How cold can you go? Is 16°C reasonable?

As we enter a cold snap, and energy prices have risen again, we are more worried about how to manage our home heating. The easiest way to reduce bills is to turn down the thermostat. Do we really need 20°C or higher at home? Perhaps this is merely a social convention. David MacKay published ‘Sustainable Energy Without the Hot Air’ in 2009, in which he proposed turning down the thermostat at home to 16°C to reduce carbon emissions. At his publicity talks he explained how his household (with wife and two children) had successfully adapted to these conditions. He also incorporated the 16°C target in the 2050 calculator for scenario planning for net zero. This was the extreme level for one of the home energy 'levers'.

I find this idea scary; thick jumpers do not seem to keep my hands warm – even at 19°C I find my fingers stiffen so that I struggle to type unless I wear half-fingered gloves. I know many people are forced to endure such conditions through fuel poverty but I cannot imagine doing it from choice. Is this reasonable?

So I was intrigued to read about a group of 23 people in Belgium who decided to experiment with heating at home – can they turn down the setpoint on the thermostat and heat the body instead, generating less carbon emissions while still enjoying their living conditions and avoiding any ill effects? [1].

By the third season some were comfortable with the mean living room temperature as low as 14°C.

Are hybrid heat pumps a good thing?

A hybrid heat pump is normally a combination of a gas boiler and an air source heat pump (HP). The simplest arrangement runs either one or the other at any one time. For example, you can use the boiler (possibly your existing boiler) for space heating when it is cheaper to run, thus reducing your bills. This typically means using the boiler when the outside temperature is low. You might also use the boiler to provide hot water, if you currently have a combi and do not want to find the space for a hot water cylinder, or if you do have a cylinder but it is not suitable for the heat pump. The hybrid approach avoids some of the disruption and expense of switching to a standalone heat pump.

Government subsidy for hybrids – off and on.

Hybrids were supported under the RHI subsidy but only with meters to see how much you use the HP side. They are not supported by the Boiler Upgrade Scheme which gives grants for heat pumps now. The consultation for the Clean Heat Market mechanism proposed including them [1]. This mechanism aims to increase the rate of heat pump installs by requiring heating system suppliers to install an increasing proportion of heat pumps rather than boilers (a bit like the old Renewable Energy Obligation scheme). It will be interesting to see what different stakeholders think about allowing hybrids to count – the responses are not yet published.

A hybrid HP could be a temporary or a long term solution.

You might opt for a hybrid heat pump as a temporary solution:

• To see if it the HP is adequate, because you have doubts.
• Because your house has high heat loss at the moment and you intend to do some more work on it, so you will get a heat pump sized for when you are finished and use the boiler as a backup in the meantime. 

You might opt for a hybrid as a long term solution:

• To minimise your energy bill – from hour to hour you can use whichever is cheaper (see below for more analysis)
• Or to minimise your carbon emissions, using predicted emissions intensity instead of price
• Because your house has high heat loss and you do not believe it is possible to make it ‘heat pump ready’. You can have a hybrid solution with no radiator upgrades and it will reduce your gas demand but not completely.

How long can we rely on cheap rate electricity?

A number of my friends are considering or have already installed a Tepeo zero emissions boiler (ZEB) with storage as a low carbon heating solution for their home. This is less disruptive than installing a heat pump but it is only 100% efficient compared to 300% or so for a heat pump. It relies on cheap electricity to keep energy bills at a sensible level – either economy 7 overnight, or similar, or relying on prices fluctuating during the day with a tariff based on wholesale prices, such as Octopus Agile. I shall call all of these tariffs ‘cheap time’, for brevity. 

Cheap time tariffs could become cheaper – or more expensive

Historically, overnight demand is lower than during the day, and hence prices are lower because there is no need to run the expensive peaking plant. This has enabled tariffs such as Economy 7 to work well for homes with storage heating. Hitherto this has been only a small fraction of heating demand: I estimate just 2.2% of domestic heating demand in England [1]. However, there is going to be lots more demand for cheap time power due to switching to low carbon transport and the need for electricity storage to take account of intermittent renewables. Cheap-time tariffs could become cheaper, because renewable power is cheap, or more expensive, because there is more demand for it. On balance I fear the latter is more likely.

The effect of reducing radiator temperature on room temperature

Heating engineer experts have been complaining for years that condensing boilers were not being installed correctly to achieve the claimed efficiencies. A gas condensing boiler that is supposed to give you 92% could be giving you 85% or even less, if it has been set to heat the radiators at 70°C or hotter. Now that we have an energy crisis, this is one of the few things you can do to make significant savings for no cost and hardly any effort (if you have a combi boiler). Nesta have just launched a campaign on this. However the Heating Hub has been giving similar advice for some time. Some of the savings are from increasing boiler efficiency and some are from indirect effects on the room temperature. There is usually only a small reduction in average temperature but it will vary from one house to another. If you find you are uncomfortable you can tweak the thermostat up a bit to compensate.

Can reducing the thermostat 1°C really save 10%?

It seems extraordinary that reducing the thermostat setting by just one degree can save more than 10% on your gas bill. A back of the envelope calculation suggests this has to be an exaggeration. However, when you take into account other sources of heat it can be an underestimate. Of course it depends on various factors and for a less well insulated house the relative saving is less, though the kWh and £ savings will be more.

How big should your heat pump be?

Most people have combi-boilers and the size of the boiler required is often dictated by the need to heat hot water for baths and showers quickly rather than space heating. With heat pumps, you normally have a cylinder for hot water and it is the space heating requirement that dictates the size you need. So how big should it be? Hearsay tells me that even with boilers, sales-people love to sell you systems that are bigger than you need. My heat pump is oversized by at least 50%.

My heat pump is much bigger than it needs to be. I have tried to make it less intrusive with some stick-on leaf patterns. Still, at least I have renewable heating :-)

Oversizing does matter, both for boilers and heat pumps.

• Large systems cost more - not as much as you might expect, but still more.
• Running at small fractions of full capacity reduces efficiency (usually).
• Large systems are physically large and, in the case of heat pumps, have a greater impact on the landscape (see picture above).

In this post I discuss a very simple rule of thumb for heat pump size - thanks to Michael de Podesta - and some reasons it gives an underestimate in my case. Also I show you how to cross check your installer's estimate using the EPC certificate for your home. I wish I had done this!

Performance of my heat pump

Last year we installed a heat pump at home and we now have almost an entire heating season of data for it. Here I present some results. Our overall mean COP (efficiency) is a little better than we were told to expect. During the heating season we have made a number of changes: increasing the night time setback temperature and reducing the configured flow temperature, with impacts on heat demand, electricity consumption or both. It is tempting to focus on efficiency alone but, being practical, it is energy use that we want to minimise. For example you can expect increasing setback temperature to increase efficiency but also heat demand - which wins? 

SMETER – measuring the energy performance of a house

I think we all realise that home EPCs are not always accurate in predicting energy bills for a house. This is a shame for a lot of reasons. It makes it hard for the government to form housing policy: how do we assess potential energy savings from energy efficiency measures on the housing stock, if we don’t know how bad it is already? It is a shame for people looking to rent or buy a home who want to know what they are letting themselves in for. So why don’t we just measure the energy efficiency instead? Would that it were that simple. The government is well aware of the problem and has provided financial support for some potential solutions making use of smart meter data. The project was called SMETER (Smart Meter Enabled Thermal Efficiency Ratings). The results were promising but not brilliant. Here I explain why it is so hard, and some of the solution strategies.

Whether you are modelling or measuring, the core of the problem is the balance of heating coming in and heat going out. This chart captures the balance from a model of a typical semi detached house with filled cavity walls, double glazed windows and 100mm loft insulation, in the East of England. The model is similar to that used for calculating EPCs. The chart shows month by month heat losses (blue and purple, downwards) through conduction through walls etc and air leakage, balanced by heat gains including solar gains through windows, use of electrical appliances, heat losses from the hot water system (mainly from hot water pipes distributing heat around the house) and of course the heating. 

Estimating carbon savings for switching to EV or heat pump

Suppose you want to invest some cash in reducing your personal carbon emissions - can you save more carbon by replacing your car with an EV or by swapping your boiler with a heat pump (and how much would this cost?). Unfortunately, the answer depends on lots of variables such as how far you drive and how much is your heating bill, do you have solar panels – if so how much power are you currently exporting? So I have prepared a tool to help you estimate your savings and in this post I use it to illustrate some examples. You probably know that energy costs for an EV are lower than for a diesel or petrol car. You may be surprised that the heat pump can reduce your energy bills too, using prices from April after the cap changes.

Click here for the EV tool and here for the heat pump one. These are only estimates! If you find them useful let me know.

Will heating your house constantly use more energy?

We are advised when we get a heat pump to change the heating schedule to be constant, or nearly so. This is because heat pumps are efficient when supplying gentle heat but not good at heating a home from cold quickly. This is completely the opposite of what we have learned about keeping our bills low when using a gas boiler. So how much are we currently saving, and how is this different with a heat pump? I have investigated this with a model of a semi detached house (using similar models to my work for BEIS [1]). In the boiler case, savings from intermittent heating are substantial - up to 21%. In the heat pump case, the difference is much less - at most 4%. 

Scaling up heat pump installation – counting the benefits

It is generally recognised that reducing carbon emissions is going to mean lots of heat pumps (or other electric heating systems) installed in homes to replace gas and oil boilers. However, heat pumps are (currently) more expensive to run and to install and not many households have been persuaded so far. The government is running a public consultation now on a market mechanism to increase the install rate [1]. By their own assessment this policy has a net social cost of £0.6 billion over 4 years. So how can this be sensible? Or is there a fundamental problem with the cost benefit analysis? - I think there is.

This chart illustrates the balance in costs and benefits – it is quite finely balanced.

Data from the consultation 'A market based mechanism for low carbon heat' cost benefit analysis [4]

Review of the Heat and Buildings Strategy

Last month the government announced the long awaited Heat and Buildings Strategy [1] which sets out plans to convert this sector to net zero GHG emissions. This came out almost at the same time as the wider Net Zero strategy [2]. One aspect I was particularly interested to see was the plan for hydrogen in heating – there is still indecision in this area we will have to wait until 2026 for more certainty. Another aspect is the balance between electricity and gas energy costs, also support for retrofitting heat pumps into homes and building up the supply chain for retrofit. There are definitely some good things in this policy but some serious gaps.

Scenarios: high hydrogen, high electric or in-between? 

The future role of hydrogen is still uncertain and the Net Zero Strategy refers to three scenarios for heating in the future. In all cases, heat pumps have a very large role. As well as the net zero in 2050 target there is an interim target to reduce GHG emissions from this sector by about two thirds by 2035.

By 2035, the target is for 13 million homes to be on low carbon heating of which two million will be on heat networks. For the rest there are different scenarios. If hydrogen does not work out, almost all the rest will be on heat pumps. If hydrogen does work out, then we can expect up to 4 million on hydrogen gas by 2035 and 7 million on heat pumps. Or, it could be something in-between.

Whatever happens, at least a third of homes in the UK need to be heated by heat pumps by 2035 – a lot more than on hydrogen.

How much does it cost to install a heat pump?

The recently announced Net Zero Strategy includes a boiler upgrade scheme with a £5000 grant towards installing a heat pump. How does this compare with actual heat pump costs? To be fair, you should bear in mind that when you upgrade your boiler you only need a new boiler slotted into the same space as the old, whereas when you convert to a heat pump there are additional one-off costs for plumbing and other work. So subsequent heat pump upgrades will cost less. Still, there is no doubt that heat pumps cost more than just replacing a boiler, which is usually £2,000 to £3,000. Here are some top level estimates for a heat pump installation – as you can see they vary greatly We will break this down in a minute.

Energy Savings Trust (2021)	£7,000 - £13,000
Renewable Energy Hub (2019)	£5,000 - £8,000
EDF Energy (allows 20% for installation costs)	£5,000 to £10,000
Heat Pump Retrofit in London (Carbon Trust, 2020)	£7,000 (3.5 kW) - £11,000 (11 kW), mean £8,800
Cost of domestic heating measures (Delta EE, 2018)	£9,000 (8 kW) - £15,000 (16 kW)
Development of trajectories for residential heat decarbonisation to inform the Sixth Carbon Budget (Element Energy , 2020) – excluding the fabric upgrades.	£10,000 (mean).

Turning down the radiators in unused rooms

To reduce our heating bills we are often advised to turn down radiators in rooms that we are not using. However, this can be a bad idea if you have a heat pump. The adjacent rooms leak heat into the colder room which means the working radiators have to work harder. This is generally OK with a boiler but not with a heat pump which gives better efficiency at low temperatures. With the radiators working harder they need to run hotter which often means you end up using more energy rather than less [1]. I have done some modelling to see what this effect looks like. The savings on the overall heating demand is probably smaller than you might think – typically 3.5-5.5%. On the other hand the impact on the radiator heat demand surprisingly high – 20% or more.

Fabric first is not the cheapest path – is it the best?

Greater Manchester has declared a climate emergency and set itself a target of delivering net zero housing stock by 2038. Now they have published a report detailing what is needed to get there. This shows that the fabric first approach is not necessarily the cheapest or even the fastest strategy to decarbonise houses in Manchester. However the authors still recommend it for a variety of reasons. They make some very good points, although there are going to have to be some trade-offs made in practice.

The report has been prepared by a consortium of very respectable consultants: Parity Projects, Energy Systems Catapult, ADE research and Bays Consulting. Their findings are in line with the results of work I have been involved in for BEIS (Cost Optimal Domestic Electrification - CODE) but frustratingly this is still not published - and nor is the government’s Heat and Buildings Strategy, probably delayed due to concerns over cost [2]. In any case, the Manchester report covers policy as well as costs.

The nub of the problem is illustrated by this one chart, representing an ‘average’ house. The height of the coloured bars shows carbon emissions, the yellow coins at the top show the capital costs and the black diamonds show annual energy bills.

Chart from the Manchester report [1]

Decarbonising your home or car - which saves more?

The global carbon budget is extremely tight and we all need to do what we can, as soon as we can, to reduce the carbon emissions from our home and other life choices. I was recently asked – will I save more by replacing my gas boiler with a heat pump or replacing my car with an EV? I was surprised to find the results were fairly well balanced. Which saves most depends quite a lot on how much gas you use and how much fuel you typically use in your car. Here are some typical figures and example calculations. 

Also there is at least one other factor to consider: lock-in. If you buy a new gas boiler today you probably will not want to replace it for 15 years. On the other hand you might buy a second hand car and expect to replace it in 5 years. So the decision on a new gas boiler might have more impact over the lifetime of the boiler even if the annual carbon savings are higher for the car.

Field tests on thin insulation for internal wall insulation

There is a new report out: Measuring Energy Performance Improvements in Dwellings Using Thin Internal Wall Insulation [1]. Researchers at Leeds University performed field tests on six kinds of thin internal wall insulation: PIR, aerogel, and EPS (all laminated onto a board), cork render, latex rolls and thermo-reflective aerogel paint. They were all compared to a typical thickness of phenolic foam insulation - the conventional option. The products were installed on three different solid-wall houses (two systems tested in each). The team measured U-values (thermal conduction through the wall) directly and also the impact on air tightness. They used simulations to study the effects on damp and risk of frost damage. They recorded costs of installation, separating out the costs of materials and labour and decorative finishes. They interviewed the installers to find their opinions - you very rarely find these reported! Here I describe the main results. 

A tradeoff between cost thickness and performance.

Reduction in heat loss (percent reduction in measured U-value) obtained from Phenolic foam board and six types of thin insulation. Paint: thermo reflective aerogel paint, Latex: latex roll, Cork: cork render. The thickness is the total thickness including air gap and plaster skim, where appropriate. Data from [1]

Renewable heat installation rates

A friend asked me if I think that heat pumps are still a 'niche' technology or are becoming mainstream. She was surprised by the number of companies able to install in this area - the MCS website finds ten within 30 miles of Cambridge. So I took a look at the statistics for take up of the renewable heat incentive for heat pumps (domestic). Here is a chart showing cumulative installations.

Domestic Renewable Heat Incentive cumulative applications by month, data from [1]. The steep rise up to April 2015 includes legacy applications.

Energy Ratings for heat pumps - what do they mean?

Example energy label from [1]

Do you check the energy labels when you buy appliances such as washing machines or refrigerators? I hope you do. To make comparisons easy each product has  a rating, which is generally between A and G except for some appliances it goes up to A+++. This is helpful only if you know what the best or typical rating is. In any case, the rating does not actually tell you what the efficiency is or typical energy use would be - hence my advice has always been to look for these on the energy label. I would love to be able to say the same for heat pumps but there is nothing like that on the energy label for heating appliances. The rating is the best we have to go on. Here is a picture of the design of the energy label for a heat pump with integrated hot water cylinder, so it supplies hot water for the taps as well as for the central heating system. To understand what this means I have been reading the legislation behind it [2].

Is the gas industry promoting uncertainty as a delaying tactic?

We have to decarbonise our heating systems, but there are competing options as to how this is best done: electric heat or green gas. Whatever happens this is going to be very disruptive for industry sectors including oil and gas supply and appliance manufacturers - some very large corporations face an existential threat. So it is hardly surprising that the gas industry is lobbying our government intensively, arguing that green gas is a practical, inexpensive alternative to electric heat. It is not clear that they have successfully driven policy decisions in their favour, but it is possible that they have achieved delays to measures that are needed urgently. Or are ministers simply putting off decisions that they believe will be unpopular? As one consultant said (of decarbonising heat policy): Anything that puts people’s bills up is a big issue. And anything involving intervening in people’s homes is a big issue. You’ve got them all. It’d be an absolute car crash. [1]

This blog post is inspired by two journal papers by UK academics Richard Lowes and Bridget Woodman from the University of Exeter, and Jamie Speirs from ICL. You can find them here:

[1] Disruptive and uncertain: Policy makers’ perceptions on UK heat decarbonisation (Energy Policy) 2020

[2] Heating in Great Britain: An incumbent discourse coalition resists an electrifying future (Environmental Innovation and Societal Transitions) 2020

As academic papers go, these are very readable. I particularly like the extensive quotes in the first one, one of which you have read in my first paragraph. Both are based around interviews with people who have influence over policy - from Ministers and civil servants to industry reps, 3rd party consultant and NGOs.