Well, should I get a heat pump?

Above: Ian McKay at the Lewes Climate Hub talking about heat pumps and what you might need to do to your home to successfully install one.

2023 may turn out to be the year when heat pumps went from being considered in the U.K. as niche heating solutions to becoming a major player in the domestic heating market. Ian McKay was heavily involved in the month-long series of events and exhibition held at the Lewes Climate Hub in November and December 2023. It culminated with a talk he gave entitled, ‘Should I get a Heat Pump?’ and was in fact an updated version of a joint talk McKay gave earlier in the year to another local audience. The talk looks in detail at some of the measures you might need to carry out to your property as well as outlining the pitfalls and opportunities of eco-retrofit and heat pump technologies.

You can watch the edited highlights of the talk, which were filmed and produced by the award-winning film maker, Rowland de Villiers by following this Vimeo LINK.

Hunting for a cheap to heat house?

Above: The award winning eco-retrofit of a 1960’s built timber frame house in Lewes, East Sussex as shown in a sectional perspective. For a budget of less than £70K including solar panels and VAT, the house was made virtually carbon neutral and almost energy cost neutral. As a mid-terrace flat roofed timber framed house, it was an ideal candidate for cost effect eco-refurbishment.

With the advent of the new PAS2035 – Specification for the Energy Retrofit of Domestic Buildings, the UK is having another go at mass eco-retrofit. Here are some top tips towards a successful eco-retrofit starting with what kind of property you should try and find if you fancy having a go.

Don’t stretch your budget too thinly

It is fair to say that the most common enquiry an architect will receive in the private residential sector is a couple who have bought a house and they want to add an extension and they want to make the rest of the house more energy efficient. Nine times out of ten, their budget will be insufficient to do both elements well and normally they end up just making the extension and the house remains expensive to heat. Everyone wants more space of course and whilst people can afford their energy bills, they will likely go for the extension over the eco-retrofit when push comes to a budgetary shove. Perhaps in the energy starved future ‘adding value’ to a house in real estate terms will not just be about how many additional rooms you may have added but to demonstrably show how energy efficient and cost effective the building is to live in.

The less surface area the better

A terrace house is one of the most cost effective typologies you can get as you only really need to treat the external facing elements of the ‘thermal envelope’. It is even better if it has a well orientated (within 30º of due south) pitched roof facing south either on the front or garden elevations.

The humble mid-terraced property is your best bet because you simply have less external wall to insulate. However, be careful of terraced housing with large rear extensions (also known as ‘outriggers) – with their shallow but long floor plans and lots of external walls they are particularly parky in winter. If you have a detached, semi-detached or end of terraced property, you will have more external surface area to treat.

Steer clear of charming but fussy architectural detailing

Sadly, if the property has lots of charming but fussy architectural articulation, this will likely be expensive to deal with when it comes to insulating the walls. If it is on the outside you may also find the local authority are not too keen on you changing it or covering it over. If you are insulating on the inside, you may override the charm of the period features or need to have them reinstated with a small loss of floor area.

Eke out a timber frame

Importantly for a refurbishment, it is usually easier to carry out an energy efficient refurbishment on a timber frame building than on a masonry one and the key to that is ensuring you keep the constructions BREATHABLE. However, in the UK there has been a lot of predominantly unwarranted resistance to timber frame construction. It is true a lightweight timber frame building can be prone to overheating if the glazing is poorly protected against too much unwanted solar gain and/or the structure is under insulated. That said it is quick and easy to heat up.

There are a lot of people who espouse the benefit of heavy thermal mass when it comes to energy efficiency – solid brick, stone, blockwork or concrete will give you that. The idea being is that once you’ve loaded up the internal fabric with the heat energy or ‘coolth’ to provide thermal comfort, the building will not be too much affected by big fluctuations in outside air temperature between day and night. Of course, this also means you have to insulate to the outside of the mass really well, so your building does not hemorrhage the energy. This has to be taken with moderation. Too much thermal mass will give you internal condensation issues, particularly in the warm season – counterintuitive though it may seem. It will be interesting to see what the insurance industry makes of poorly conceived eco-retrofits of traditional brick buildings relative to well-designed timber framed houses in the years to come.

Try to avoid buildings where internal insulation is the only allowable solution

In many instance, insulating an existing external wall is only practicable from the inside. In this case it is solid brick.

When it comes to insulating external walls, it is always more efficient and technically safer to insulate on the outside of a property than on the inside faces. If you insulate the inside faces you will have the risk of creating stale moisture behind the cladding and this may lead to mould growth and possibly fungal attack of the building fabric. Conservationists should take note that if you insist on a brick or stone building being insulated internally, that stone or brick will stay colder and wetter for longer with an increased risk of frost damage. If you have timber joists socketed into the brick, these should be cut back and the load transferred to the sidewalls and that is expensive to execute. The best type of internal insulation is a breathable one. If you can get the moisture in the wall to transpire to the inside as well as the outside, it should be safer. The only other watch point is not to insulate internally too much. Super insulating on the inside of a masonry building will increase the risk of defects arising. The aim should be to just to take the edge off those ‘stone cold’ surface finishes. Calcium silicate boards, timber fibre insulation with lime plaster and cork linings or cork plaster can achieve just that.

Hunt for a property with plenty of access to the sun

Roof typologies which are well suited to collecting free solar energy.

If you want to take advantage of free energy from the sun, look for a property with plenty of sun. A south-facing slope with little overshadowing buildings or trees is ideal. Also check how the building is orientated with the sun. If you have large amounts of glazing facing south, then you can look forward to plenty of free passive solar heating in the colder months. South facing glazing is also easier to protect against too much solar gain in the warmer months with external blinds, shutters or even foliage. East and west are tricky as the sun is lower in the sky when it enters the glazing and easy horizontal solar shading solutions above the glass do not tend to work. Large amounts of west facing glazing are perhaps to be avoided or changed. Check out the roof and make sure it can accept plenty of solar panels with minimal overshadowing and predominantly facing south. Hips, dormers and chimneys are not great when it comes to an efficient array of solar panels.

Awkward roof forms will also tend to limit the amount and the efficiency of a solar panel installation.

Glazing

If heritage styling is important, remember that it can negatively effect the outcome of a property’s potential to exploit natural daylight and beneficial solar gains.

For years, double glazing was always seen as one of the biggest energy-saving features you could install on a property. Nowadays, almost all dwellings in the UK have double if not triple glazing so it is a case of diminishing returns if you replace knackered units. One good rule of thumb with glazing is to maximise glazing on the south side (as mentioned above) and have smaller penetrations on the east, west and north elevations. Some energy consultants also promote running with double glazing on the south side and having triple glazing on the other elevations. If the passive solar gain on the south side works well through the year then this makes a lot of sense as triple glazing will actually reduce the solar gain coming through. Triple glazing is more expensive than double glazing, but it may surprise many at how little extra it is.

Heating and power

In 2020 the electricity grid is only partially decarbonized. Until it is more fully carbon neutral, if your property has mains gas, you might be best off sticking with a high efficiency gas condensing boiler for a little longer. Heat pumps work off electricity and crudely they work a little like a refrigerator in transferring heat away from either outside air, the ground or a body of water and, with a bit thermo-dynamic magic, dump that heat into your space heating and domestic hot water tank. The technology is still very expensive to purchase compared with a gas boiler and, until the grid is more fully decarbonized, it is perhaps more carbon beneficial to stick with a gas boiler, although that argument will not be valid for much longer.

Solar thermal panels are good value. If you have room to mount a couple of panels on the roof and have a hot water cylinder in the house, you can get about 50% to 60% of your dwelling’s domestic hot water needs through a solar thermal installation.

Solar photovoltaic panels are the type that make electricity. That said, you need about 8 to 12 panels to have a worthwhile amount of power generated for a house. You also need what is called an inverter internally. This takes the DC power from the panels and converts it to AC power so it can go into the mains power supply and also be fed back into the grid. You can fit electric storage batteries or dump excess electricity into your hot water cylinder, though arguably that is a very poor conversion of energy.

In traditional central heated house, we have relied on a boiler using bio-mass or fossil fuel to provide the heat energy input to provide thermal comfort when the rate of heat loss through the building’s fabric in cold weather starts to accelerate.

In a super-insulated house the rate of heat loss is slowed down to such a point that in some instances it is possible to heat the building with body warmth, cooking and the odd electric heater. This is the principle behind the Passivhaus standard. To do this with a retrofit project is a big and possibly very expensive undertaking.

It is always worth remembering when thinking about heating and power, the so-called, ‘fabric first’ principle. That is to say, try and get the heating and power load of the building as low as practicable before you look at other technology.

Ventilation

It is important to ventilate your habitable spaces. Stale air can be unhealthy for the occupants. In winter, even if you have a well-insulated house, you still have to bring in cold fresh air. For this reason, you might want to consider mechanical ventilation. If you do a really good low carbon refurbishment, you should also try and eradicate unwanted air leaks through the fabric of the building – areas between external windows, doors and walls or service penetrations. If you can get the air leakage rate down to under one air change per hour (difficult to do) then you might want to consider mechanical ventilation with heat recovery. This expensive and very invasive technology may not be all that practicable to install in an existing building as you have to run air ducts to the habitable rooms, bathrooms and kitchen. It is however considered more energy efficient than passive ventilation. If your budget or the constraints of the project do not allow for this, ensure the glazing installed has trickle vents.

Low energy lighting

The advent of LED lighting technology has revolutionized how we light buildings in recent years. Even in the early 2000’s LED lighting was not really capable of being the main lighting source for a room. They just were not bright enough. All that has changed. They provide superb quality and quantity of light without flicker and incredibly long service life – no more changing bulbs. Best of all though is how much more energy efficient they are than the old tungsten filament lamps. You can light a three-bedroom house for under 200 Watts!

Climate change adaptation 

The experts say our weather patterns will change quite significantly over time. There will be longer and more intense periods of sun and rain that in turn will stress a building’s fabric yet further. Rainwater goods may need to be replaced with larger profiles to handle more intense rain events. Increased ultraviolet intensity will accelerate the degradation of some external materials. Older buildings may yet perform better than some of our more recent commercial offerings particularly in protecting the occupants against unbearable overheating. If many of our houses then require energy for comfort cooling in the summer as well as for heating in the winter, our efforts towards creating cheap to heat buildings will be environmentally meaningless.

Switching from a gas boiler to a heat pump

“A heat pump is in principle a refrigeration cycle operating in reverse by extracting heat from a low-temperature source and upgrading it to a higher temperature for heat emission or water heating. The low-temperature heat source may be from water, air or soil which surrounds the evaporator.”[1]

Introduction

Should I be switching my gas (or oil-fired) boiler for an electric heat pump? It is a question a lot of people are asking and the considerations involved in answering it are not entirely straight forward. For years, the adage has been, “if you are on mains gas, then don’t change to a heat pump”. As the electricity grid continues to reduce its carbon intensity, that argument is becoming increasingly tenuous and soon it will be just plain wrong advice both in terms of cost in use and relative global warming potentials. Let us have a look at some of the issues involved and compare how gas or oil-fired boilers and electric heat pumps work.

Gas boilers and heat pumps briefly explained

Most of us are familiar with a gas boiler. By definition, a boiler heats water for use in space heating and the domestic hot water supply. Very similar boiler designs are used to work off liquid propane gas (LPG) and domestic heating oil. Some are designed to work with a hot water cylinder whilst others, known as combination boilers, make the hot water for immediate use in the property. Most boilers in the modern era work off a balanced flue whereby intake air comes in around a sleeve containing the hot exhaust gases. The most efficient are condensing boilers whereby that exhaust gas provides something of a preheat to the cool water return pipe. In so doing it the water vapour in the combustion gases condenses and thus the name. These generally have rated efficiencies of up to 90% although the real-world figure is likely to be more like 85%. A typical installation cost might be between £1,900 to £3,000 depending on the make, range and power output.

The essential difference between a gas boiler heated home and an air source heat pump heated home is that the heat pump would struggle to make 70ºC water and as such the house itself needs to have a reduced rate of heat loss (usually requiring some thermal efficiency upgrades to the external building fabric) and having larger ‘heat emitters’ like underfloor heating and extra large radiators.

Heat pumps are electrically driven. Essentially they work very much like a refrigerator in that they take heat from one area and release it in another area. In most cases a heat pump for a building will transfer heat from outside air and concentrate it into a hot water cylinder inside the property. This would be an air to water heat pump. Air to air heat pumps are also quite common and often used in commercial premises. For those with deeper pockets and a context which allows for the physical interventions required, there are also ground source heat pumps and even water source heat pumps. In most cases, these more expensive units have lower running costs and you will see that quoted in the equipment’s declared ‘coefficient of performance’ or CoP for short. More on that later.

The main heat source varieties of heat pumps for buildings.

To do this thermal-dynamic alchemy, they use a refrigerant gas in a closed circuit and a compressor, just like a fridge. The electrically driven compressor has the effect of raising the temperature of the refrigerant so that even if say the air outside is 5ºC, it can raise water temperatures in the tank to around 40ºC via a heat exchanger. Amazingly a typical air source heat pump produces about three units of heat for every unit of electricity inputted.

Physically, a gas boiler for a private residential premises will be wall hung and can easily fit within a 600x300x900H mm wall cabinet so it can be seamlessly hidden in a kitchen, utility room or airing cupboard in a most unobtrusive manner. A heat pump on the other hand needs more thought about its spatial requirements. An air source heat pump has an external unit about the size of an old pavement mounted telephone exchange, typically 1000x300x1000H mm. The hot water cylinders which work with heat pumps also have extra kit on them so possibly a little larger than people are used to. With ground source heat pumps there is no ‘external unit’ as such but you still have to have a hot water cylinder and a circulation pump unit which is usually floor standing and about the size of a large fridge freezer.

For a private residential project, the cost of an installed air source heat pump (ASHP) might be in the region of £10K to £13K whereas the cost of a ground source heat pump (GSHP) is typically around £25k - £40K depending on the size and type of installation.

Ground source heat pumps normally require a significant amount of external space to be created. This is needed either for the borehole typology whereby the heat is extracted with the ground down to perhaps 100+m below the surface. Alternatively, there is a horizontal type with relatively shallow excavations needed to install a pipe working just out of reach of winter frost ground penetration. Generally, this type needs a very large external area to work within.

Operationally, probably the most important difference between a heat pump and boiler is that the later works with an ‘energy dense’ fuel source such as gas or oil and has no difficulty at all in raising water temperatures to around 70ºC. A heat pump, on the other hand, will struggle to get water temperatures above 40ºC. The knock-on effect is that it is usually necessary to increase the size of the heat emitters in your home (ie. Radiators) or put underfloor heating in. Furthermore, if your house has single glazing and is poorly insulated then you might struggle to have a heat pump at all. As a rule, you should reduce the rate of heat loss in your home to a point whereby a heat pump can efficiently maintain thermal comfort conditions.[2]

When is the right time to switch to a heat pump

So, the key message is – if you do nothing else, make enough fabric improvements to switch to a heat pump and avoid putting in fossil-fuel systems at all costs.

LETI, LETI Climate Emergency Retrofit Guide, October 2021. Pg. 9[3]

This illustration shows relative costs of eco-retrofit investment with running costs over time showing the cumulative effect of each measure. This is set against retention of a gas boiler, the fuel source of which, is likely to be subject to more cost inflation as global supplies diminish. The more energy efficient the building, the more protected the occupants will be to rising energy costs. Projections beyond 2022 (Year 1) are speculative.

The big question is, when is the right time to make the switch to a heat pump? If you already have a well-insulated house, it might be that you should crack-on and get one put in soon. If not, best to wait until you have made at least some basic thermal efficiency improvements.

There are two other issues to consider. One is the cost of the fuel source and the other is the relative environmental impact which is usually expressed as ‘CO2e’. It is a measure of global warming potential.

According to the Energy Saving Trust who do a useful online comparison, as of October 2022 it is still slightly cheaper to run an A-rate gas boiler over an ASHP, but cheaper than almost any other form of heating.[4] Curiously, domestic heating oil is now about the cheapest form of heating whereas in the early 2000’s and within the space of a few years, its cost increased over 400% and led many off-grid homeowners to carry out deep retrofit makeovers of their homes. However, if you want to do the right thing for the planet and reduce overall carbon emissions, an ASHP is already rated better than all other heating typologies.

This illustration depicts how gas boilers are no longer as carbon beneficial for providing heating as heat pumps. A similar tipping point is shown for the relative fuel cost. Note the effect of the war in Ukraine. Projections beyond 2022 are speculative.

According to provider Bulb, UK electricity production went from 0.233 kg of CO2e per kWh in 2020 to 0.193 kg in 2022.[5] This reduction is down to more renewable sources of energy production, like wind turbines and solar panels, coming online and increasing their share in the national electrical grid. This is known as the ‘carbon intensity of the grid’ and it is important as, up until recently, it was more carbon beneficial to have an efficient gas boiler doing your home heating rather than a heat pump. As recently as 2014, a respected information source on eco-retrofit stated, “With the current mix of fuels for electricity production, heat pumps result in approximately the same fuel costs and levels of emissions as heating by gas-fired condensing boilers, so it is not appropriate to install heat pumps in dwellings that have mains gas supplies.”[6]

That is largely because for every unit of grid supplied electricity coming out of your plug socket, about three units of power are required to make it back at the power station. It is to do with losses in conversions and the power being bumped-up and then bumped-down as it is passed around the grid’s distribution infrastructure. To illustrate the point, think how much more efficient it is to burn a unit of gas to make heat in your home in a boiler than burning the same unit of gas at a power station to make electricity which you then use in your home to make heat.

So, whilst it has very recently become more carbon beneficial to heat your home with a heat pump than a gas boiler, we are approaching a similar tipping point whereby it will soon be cheaper in running costs as well.

Shifting sands and diminishing returns

We have already looked at how the carbon intensity of the grid has been lowering and thus making the case for switching to a heat pump stronger. It is also useful to check what forms of Government subsidy might be available for an ASHP installation. Sadly, there is very little financial help left with the Feed-in Tariff scheme ending back on 31 March 2019 and the Government’s reduced VAT rate of 5% is no longer available to all income groups for eco-retrofit work. Happily, as of October 2022 there are still grants available for ASHPs under the Boiler Upgrade Scheme of £5,000 and of £6,000 for GSHP. It is always worth doing fresh research on this when you make your evaluation.

Curiously, if you have shelled out tens of thousands of pounds on eco-retrofitting lots of insulation and replacement double or triple glazing to your house, to then fit an ASHP will in effect constitute a diminishing carbon v cost return. Say you reduce your heating demand by 75% with thermal efficiency upgrades, the heat pump can then only provide an efficiency saving on the remaining heating demand which might be awkward for those who fuss over pay-back periods.

Another adage which might be about to be challenged is the ‘fabric first’ approach to low carbon refurbishment. It can be shown that to achieve a given carbon saving, you can do it more cheaply in capital cost with an ASHP than with a thermal upgrade of the external building fabric. The problem with this argument is that as fuel prices continue to rise, a relatively uninsulated building with a heat pump is going to be far more exposed to unaffordable heating bills than a well-insulated one. Relatively, in a few decades time, it might be more costly to do a low carbon refurbishment than it is nowadays so perhaps ‘fabric first’ still makes sense.

Is a heat pump noisy?

Refrigerators have been around for over a century now but heat pumps, which use a very similar form of operation, are perceived as new technology. It is fair to say that there are plenty of misconceptions about them, particularly around the issue of how much noise they make.

For most of the year the external unit, which in effect is a big box with a fan inside, works pretty much near silent. In fact, inaudible over most background noise conditions. Periodically, in cold weather or if it is having to make high temperature water, the fan must work harder and the noise of the air being moved across the heat exchanger will become audible from a few metres away. ASHPs also have a defrosting cycle which will need to run periodically for around half a minute or so on damp cold days. Let us remember though that a balanced flue boiler also works with a fan and this produces noise of similar intensity as an ASHP when working at peak.

Such is the concern over noise that on a recent Deeper Green private residential project, the Environmental Health Officer demanded an expensive noise assessment (costing the applicant an additional thousand pounds or so) be undertaken for a proposed ASHP before allowing a planning permission in a conservation area to proceed.

Who can install a heat pump?

There is no question that an air source heat pump is a more involved consideration for a heating engineer than a conventional gas boiler installation. It follows that not all installers think each job through as well as another and many are still on a learning curve. In 2022, it can be hard to find a good installer and some of the lead times for the equipment and products are considerably longer than for boilers. Perhaps not so obviously, an ASHP installer needs to produce a very accurate heat loss calculation. Because a heat pump works at a lower flow temperature, the margin for error in the heat loss calculation that a heating engineer should do, is much smaller than for a gas boiler. Boilers, with their energy dense fuel supply, can easily cope with slightly higher or lower heating demands in operational use. Therefore, ensure that you ask that a room by room heat loss calculation be prepared in accordance with best practice CIBSE guidelines[7] if you wish to pursue ASHP.

[1] Hall, Fred and Roger Greeno, Building Services Handbook, Routledge, Abingdon, 2017. Pg. 332.

[2] LETI, LETI Climate Emergency Retrofit Guide, October 2021. Pg. 9

[3] LETI, LETI Climate Emergency Retrofit Guide, October 2021. Pg. 9

[4] https://energysavingtrust.org.uk/advice/air-source-heat-pumps/#:~:text=Heat%20from%20the%20air%20is,transfers%20that%20heat%20to%20water.

[5] https://bulb.co.uk/carbon-tracker/#:~:text=With%20an%20average%20supplier%2C%20annual,kg%20per%20kWh%20of%20gas.

[6] Rickaby, Peter et al, An Introduction to Low Carbon Domestic Refurbishment, Construction Products Association / RIBA Publishing, 2014. Pg. 55.

[7]  CIBSE, Domestic Heating Design Guide 10th Edition, 2021. Section 3.

Acknowledgements: I am indebted to the observations of Neil Williams and Mischa Hewitt in helping to direct the research for this article.