How to heat your home with the minimum amount of energy, and save the planet
There are many different motivations for a householder wishing to consider measures on their home to improve its energy performance: reduce energy bills, improve health and comfort, reduce the carbon footprint of heating, to name just a few.
In this context, the term ‘retrofit’ refers to any improvements to a building to reduce heat loss from a building’s walls, roof, doors, and so on; what are collectively termed the ‘fabric’ of a building. But the term is much broader than this, and can be applied to technology to remove moisture and stop condensation issues, and to alternatives to a gas boiler to provide a source of heating.
The media talk about retrofit in negative terms: “it’s too hard”, “there is a lack of skills”, etc., but there are many simple measures that can bring great benefits, like increasing loft insulation to current standards. People experience benefits from having homes that are comfortable and healthy to live in, during the winter and summer, and with reduced bills. There are plenty of experienced people out there who can advise on cost effective measures.
Despite the fossil fuel industry’s PR that gas is ‘clean’, the truth is that domestic heat accounts for 13% of the UK’s annual emissions footprint, and for the majority of us, that is the result of burning natural gas in our home boilers.
Fabric first
The mantra amongst retrofit professionals is ‘fabric first’, and that is right [1]. We need to reduce the heat loss from the UK’s leaky housing stock, whatever source of heating that is used. But the heat loss can never be reduced by 100% and in the majority of cases it is unlikely to be better than a 50% reduction.
So what do we do about the 50% of the carbon footprint that remains?
The answer is simple: we need to get off gas. It’s that simple. According to the Committee on Climate Change, heat pumps will play a major role in achieving this move away from gas (and other fossil fuels) [2], but what is a heat pump?
A new study from the Department of Business, Energy and Industrial Strategy (BEIS) states:
'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.'
What is a heat pump?
You have one in your home: it is the fridge. Heat pumps use a substance called a ‘refidgerant’ that is very good at absorbing heat, and through a clever process invented in the 19th Century, raises the temperature by using a compressor (which you can hear when the fridge is working). This moves the heat from one place to another. In a fridge, the heat is moved from inside the fridge to the panels at the back (which feel warm). For heating homes, the term ‘heat pump’ is used for a system that does this in reverse: It moves heat from the environment outside the house to inside the house. The heat outside the house comes from the Sun that warms the ground and air. Just as a fridge keeps moving heat even when the inside of the fridge is very cold, so does a heat pump designed to heat a home: It continues to work in the middle of winter.
Heat pumps are not an optional extra on top of ‘fabric’ measures, they are an essential and equal partner in the endeavour to reduce the carbon footprint of heating. Reducing heat loss and changing the source of heat are two sides of the same coin, but while most people have some understanding of fabric measures such as loft insulation they will admit to being a little mystified by heat pumps. An important point about heat pumps is that while they need electricity to power them, they create at least 3 times as much heat energy as the electrical energy put into them [3].
Dispelling myths
There is often a narrative around heat pumps that repeats many myths about them: “they can’t heat older buildings”, “they need backup heating in winter”, “they are noisy”, etc. (something we explored in a previous blog ‘What if we could all heat our buildings with renewable technologies?’). These myths are often based on anecdotal experiences that predate modern installation standards and technology.
One of the problems with the debates heard around retrofit in general is that it is easy to make bold statements that are not always backed up by numbers. This is important because the numbers translate into pounds Sterling and kilograms of carbon dioxide.
The report Reinventing Retrofit by the Green Alliance makes clear why we must address the two sides of the decarbonising heat coin:
If every UK home had cost effective, conventional energy saving measures installed, energy use would fall by 25 per cent. Current UK ambitions are about half this, so would reduce energy use by about 12 per cent.
If the full technical potential of conventional measures was realised, regardless of cost, energy use could be reduced by about 53 per cent by 2035.
The report goes on to describe the Dutch Energiesprong measures, that includes ambitious fabric measures, heat pumps and solar energy, not in competition, but working hand in hand.
What can I do?
But what is a UK householder to do - without a similar level of ambition and investment from our Government - faced with the desire to reduce the heat loss of their building and get off gas?
If one was to do just one thing to reduce the carbon footprint from heating our home, then replacing a gas boiler with a heat pump is the single most effective measure available by a country mile [4]. But clearly, it makes sense to reduce the heat loss whatever the heat source! The question is only how much one does, that is practical and affordable, and finding the right balance between benefits and costs.
Given the relative costs of gas and electricity, and even with the greater efficiency of heat pumps, the retort to this is often ‘but the running costs will be higher, so that despite the carbon savings, people will be put off installing a heat pump’. Maybe, maybe not.
Let’s take a real world scenario.
An old house with solid walls has a 23 year old gas boiler that is 70% efficient, poor loft insulation, drafty doors and windows, and poor thermostatic heating controls [5]. The good news is that the house has been well maintained - lime mortar for Cotswold stone walls, gutters and downpipes that are kept clear, and so on - all helping to reduce heat demand (poorly maintained or damp walls and roofs will increase heat loss).
Let’s suppose a householder now upgrades the loft insulation to current standards, and fixes the worst of the drafts (weather strips around doors, brushes for sash windows, etc.), and better controls will come with the new heating system, and this might well lead to a 20% reduction in the building’s heat loss.
The householder wants to maximise the reduction in their carbon footprint, so what if they replaced the old boiler with a modern air-source heat pump (ASHP) that is installed by a professional MCS certified firm (including radiator upgrades where necessary); what impact would that have on heating bills?
Pre-retrofit, the householder would be paying (at current rates) 3p per unit of gas, and because of the 70% efficiency of the old boiler, they would be paying 4.3p per unit of heat delivered/ required [6].
Post-retrofit (with the retrofit measures, including a modern heat pump), the household would effectively be paying 5p per unit of heat delivered but the effective cost is reduced to 4p because of the 20% reduction in heat loss [7], which suggests the running costs will be no worse and even slighter better.
So it is simply untrue that a householder cannot find ways to maintain current running costs to heat their home when combining affordable fabric measures along with replacement of a gas boiler with a modern Air-Source Heat Pump (ASHP). And this is achievable despite the current ratio of unit prices for energy from electricity and gas (and this will in the future be less skewed towards fossil fuels that are currently heavily subsidised).
If in addition the householder has some solar PV (PhotoVoltaic) installed, it can help to power the heat pump. Now, while the season when the heat pump works hardest is in the winter when solar PV will deliver the least amount of energy, in the ‘shoulder seasons’ (Spring and Autumn) heating is still needed and the solar energy will provide a significant contribution over the year (perhaps reducing reliance on the grid by as much as 25%).
A ‘whole house’ approach
There are several other topics to explore in the world of ‘retrofit’ in addition to the ones already mentioned.
For example, when there are moisture issues, we may need mechanical ventilation and heat recovery (MVHR), particular where an older building becomes less drafty after measures to prevent leaks, and moisture problems result.
When we need to reduce heat lost from windows, we can use secondary glazing as an alternative to completely replacing them (which may not be an option in a conservation area).
The message is that we need a ‘whole house’ approach to retrofit, with fabric first, but not fabric only, and a good dose of pragmatism in exploring options.
For individual householders - especially those that are wishing to reduce the carbon footprint of their heating - heat pumps, and particularly air-source heat pumps, will need to be considered alongside affordable fabric and other measures.
Perhaps the new mantra should be “Fabric first, but not only”
. . . o o O o o . . .
Notes
[1] The report Reinventing Retrofit by the Green Alliance makes it clear how important fabric retrofit measures are:
To meet its climate targets, the UK has an ambition to retrofit all homes to EPC band C standard by 2035. But only 29 per cent of homes today meet this standard, and the UK’s current policy approach is nowhere near ambitious enough to tackle the remaining 71 per cent. In fact, energy efficiency improvements have stalled, measures are expensive, industry is underinvesting and householders still find retrofits a major hassle.
So it is imperative that householders seek out those with the knowledge to help them overcome this ‘hassle’. There are no shortage of excellent organisations working to disseminate and share that knowledge, including AECB, Carbon Coop and The Retrofit Academy, to name just a few.
[2] The Climate Change Committee’s Sixth Carbon Budget states that
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.
Some have estimated that by 2030 more than 600,000 household boilers per annum will need to be substituted for heat pumps to meet the Government’s net zero goals. This is a major undertaking.
Note that the ‘district heating’ refers to the method of distributing heat, not the source. The source of that heat might well be, in many cases, a large heat pump (air, ground or water sourced).
[3] As already explained, heat pumps used to heat homes are like fridges in reverse, and most people are happy to use a fridge without really knowing how they work. Why not with heat pumps?
Put simply, heat pumps harvest energy from the environment (heat that ultimately comes from the Sun), and use it to heat a house, moving it from outside a house to its inside (both for space heating and water heating).
A useful not too technical, fuller explainer is provided by Pia Balaam at Your Energy Your Way in a blog How does an air source heat pump work?
While they use a lower ‘flow temperature’ than with a gas boiler system it is more than enough to do the job, because this is compensated for by having (typically) fatter radiators with more surface area.
For every 1 kiiowatthour (kWh) of electrical energy supplied to the pump, it generates at least 2.5 kWh of heat energy (that is described as a Coefficient Of Performance (COP) of 2.5). Over the year, the average is now typically higher. A modern ASHP can achieve a Seasonal COP (SCOP) of at least 3; sometime this is described as an efficiency of 300%.
There are air-source (ASHP), ground-source (GSHP) and water-source varieties of heat pump.
In ‘Retrofitting Domestic Heat Pumps – methods and lessons from Ireland’ (Carbon Coop, 2020), Paul Kenny described a trial in Ireland with a large number of ASHP installations designed to provide sufficient heat for a -3C winter. They performed extremely well even during the ‘Beast from the East’ with -6C for two weeks, and the COP never dropped below 2.5! Despite it being received wisdom in many circles that supplementary heating is required in such circumstance, none was required.
Whereas a gas boiler will take 1 kWh of primary gas energy and never produce more than 1kWh of heat energy (for a modern boiler this can be 0.9 kWh which is also described as 90% efficient; but much less for an older one).
[4] This was explored a in blog ‘Are Air-Source Heat Pumps (ASHPs) A Silver Bullet?’ showing that ASHPs are far more cost-effective than GSHPs and a less complex undertaking. The current blog tries to explore this from a ‘fabric first’ perspective. The Committee on Climate Change also see a big role for so called ‘district heating’ which refers to a community heating source distributed to a number of dwellings such as flats (even here, the source will in many cases be a heat pump, albeit a shared resource). This blog is focused on individual households.
[5] Heating controls can mean that too much energy is being used to heat a home. For example, with the old gas system, a single thermostat in a cold hall could mean that a sitting room is heated more than is needed. Better controls can optimise the use of energy. Heat pumps have smart controls but not in the way people are used to. They compensate for changes in the weather and adjust their operation to be as efficient as possible. Whereas with gas boilers we are used to programming them to come on with a big burst of heat in the morning and again in the evening, with a heat pump it takes longer to go from a cold to a warm house, but that is why they are designed to stay on longer and avoid the peaks and troughs in temperature that we have become used to with gas. This makes the home heating more comfortable, not less. A professional installation will tune the system to home’s needs (e.g. depending on whether it uses radiators or underfloor heat, the flow temperature can be adjusted).
The householders job after this is NOT to fiddle with the system - with thermostats, programming, etc. - and to ‘unlearn’ the behaviour we have adopted with gas boilers. If the system is not operating as expected, then get the installer back in to fix the tuning.
The other tip is NOT to put your hand on the radiator to judge whether it is working! A gas system sends water at around 70°C to radiators, and we have got the habit of feeling radiators. With a heat pump the system operates at lower flow temperatures - the lower the better if higher efficiency is desired. So the radiators might be anywhere between say 35°C and 55°C. But the only readings that matter are the ones from a thermometer in the room. If the design temperature for the room is 21°C and that’s what the thermometer says, then job done! Even if the radiator “feels cold”; remember a human’s normal temperature is 37°C, so 35°C 'is bound to feel cold in comparison. Whereas 35°C is a lot greater than the target of 21°C, so we can see how ‘common sense’ can be a poor guide to judging if the system is working OK.
[6] The 70% efficiency of the old boiler means energy is wasted so the net cost of a unit of ‘heat energy’ delivered is greater than 3p (the current unit price for a kWh of gas), so 3p/0.7 = 4.3p per kWh of heat delivered/ required.
[7] The costs of electricity to run the heat pump (at current 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. But remember we need 20% less heating due to the fabric and heat control measures, so we would paying effectively 4p instead [8] for the heat required, which is less than the old gas running cost [9].
UPDATE The energy crisis that is impacting UK in Autumn 2021 is raising the price of gas, and is due to demand outstripping suppy and other reasons. This is without a carbon tax on gas. So the ratio of the price of electricity to gas could come down. There is also talk of the extra ’environmental’ levies that the Treasury and regulator had strangely put on electricity will be put on gas instead (or some of it, or removed altogether). In any case with the increasing need to ‘electrify’ both heating and transport, and the increase in renewable electricity generation, the time should not be far off when electricity is no longer penalised but gas will be (although, the Government needs to protect the poorest in our society).
[8] And if the electricity used to power the heat pump comes from renewable sources, the householder can have a very small carbon footprint with a lower running cost. Even if taken from the electricity grid’s mixed sources, this is becoming increasingly lower in its carbon intensity. So a heat pump gets greener every year, without the householder having to do a thing.
[9] Of course, some might argue that the householder can upgrade to a modern gas boiler, with a 90% efficiency which would bring the cost down to 3.3p (3p/0.9), and with the benefit of the 20% heat demand reduction, an equivalent figure of 2.7p. But there are a few reasons why someone might choose not to stick with gas:
they are very concerned about the climate emergency and want to do their bit;
given that with the little bit of fabric now completed, a householder can stop the running costs being higher than pre-retrofit, that may be enough for most people;
they can see signs that the price differential between gas and electricity could change, perhaps sooner than many think (and the ratio does not need to change very much to swing the running costs back in favour of a heat pump);
in the future, gas may be a liability for a home, and harm its resell value - more and more buyers will see a well functioning installed heat pump as a bonus.