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.