also a promising alternative in the UK in some instances—especially where space is limited, for instance in small flats (see here). They may also be the better solution for homes with particularly poor fabric efficiency.
Any home space heating system should be specified such that peak winter heating needs can be met—and those needs derive from the underlying fabric efficiency of a home. In the case of a heat pump connected to a wet central heating system, the main salient factors concern: the heat pump’s capacity; the temperature to which it heats the water (the flow temperature); and the output capacity of the radiators.
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| Air source heat pumps. Photo by rawpixel.com, CC BY 1.0 |
So-called low-temperature heat pumps heat water to 50°C and below. High-temperature heat pumps heat water to 60-70°C.
With that in mind, though, it is important to understand that heat pumps can be deployed in any building, regardless of its underlying fabric efficiency. In a recent report for the Regulatory Assistance Project (RAP), Richard Lowes mentions a ground-source heat pump in an eleventh-century church in Kent—“a totally uninsulated, thousand-year-old building.”
However, because you can install a heat pump with no fabric efficiency improvements, does not mean that that is a good path to follow.
To judge the performance of a heat pump, we look at the thermal energy it transfers for each unit of electrical energy consumed. This effective efficiency is called its “coefficient of performance” (COP).
Because heat pumps transfer thermal energy from one location to another, the amount of electrical energy they consume varies according to the difference between the outside temperature and the target indoor temperature—the so-called “heat gradient”.
However, the COP itself of a heat pump is greater at gentler heat gradients—you get more thermal energy transferred per unit of electricity consumed. For example, the warmer it is outside, or the lower the target flow temperature indoors, the higher the COP will be, and vice-versa.[8]
A “Seasonal Coefficient of Performance” (SCOP) is the effective efficiency of a heat pump, averaged over the whole course of a year, for a given climate and target temperature.
Many heat pumps can heat water to a wide variety of temperatures. Moreover, it is now common for heat pumps to vary their target temperature depending on the season, in order to maximise efficiency.
The performance of domestic heat pumps on sale in the UK is regulated, such that an ASHP must have a measured SCOP of 2.5, i.e. an effective efficiency, over a year, of 250%. A recent study of 549 ASHPs installed during 2020-21, commissioned by the government, found that they had an average SCOP of 2.9.[9]
That means that ASHPs in the UK are generally at least 3 times more efficient, in providing heat, than a gas boiler, over the course of the year. The CCC refers to heat pumps as being 3-4 times more efficient than a gas boiler.[10]
The diagram below compares the efficiencies of a domestic heat pump and an average domestic gas boiler in the UK. The vertical bars show the relative scale of energy consumed (C) to useful “energy out” (U). The units here are unspecified—the salient issue is the ratio of C to U. Heat pumps consume far less energy to provide a hot water system with the same amount of heat.
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| Comparing the efficiency of an average gas boiler and a heat pump |
For example, for 85 kilowatt hours (kWh) of useful thermal energy out, you would need to burn 100 kWh-worth of gas with the average UK gas boiler; you would only have to consume 34 kWh-worth of electricity powering a SCOP 2.5 heat pump. That is a 66% reduction. (And SCOP 2.5 is the minimum regulated standard for an ASHP.)
It follows that, if everyone in the UK were simply to convert their sources of home heat to heat pumps, the amount of energy consumed (C) would also decline by about 66%. This would be the case even if there were no change in the amount of energy used (U). However, in practice, more energy might be consumed to bridge the existing “need gap”, and at least some fabric improvements would and should take place, thereby reducing the amount of useful thermal energy needed.
All other things being equal, such a reduction in energy consumption (C) would also make home heating 66% cheaper. It would additionally provide more “consumption space” for the fuel poor to heat their way to thermal comfort.
However, all things are not equal—most notably, cost. The most obvious factor here is up-front cost—and this is what people tend to focus on with heat pumps.
The average cost of a first-time ASHP installation appears to vary wildly, between about £10,000 and £20,000. The recent Seventh Carbon Budget (7CB, 2025), suggests that the average cost of the ASHP itself is £10,900, but once various ancillary measures are included (“such as a hot water tank or radiator upgrades”) the average total cost rises to around £15,000.[11]
Recent research commissioned by the government suggested total costs averaging about £16,000 for a system using a low temperature heat pump, and around £20,000 for a system using a high temperature heat pump.[12] Other recent research commissioned by the National Infrastructure Commission (NIC) put the total cost much lower, at between £9800 and £12,100.[13]
In the UK, the Boiler Upgrade Scheme (BUS) presently pays just £7500 towards the installation of a heat pump.
Equally important to upfront costs, however, are the relative prices of electricity vs gas. In the UK, as in many places, fossil fuels are still considerably cheaper than electricity, on an energy-content basis.
At present, electricity in the UK costs the domestic consumer about 4 times as much per kWh as natural gas. This is mostly because electricity bills are taxed extra by the government, to pay the energy companies to expand renewables generation and grid capacity. Gas bills are not similarly taxed, despite gas being the climate villain.
There is widespread recognition of the need to “rebalance” these “policy costs” soon.[14] Energy company profits are unacceptably high.
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| Converting from a gas boiler to a heat pump |
The next diagram compares the energy efficiency of heat pumps and gas boilers, with the additional parameter of cost. This varies with the amount of energy consumed—but in the case of electricity, it costs approximately 4 times more per unit of energy content than gas does. (The “£” figures here are illustrative, not based on actual costs.)
The illustration shows, in stylised form, how energy consumption (C) and the cost of that energy (in grey) would change, if a household switched from a gas boiler to a heat pump. The amount of heat used (U) remains constant. Supplemental heating efficiency improves by about 300%. However, all other things being equal, you would still pay about 36% more to heat your home, compared with your old gas boiler! The 400% price differential of electricity vs gas still outpaces the energy efficiency saving of a heat pump.
The higher operational costs of running a heat pump continue to be a large barrier to adoption—even before we include the far greater upfront costs of a heat pump, compared to a gas boiler.[15]
One engineering downside of heat pumps has been that the refrigerants used tend to also be very potent greenhouse gases —and heat pumps can leak refrigerant into the air. Heat pump manufacturers are increasingly stepping around this problem, by using alternatives that have lower global warming potential, such as CO2 and propane (!).
Another downside of domestic heat pumps in individual homes is that they need replacing every 15-25 years—although one would hope that this improves.[16] Like domestic gas boilers, they therefore imply a significant “churn” of materials over the lifetime of a building.
It is also worth considering the emissions arising from the production of a heat pump. Those “embodied” emissions—for a heat pump on sale in the UK presently—stand at around 1500 kilogrammes of carbon dioxide equivalent (kgCO2e), according to Jan Rosenow of the Regulatory Assistance Project. That is comparatively small compared to the potential emissions savings.
According to Rosenow, the “carbon payback” of an average heat pump in the UK is presently around 13 months, when it replaces a gas boiler. He suggests that it is worth retiring a gas boiler in favour of a new heat pump, even if the boiler still has a useful life of ten years or so remaining.
Moreover, those emissions will only decline as production is decarbonised—which means electricity at the site of production, and the upstream production of metals, plastics, and other materials. Note, however, that the scale of those embodied emissions depends on the size of the heat pump, and on where it is produced.
Heat pumps in individual homes are the focus of home heating decarbonisation policy in the UK. The “Balanced Net Zero” (BNZ) pathway in the Sixth Carbon Budget (6CB, 2020) forecast heat pumps in about 18% of homes by 2030, 54% of homes in 2040, and 76% of homes in 2050. However, as of the end of 2024, there were only about 200,000 certified home heat pump installations in the UK—about 0.7% of homes.[17]
The Seventh Carbon Budget (7CB, 2025) therefore moderates expectations for the rate of deployment to 2030; but it also argues for marginally more homes to receive a heat pump by 2050, compared to the 6CB. In the 7CB, the BNZ pathway models heat pumps in just 6% of homes by 2030, in 52% of homes in 2040, but in 80% of homes in 2050.
Plainly the pace of deployment needs to be stepped up, significantly. This will require grants to help households pay the upfront cost of a heat pump; and it will also require some “rebalancing” of energy costs, to make electricity cheaper. The government should also focus on pushing heat pumps into homes presently not connected to the gas grid—often relatively isolated homes, reliant instead on fuel oil, wood combustion, and other carbon-intensive fuels.[18]
The requisite pace of roll-out meanwhile implies a phase-out date for the installation of new gas boilers. The Sixth Carbon Budget (6CB, 2020) recommended that the sale of gas boilers be phased out in 2033; and the last Tory government finally set up plans to do that in 2035.[19] However, the present Labour government recently cancelled the 2035 gas boiler phase-out plan. The government also said that gas boilers will now be allowed in new homes under the forthcoming “Future Homes Standard” (FHS).
In my view, that is an outrageous move. The danger now is that the abandonment of the 2035 phase-out date indicates that Labour is lining itself up to also abandon any ambition to wholly electrify home heating by 2050.
District heat networks
District heat networks provide heat to many homes or buildings from a centralised source, by pumping hot water through insulated pipes. The heat is then used to provide space heating and hot water. Again, in the UK, that primarily means relaying network heat to a home’s wet central heating system. Hot water temperatures are topped up using an immersion heater. Although widespread elsewhere in Europe, in the UK heat networks are rare.
District heat networks are best suited to feeding buildings and homes clustered close together, in towns and cities—areas of high “heat density”. They are not suitable for isolated dwellings. Large buildings, like hospitals and schools, can serve as “anchor loads” for the network, reliably drawing consistent volumes of heat.
District heat can be fed by heat pumps powered by green electricity. In such cases, the heat for the heat pump can be sourced from the air, or from industrial waste heat, water from rivers and lakes, or sewage. Moreover, the source of heat can be changed according to availability and shifts in technology.
District heat networks can additionally employ thermal energy storage to buffer variations in the demand for heat—for example, using batteries of heated rocks or molten salt.
Some important further advantages of district heat derive from the implied materials and cost savings—indeed, I think this is the main advantage over heat pumps in individual homes. The useful lifetime of a heat network is much longer than the 15-25 years of a consumer heat pump. The various components of district heat networks can be serviced, replaced, and updated as needed, but the capital cost and the hassle of that do not devolve directly onto individual households.[20]
Heat networks can alternatively be used to feed ready-heated water to water-source heat pumps in individual buildings, thereby allowing more tailored local temperatures, while boosting in-building heat pump efficiencies.
Wherever it is physically and organisationally possible, I see little reason – from the position of use-values – not to prioritise district heating.
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| New district heating pipes, near to Leeds. Photo © Stephen Craven, CC BY-SA 2.0 |
The UK’s Climate Change Committee (CCC), and the National Infrastructure Commission (NIC) both recommend a wider use of heat networks. The CCC have, however, recently revised downwards their recommendations for the use of heat networks in homes in the UK.
Whereas their BNZ pathway in the Sixth Carbon Budget (6CB, 2020) saw about 12% of homes in the UK connected to a low-carbon heat network in 2040 and about 18% by 2050, the BNZ pathway in the Seventh Carbon Budget (7CB) sees only half this: about 6% of homes in 2040.[21] I think that the UK should aim for about 13 million home heat network connections by 2050 (35% of homes).
The NIC, meanwhile, thinks that the per household subsidy for district heat connections should be matched to the scale of upfront grants for heat pumps (where presently it is not).
One challenge for district heat and cooling networks is that they require large upfront physical deployments of resources, and significant upfront political will to roll out the necessary infrastructure. This is where the role of government is crucial.
Unfortunately, however, successive governments (Labour, and Conservative before them) have so far shown very little readiness to fund and coordinate the large-scale roll-out of district heat networks. The cost- and energy-efficiencies of district heat are also habitually underestimated by government.
Most notably, the Treasury’s “Green Book” costing methodologies appear to mandate that the CCC apply a 7.5% discount rate in the cost of capital for district heat, instead of the 3.5% applied to heat pumps in individual homes. That suggests an assumption that district heat will only be funded by private capital, which will bear all of the investment risk.[22]
Moreover, the CCC assumes that district heat investments only have an “economic lifetime” of 20 years—an absurd proposition for infrastructure investments of this kind. These “value for money” determinations need to be rethought.
We need to be clear about the wider benefits of district heat, from the standpoint of use-values—over the next 50 years and more, and not just the next 20 years.
Wherever cooling is required in homes and non-residential buildings, district heat networks should also be developed with the option, if possible, for including a parallel cooling network.[23]
Electric storage radiators
Electric storage radiators are situated in the home, and use electric resistive heating to heat (for example) a well-insulated ceramic block. The heat is then released on demand via a fan.
The effective efficiency of an electric storage radiator is always less than 100%, because, although electric resistive heating is about 100% efficient, some undesired heat loss is inevitable. Good home fabric efficiency, the presence of thermal massing in the building itself, and storage radiators and other thermal stores, all assist in the retention of heat, however.
Storage radiators—and other similar forms of thermal storage—are helpful in buffering demand for heat when you are concerned with managing supply. District heat networks benefit from thermal storage, and the same is true within homes.
That is good for the national heating system, because it helps to reduce energy consumption when the need for heating peaks. It can also help households reduce their energy bills—with lower off-peak tariffs designed precisely for the purposes of encouraging such a shift.
Arguably, storing heat with a heat store is also better than storing electricity with an electric battery—simply because the latter remain bound up with the problems around critical minerals. Electric storage heaters are also good for very small homes, where space constraints make heat pumps or central heating unviable.
Another benefit of storage heaters, according to a BEIS-commissioned report linked above, is that storage radiators—unlike heat pumps—are unlikely to require any ongoing maintenance or replacement over a 25-year time horizon.
Central heating (in)efficiencies
The efficiency of a central heating system concerns its capacity to transfer heat from a source of heat to where you want it in a home. Conversely, inefficiencies in the system concern heat loss between the heat source and the target locations for heat.
Ideally, whatever heat is lost will end up somewhere else in the house, and be useful somehow. However, dissipated heat may also end up in areas of the home from which it can be more easily lost to the world outside—and that depends on the underlying fabric efficiency of the whole building “envelope”. Heat is not delivered where you want it.
Inefficiencies in wet central heating systems are very common. They can arise, for example, from poor “hydraulic balancing”. This is when there are imbalances is the distribution and flow of hot water through a central heating system, so that thermal energy is misdirected and lost. According to a recent (2021) report by the BEIS, this can reduce performance by 10%. A survey of the plumbing profession also commissioned by the BEIS found that only 25% of plumbers say that they always carry out hydraulic balancing, versus 32% that say they never do.
Other, perhaps more severe, causes of inefficiency include: the build-up of sludge, air, and limescale in water pipes. According to the same BEIS report, each of those can substantially reduce the energy efficiency of a central heating system—by 15%, 6%, and 15% respectively.
It is unclear to me how severe the compounding effects can get if all of these maladies are present together. The consequences could be fairly pronounced, in terms of operational heating costs and thermal comfort.
Various devices can mitigate these problems. The performance-enhancing measures “with the most robust evidence base” are thermostatic radiator valves, according to the BEIS report. These provide a 3% energy saving on the norm. Other measures require “further trials and testing”.
In addition, it is thought that radiators are frequently oversized in the UK, relative to how much heat is needed on the coldest days. The same BEIS survey found a typical oversizing of 20-40%. Rules-of-thumb among plumbers apparently tend to stand in place of rigorous in-situ measurements and calculations. On the plus side, however, these extra sizing margins permit a home to be heated faster when heating from a “cold start”—which can benefit thermal comfort.[24]
The main issues with central heating inefficiency, therefore, appear to concern two types of problem: poor standards of installation (absence of thermal balancing, oversized radiators); and the accumulation of unwanted sludge, air and limescale in the pipes.
All these problems, except radiator sizing, can be readily diagnosed and remedied in a routine service. Efficiency measures like thermostatic radiator valves could also be added at the same time. The BEIS recommends that an annual service for central heating systems should be standard. As things stand, however, the BEIS says that servicing is rare, and that only about 20% of dwellings have their central heating serviced annually.
Low flow temperatures[25]
The reality is that heat source efficiency and heat distribution efficiency can often interrelate. An important example of this concerns the flow temperature in a central heating system, i.e. the temperature of the water as it leaves a heat source. (The temperature of the water on its return trip from the emitters is called the “return temperature”.)
The average domestic gas boiler in the UK has an energy-efficiency of about 85%. Modern (condensing) gas boilers can have efficiencies above 90%. To attain those higher efficiencies, however, condensing gas boilers need to be able to heat water to cooler temperatures than is presently standard—50°C and below, instead of the present norm of 60°C and above.[26]
Heat pumps and district heating also work better at lower temperatures—though for different reasons.
In the case of a heat pump, it stands to reason that—all other things being equal—heating water to a lower target temperature will require less energy. However, a smaller temperature gradient in itself increases the effective efficiency of a heat pump—you get more useful heat out per unit of electrical energy consumed. In terms of heat pumps, therefore, low flow temperatures provide a double whammy of energy savings.[27] These apply to heat pumps in individual homes, and to heat pumps used to feed heat into a district heat network.
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| A standard hot water radiator. Photo by Christian Ohde / picturedesk.com, CC BY-SA 2.0 |
Low flow temperatures also reduce unwanted heat loss from the pipes in a wet central heating system, or in a district heat network.
Ultimately, the permissibility of a given flow temperature is dependent on two factors: the underlying fabric efficiency of a home, and the overall capacity of the heating system to deliver heat.
For the average home in the UK, lower flow temperatures will tend to require some combination of fabric efficiency improvements and enlarged radiator capacity.
Recent BEIS survey research indicated that radiators in the UK tend to be oversized, by 20-40%. Nevertheless, that same research suggested that, on the average, converting to a low temperature central heating systems in the UK still means doubling the existing radiator capacity: that is, adding more, bigger radiators.
On the other hand, modelling compiled concurrently by BEIS offered some more optimistic results. It suggested that, on an average winter’s day in the UK, as many as 53% of existing dwellings could be heated using a (fairly low) 55°C flow temperature, with no alterations to their existing radiators—and, by implication, no alterations to their underlying fabric efficiency either. As many as 10% of dwellings could do the same on the coldest of winter days.
These conclusions appears overly optimistic, in my view, because they are based on poor quality Standard Assessment Proceedure (SAP) data. The BEIS themselves caution that the SAP likely significantly underestimates rates of heat loss from homes—by up to 45% on average, according to one study.
When accounting for those likely underestimates (and for underperforming radiators), the BEIS provide revised estimates for the percentages above: 8% of existing homes could be adequately heated, on an average winter’s day, using a 55°C flow temperature; only 1.5% of homes could be adequately heated on the coldest of days.
Very few existing homes in the UK seem fit for low flow temperatures, without some adjustments to radiators and/or homes’ underlying fabric efficiency.
High flow temperature and low flow temperature systems tend to require distinct models of heat pump—the high temperature and low temperature models mentioned above. As stated, low-temperature heat pumps heat water to 50°C or below; high-temperature heat pumps heat water to 60-70°C.
For the time being, a low temperature ASHP costs an average of £6200 in the UK and requires additional radiator upgrades costing £1900. High temperature heat pumps run at existing (high) flow temperatures, require no radiator upgrades, but tend to be more expensive—around £10,000. New heat pumps that use propane (R290) as a refrigerant can readily heat water to 70°C and above.[28]
Many modern heat pumps, as noted previously, can provide variable flow temperatures, for purposes of seasonal efficiency (“weather compensation”). For instance, the BEIS reference a ground-source heat pump that can provide flow temperatures of 35-70°C.
Variable flow temperatures can go some way to mitigating the need for home fabric upgrades and extra radiator capacity. But higher flow temperatures will still tend to be more inefficient than low flow temperatures. Weather compensation offers a workaround for those months of the year when the combination of building fabric, radiator capacity, and low flow temperatures, is simply inadequate for maintaining thermal comfort indoors.[29]
Similarly, there may be a limited role for hybrid heat pump systems—although I do not support that. In these systems, a traditional fossil-fueled boiler runs in parallel with an electrical heat pump—and can kick in on the coldest days, to raise the flow temperature. Plainly, such systems are not fully electrical, and still produce greenhouse gas emissions.
Once a low-temperature central heating system is in place, needing less energy for heating naturally brings energy and cost savings. Low-temperature heat can therefore be an important route to reducing household energy costs and reducing fuel poverty—so long as that is done on the basis of thermal comfort.
Low flow temperatures also bring efficiency benefits for homes that use a condensing gas boiler heating water to 50°C. So long as heat pumps use electricity generated with greenhouse gas emissions, any reduction in energy consumption also serves to mitigate greenhouse gas emissions from electricity generation and transmission.
Moreover, all savings in energy consumption also suggest possible wider savings in the upstream electrical grid. These could be crucial if we are to electrify the entire UK economy, and simultaneously decarbonise all sources of electricity. They are also important for constraining overall material consumption, and the use of critical minerals.
However, since low temperature heating does tend to require additional radiator capacity, alongside some fabric improvements to the home (e.g. insulation), the salient issue from the perspective of the energy system as a whole is whether the material and environmental footprints of those things outweigh the sum of burdens avoided in future—the extra grid generation, transmission and storage capacity foregone, with all the associated upkeep and land use.
It seems intuitively likely that in-home improvements would present a net material saving for the energy system.
Looking now at heat networks: when fed by heat pumps, and working to a lower target temperature, low flow temperatures bring all of the benefits listed above, plus some additional ones.
Low-temperature district heat means reduced “transport losses” of thermal energy from water pipes. Lower temperatures mean lower investment costs. Lower target temperatures additionally enable a wider and more flexible use of excess, ambient, and renewable heat sources than when the target temperature is high.[30] This increases the scope for the deployment of low-carbon district heat networks.
In pure energy efficiency terms, all of the above factors point strongly towards a principle of “the lower the flow temperature the better”—so long as thermal comfort is maintained.[31]
The Regulatory Assistance Project and the Institute for Energy and Environmental Research Heidelberg (RAP/IFEU) define low-temperature heating as a flow temperature of 35-55°C on the coldest day of the year. They propose a scale for homes, rating their “low temperature readiness”:
Figure 2. Draft “low temperature readiness” scale[32]
If we accept that high levels of heating efficiency are desirable, and that low flow temperatures are a part of that, we then have to ask: how do we get there, for the UK’s existing housing stock?
In England and Wales, “Part L” of the recently updated building regulations (in force from June 2022) requires that all new space heating systems should be installed to run on low flow temperatures—a maximum of 55°C, down from 80°C under the previous standard. Part L also stipulates some minimum fabric efficiency standards in the case of renovations and extensions to existing homes. Part L is a good starting point with regard to low flow temperatures, but it is not enough. In my view, a universal home retrofit programme is in order too.
Phasing out firewood use in homes
Burning wood remains a comparatively popular way to keep warm. Wood burning stoves are also widely considered to be a “green” form of home heating, but they are not.
It is true that all forms of plant biomass combustion, including wood burning, are “net zero” for CO2 over the whole lifecycle of growth and combustion. This can be viewed in two ways. Either CO2 is absorbed by plants from the atmosphere, and then simply released again when that plant is burned (“dividend-then-debt”). Or the CO2 released on burning is later re-absorbed, once new plants are grown to replace those culled for fuel (“debt-then-dividend”, or “carbon repayment”).
Either way, however, that rate of CO2 “flux” between plant matter and the atmosphere depends entirely on the rate of plant growth. In the case of wood, plant regrowth typically takes at least several years, and often a couple of decades or more.[33] But in a climate emergency, we need to curtail additions to atmospheric CO2 now.
More important still is that, when it is burned, firewood produces more CO2 combustion emissions than coal does, per unit of energy released—even though coal itself derives from ancient wood.[34] Moreover, those are CO2 emissions that might otherwise remain locked up in the wood, were another use found for it instead.
This is not just about greenhouse gas emissions. Firewood also does not burn cleanly: it gives off smoke—and that is a complex mix of gases and fine particles, many of which are carcinogenic or otherwise hazardous to human health. Wood burning has been linked to heart and lung disease, and to dementia. It has also been causally connected to mental illness among children.
One study found that 284 Londoners are dying early every year, just from the outdoor pollution caused by wood combustion in the home.[35] (In the countryside in the UK, twice the proportion of homes burn wood and coal as in cities.) A recent article reported on a US study that found that using a wood stove or a fireplace for heating a home increased occupants’ own risk of lung cancer by 43%.
There is some limited argument for burning biofuels and biomass in energy systems—when they are used parsimoniously, and only when essential, as a source of dispatchable power, at dedicated power stations. There is also good reason to burn some forest residues and woody waste from forestry and industry, to prevent it rotting and releasing methane, a far more potent greenhouse gas than CO2. However, there is no place for wood combustion in the home, and no case for burning wood close to homes. Time is long gone when any community in a country like the UK—urban or rural—should depend for survival on burning forest wood.
From a land-use perspective, it is also better to favour the expansion of healthy forests, and selectively cultivate wood and fast-growing crops such as hemp, bamboo, and willow, for more useful, long-lasting purposes than combustion—such as construction materials. The UK already imports most of the timber it uses in construction, due to a paucity of high-grade timber available in long lengths in the UK. More native timber could be used, given a willingness to specify for lower grades of timber, and use the shorter cuts that are readily available.[36]
As of May 2021, the sale of coal and “wet wood” for home burning was outlawed in the UK. Households with wood fires and wood stoves can still buy low-sulphur solid fuels like dry wood, which produces less smoke and more heat. However, that too may be legislated away in due course. In London, wood burning stoves were effectively banned in 2023, for all new and refurbished homes.
The sale of firewood could be further restricted. It may well be challenging to fully outlaw wood burning on small scales, from agriculture and garden waste. Nevertheless, I think that for the vast majority of the population of the UK, wood combustion should be effectively outlawed from around 2030, through a ban on selling it, combined with stricter rules on wood combustion. Something like a “permit to burn” should be introduced by 2035, if not earlier.
All the poisonous compounds contained in wood smoke need to remain locked up in wood, along with all the biogenic carbon, instead of being thrown into the atmosphere.
🔴 This is the third excerpt from Remaking Home Heating in the UK, a People & Nature pamphlet by Tom Ackers, which can be downloaded for free here.
🔴 The first two excerpts, (1) on policy proposals and (2) on fuel poverty, were published by People & Nature yesterday. The fourth excerpt, on retrofit and insulation, is here.
🔴 More People & Nature commentary and analysis of the decarbonisation of home heating is here.
References
[1] Whereas the CCC’s Sixth Carbon Budget (6CB, 2020) equivocated on hydrogen boilers, seeing a possible role for them in the form of “hybrid” heat pump systems, the Seventh Carbon Budget (7CB, 2025) recommends “no role for hydrogen heating in residential buildings” (see also here). For a broad introduction to heat pumps, district heat (and district cooling), and the disadvantages of hydrogen-based home heating systems, see Part 10 of Decarbonising the Built Environment: a Global Overview.
[2] See this diagram from the Regulatory Assistance Project (RAP), representing a heat pump central heating system with different “H” boundaries (RAP, 2022c)
[3] Despite their name, radiators typically transfer only about 30% of their heat radiatively; 70% is from the combined effects of conduction and convection, according to modelling parameters used by the BEIS
[4] Natural gas, fuel oil, electricity, etc., when used as sources of energy, are said to be “energy carriers”. In a wet central heating system, the water is the “transport medium” for heat
[5] Since 2018, all gas boilers installed are required to have an efficiency of 92%
[6] In the case of an ASHP, an outdoor evaporator (with a fan) typically connects to a condenser indoors, which has an integrated heat exchanger
[7] A2A heat pumps are effectively reversible air-conditioners, with the added bonuses of greater efficiency, and readier “reversibility” to permit rapid cooling. They are better at this than water-based systems. However, they cannot also provide hot water without a small hydronic heat pump or an immersion heater working in parallel. See this November 2023 report from the RAP
[8] “[F]or each degree of flow temperature reduction, the efficiency of the heat pump increases by 2-3%” (RAP/IFEU, 2023). At lower external temperatures, the overall heating capacity of a heat pump is also reduced, relative to its rated capacity
[9] When all components of the home heating system are included, the SCOP fell to 2.8. These are median values, out of 549 ASHPs. See Energy Systems Catapult (2025), pages 6 and 32
[10] 7CB Advice Report (2025), p.311
[11] 2023 prices. See 7CB Advice Report, p.162, 306. These estimates relate to a 12kW ASHP
[12] Converted to 2023 prices. See Energy Systems Catapult (2025), page 6
[13] Converted to 2023 prices. See Aurora Energy (2023), page 63. A low temperature ASHP was assessed to cost £6200 on average, and required additional radiator upgrades costing £1900 on average. High temperature ASHPs cost more—about £10,000—but those do not tend to require radiator upgrades.
[14] There are various proposals about how to do it. As of March 2024, the government’s Review of Electricity Market Arrangements (REMA) remains ongoing
[15] Many heat pumps have a SCOP greater than 2.5, and many gas boilers are more efficient than 85%. The CCC’s most recent Seventh Carbon Budget (7CB) models the energy bill when using a heat pump as 16% higher than when using a gas boiler in 2025 (as opposed to the 36% shown here). Note, however, that this is for total home energy costs (space heating, hot water, cooking, lighting, and other home energy use), and includes standing charges (but excludes car charging costs). See 7CB Advice Report, Figure 8.4(a), and underlying data. According to the 7CB, “Without policy action [that is, under the present energy market pricing structures and forecast changes in pricing over time], the price of running a heat pump will not reach parity with the price of running a boiler until 2035.” (7CB Advice Report, p.311)
[16] The UK’s Climate Change Committee (CCC) said in its Sixth Carbon Budget (6CB, 2020) that – as with gas boilers – the lifespan of a domestic air-source heat pump is generally around 15 years, and that a ground-source heat pump lasts perhaps 20 years before it needs to be replaced. The UK’s Energy Saving Trust (2021), said much the same. The IEA (2022) estimates a 17-year lifespan for the average gas boiler, 15 years for air-to-air heat pumps, and 18 years for air-to-water heat pumps. One 2013 study, citing earlier data, suggested that “30 years […] is a standard estimated lifetime for GSHP systems”. The Seventh Carbon Budget (7CB, 2025) appears to use the 15 years figure. Jan Rosenow, an energy systems researcher and a Director of the RAP, says that heat pumps can indeed last longer than 15-20 years, if they are properly maintained. The RAP base their projections for the cost of heat pumps on a 20-year timespan of operation. Meanwhile, the heat pump industry in the UK says that recent technological developments mean that the lifespan of a new heat pump is now 20-25 years. Publicly accessible evidence for that seems to be thin on the ground. However, one manufacturer estimates that 80-90% of their heat pumps last longer than 20 years – dependent on proper installation, “reasonable conditions”, regular servicing, and prompt repair when problems arise
[17] See the Microgeneration Certification Scheme (MSC) Data Dashboard
[18] This policy emphasis is already contained in the Home Upgrade Grant (HUG) scheme
[19] See the 6CB Advice Report, page 28; Heat and Buildings Strategy (2021). The 7CB (2025) now simply urges the importance of setting a new phase-out date
[20] A recent paper in the journal Applied Energy surveyed options for decarbonising heating systems in the UK, and found that district heating fed by heat pumps would be the cheapest option overall because of its economies of scale – about 11% cheaper than fitting heat pumps in every home
[21] Author’s calculations based on more limited published data
[22] A joint report by the Regulatory Assistance Project and the Institute for Energy and Environmental Research Heidelberg (RAP/IFEU, 2023) recommends that district heat networks should be run not-for-profit, and paid for up-front by government
[23] District heating and cooling networks can be combined to increase efficiency, as district heating and cooling (DHC). London’s Olympic Park has a DHC network. Cooling networks are less physically efficient than heat networks, and they have greater upfront costs, so building-based cooling may be more practical and economical. (See here for more.)
[24] The 20-40% oversizing figure comes from “stakeholder” evidence. Supposedly more rigorous modelling by the BEIS indicates mean oversizing of 50%, and median oversizing of 30%. Here, “oversizing” is calculated as “the rated thermal power output of the radiators in a dwelling divided by the dwelling’s peak steady state thermal demand (kW)”. However, these calculations derive “peak steady state thermal demand” from the UK’s Standard Assessment Procedure (SAP) for homes—a standard which is widely regarded as not fit for purpose
[25] The section draws primarily from a June 2023 report compiled by The Regulatory Assistance Project and the Institute for Energy and Environmental Research, Heidelberg (RAP/IFEU, 2023)
[26] “Condensation only starts at temperatures below 56°C. In a typical building, reducing the flow temperature from 80°C to 55°C improves boiler efficiency by 6%.” (RAP/IFEU, 2023)
[27] Smaller benefits are also available by increasing the “flow rate” of the water through the system—although the benefits decline as flow temperature is decreased (BEIS, 2021)
[28] For example, see here
[29] The span of flow temperatures available from a single heat pump seems to be increasing, as heat pump technology advances. It may be possible, at some point, for a mass-market air-source heat pump to function first as a high-temperature heat pump, and later transition to work instead as a low-temperature heat pump—once suitable fabric or radiator renovations are in place, and with little loss of efficiency compared to a dedicated low-temperature device
[30] In a low temperature district heat network, heat pumps in individual buildings can then also boost the incoming water temperature locally—for example, to provide hot water
[31] For reasons of sanitation, all central heating systems require a weekly “Legionella cycle”, with the hot water tank heated up to 60°C. With low flow temperature systems, this task may be accomplished using the heat pump, if it is capable of a suitably variable range of target flow temperatures; otherwise, an immersion heater can be used
[32] “The numbers are indicative only and need to be adapted to each country.” (Source: RAP/IFEU, 2023)
[33] For instance, hybrid poplar trees take about 20 years to bring to harvest; other trees can reach “carbon sequestration parity” only after something like 40 years. Short rotation forestry—for example, cultivation of willow—permits harvesting after 3-5 years
[34] See the UK government’s standardised tables of “emission factors”. The comparison here is between the “scope 1” CO2 emissions of coal, and the “outside of scopes” CO2 emissions of (e.g.) dry wood pellets. For more on this topic, see Decarbonising the Built Environment: a Global Overview, in particular Part 8
[35] See here for a similar study from Australia
[36] See Material Cultures (2022), Material Reform, p.63
The case for insulation is a no brainer. I worked for Friends of the Earth in London in the early 1980s and it was one of our main campaigns. If I remember correctly someone did the sums to show that Sizewell B would have been unnecessary if there had been a serious effort to insulate. Nothing much seems to have changed except for new build regulations.
ReplyDeleteAnother mainstay of the campaigns at the time was returnable bottles. That didn't really get anywhere. In the old days you could still get to 10 Downing Street, and one stunt was when we all dressed up as bottles and protested outside. Ah, those innocent days when we thought something like that might change the world.