Ground Source Heat Pump Comparisons

Trying to find the best heating system for your property? Thanks to the efficiency of ground source heat pumps, running costs can be reduced by as much as 30 – 50% compared to fossil fuels. See how they compare to other heating systems:

•  Gas boilers

•  Air source heat pumps

•  Hydrogen

•  Hybrid heat pumps

•  Oil & LPG boilers

•  Geothermal heating

•  Biomass

Kensa Ground Source Heat Pumps Diagram for Self Builds and Renovations with Renewable Heat Incentive

What is the best heating system?

Ground source heating

A ground source heat pump delivers the best low-carbon heating. It’s an affordable and eco-friendly alternative to fossil fuels such as gas, oil and LPG, which typically contribute to carbon emissions, air pollution and ultimately, play a part in climate change and global warming.

Achieving a net-zero carbon future

Heating and hot water make up 25% of total energy use in British homes, equating to 15% of our greenhouse gas emissions.¹ To reduce the UK’s carbon emissions, the best heating systems are being encouraged by the government.

This starts with the ban of fossil-fuel heating in domestic new-build properties from 2025 – forming a part of the government’s plans to achieve net-zero by 2050. Gas and oil boilers are some of the fossil fuel systems that the government is banning from 2025.

What is a heat pump?

In brief, a ground source heat pump will:

See how a heat pump works

What are the advantages of ground source heat pumps?

The advantages of ground source heat pumps vs. other heating systems include:

  • One of the lowest in carbon and running costs
  • Can be used to provide passive and active cooling
  • No planning permission required
  • Quiet operation
  • Safe and easy to operate
  • Simple to install
  • No noxious gasses or risk of combustion
  • No point-of-use emissions such as Nitrogen Oxide (NOx), Sulphur Oxide (SOx) or Carbon Dioxide (CO²).

Having a ground source heat pump means the property is much more energy efficient and less reliant on fossil fuels.

Alistair Mackintosh, Selfbuilder

Gas boilers vs. ground source heat pumps

As gas is a fossil fuel, it releases carbon dioxide (CO2) – a greenhouse gas that contributes to climate change. To achieve the UK’s legally binding target of net-zero carbon emissions by 2050, it’s vital that fossil fuels, including natural gas, are phased out and replaced with renewable alternatives. Delivering a whopping 77% saving on emissions versus gas, see how ground source heating compares to the familiar boiler.

See gas vs ground source

Air source heat pumps vs. ground source heat pumps

Although both air source and ground source heat pumps are renewable alternatives to fossil fuels, they hugely differ in efficiency, running costs, carbon savings and durability.

Unlike air source heat pumps, the inherent efficiency of ground source heat pumps means they can deliver the lowest running costs of any type of heating system.

See air source vs ground source

Hydrogen vs. ground source heat pumps

The future vision of heating: heat pumps or hydrogen?

Both hydrogen and ground source heat pumps hold the potential to decarbonise the UK’s heating on a large scale. However, there are fundamental differences in the way these technologies are implemented and the choices made to reach net-zero carbon.

Although only prototype hydrogen boilers have been developed, some would like to see hydrogen replace gas – eventually delivering it through the existing gas network to help achieve net-zero carbon heating. As we stand on the brink of a radical shift in how we heat our buildings, it’s important to understand the differences between the options being considered.

Here, we compare the two technologies to show why ground source has greater potential to decarbonise heating; even better, how it is tried and tested and ready to deploy right now. The choice is not binary; we also observe how the two approaches could work together to help get the UK to net-zero carbon emissions.

What is hydrogen?

There are several different types of hydrogen:

  • Green hydrogen is produced through the process of electrolysis, using renewable electricity to split water into oxygen and hydrogen.
  • Blue hydrogen is produced using Steam Methane Reforming (STR) or Auto Thermal Reforming (ATR), which is extracted from natural gas. The CO2 released as part of the process is captured and stored.
  • Grey hydrogen is similar to blue hydrogen, where STR OR ATR splits the natural gas into hydrogen and CO2. However, CO2 is not captured – it’s released into the atmosphere.
  • Pink hydrogen is made via electrolysis, just like green. But it uses nuclear energy as its source of power.
  • Yellow hydrogen is also made using electrolysis, using only solar power.

Efficiency of hydrogen vs. ground source


This article suggests that using green hydrogen to heat buildings is roughly four times less energy efficient than using heat pumps, which can in fact produce three to four times the amount of energy that it consumes. Although impressive, even this calculation may be modest for ground source. Another study suggests that blue hydrogen can be 58% efficient, while ground source heat pumps have been proven to achieve efficiencies of 400%.²

Ground source

Ground source heat pumps can be 400% efficient, which means they can produce four times the amount of energy they consume from electricity. They do this by using a small amount of electricity to harvest a much larger amount of heat energy locally. The less renewable electricity required, the more likely the fuel costs will be lower for the household. It also means that the electricity production and distribution system has to deliver only a fraction of the energy required.

Efficiency increases even further when heat pumps are deployed at district heating level (known as Shared Ground Loop Arrays) where individual heat pumps are installed in individual homes and connected to a shared ground array serving the whole street or neighbourhood with ambient-temperature district heat – sometimes known as Fifth Generation District Heating. When sources of waste heat (e.g. data centres, supermarkets, solar, air conditioning) are incorporated, the efficiency of these heat pumps can exceed 600%. The near future of ground source heat pumps could therefore be ten times more efficient at turning renewable electricity into heat energy than using hydrogen.

Cost of hydrogen vs. ground source heat pumps


When looking at the cost of hydrogen, you must consider all of the costs associated not only with the hydrogen boiler itself, but the investment in the wider network: upgrading the distribution pipework, energy storage and the increased capacity of electricity generation needed compared to ground source heat pumps. This increased capacity will come from sources such as wind turbines, which will also require much more space.

It’s not easy or straightforward to convert the gas network to hydrogen. The production of hydrogen itself creates carbon dioxide as a byproduct, requiring large-scale carbon capture and storage (CCS) technology. CCS has only been demonstrated on a very small scale and will require considerable investment and costs to allow hydrogen to play a major role in the decarbonisation of heat.

There are also hefty costs associated with hydrogen production itself. In cases where hydrogen is extracted from other substances, such as methane, the extra steps involved in extracting the hydrogen would push fuel prices up compared to natural gas.

The case is slightly different for hydrogen produced via electrolysis, which doesn’t create carbon dioxide in the process. The electricity used to power the electrolysis must then be decarbonised via renewable energy generation. The costs for solar and wind power are falling but still need to be accounted for. The low efficiency of hydrogen compared to ground source means that a far larger investment in low-carbon electricity generation would be required.

Ground source

Ground source heat pumps running costs already compete well with mains gas but the installation and infrastructure costs are currently higher. These costs vary depending on factors such as the scale of the installation, available land area and how energy efficient the property is.

To reduce upfront costs for the householder, Kensa is working towards a model where the price of the ground array infrastructure is separate from the price of the heat pump. For example, the ground array could be funded by a separate entity in return for a charge connection fee. Mimicking the gas infrastructure, a consumer can simply ‘connect’ their pump to the ground array, only having to pay for the heat pump itself.

Ground source heat pumps can also be coupled with smart controls, making use of time-of-use tariffs to shift the heat pump’s consumption to when electricity is cheap and low in carbon. The energy can then be stored, ready to use in line with the household’s usual routine.


Unlike electric heat pumps and heat networks, the feasibility of using hydrogen for clean heat needs further testing and development. The practicalities and cost of safely converting or replacing existing networks and appliances to operate with pure hydrogen need to be fully evaluated.

Business, Energy and Industrial Strategy - Energy white paper: Powering our net zero future

Installation & deployment of hydrogen vs. ground source


The greatest attraction of hydrogen is that, hypothetically, it could have the potential to flow through the existing gas network – supposedly minimising change for consumers, replacing gas and decarbonising heat across the UK. It’s not as easy as it sounds though. Vitally, the greatest obstacle is that there is no proven blueprint for this conversion. In fact, the UK would have to pioneer this; as of 2020, not one place in the world supplied pure hydrogen to homes or businesses.³

A dedicated study into hydrogen and heat decarbonisation suggests it’s unlikely that zero-carbon hydrogen supplied via a repurposed mains network will be available for the foreseeable future. There are a number of reasons to support this: it could be a major delivery risk to incorporate a new infrastructure for an unproven technology on a large scale and it is unclear who will guarantee and accept liabilities for in-building gas pipework switchover.4 The mass deployment of hydrogen relies on numerous issues being solved – if we wait for the technology to be ready but all of these issues cannot be solved, the delay could be detrimental to the planet.

What happens in the next decade could be the measure of success or failure in achieving net-zero carbon. The longer we wait for hydrogen to create a solid solution, the closer we are to the tipping point.

Without further delay in the journey to decarbonisation, we see a great role for hydrogen alongside ground source heat pumps: to produce electricity and low-grade heat when the local grid is under strain. A hydrogen-powered fuel cell or generator could be connected to a Shared Ground Loop Array. This fuel cell can deliver electricity into the grid around, say, 10% of the time in rare cases when wind and solar output is low. As well as this, the waste heat that comes out of the fuel cell can be delivered into the ground array, which improves the efficiency of the heat pumps and reduces electrical demand all at once.

Rather than competing, the two technologies can in fact work together – with hydrogen improving the efficiency of the ground source heat pump system further and vice-versa. Hydrogen is going to be expensive to make; it makes sense to burn the expensive hydrogen during low renewable generation periods and only support the grid when it really needs it.

Ground source

Heat pumps are ready to be deployed now. Compared to gas, they can already reduce carbon emissions by 77% in properties. As we build more and more renewable electricity generation onto the grid, the carbon savings will continuously improve – and there will be no need to revisit the properties in order to reach net-zero carbon.

In Kensa’s view, the key to the mass deployment of ground source heat pumps is the ground array infrastructure. The low-carbon alternative to traditional district heating, Shared Ground Loop Arrays, can be implemented on a street-by-street basis, mimicking the gas network and connecting to heat pumps in individual homes. An ambient temperature circulates around the distribution pipework at -5°C to 20°C, with each heat pump upgrading the heat to the required temperature for heating and hot water.

This will need infrastructure and investment in the electric grid and in the low-carbon generating technology attached to it, but nowhere near as much as you might think. Grid balancing concepts such as local energy storage, grid-storing batteries and ‘load shifting’, which a ground source heat pump can do when coupled with smart controls, will reduce strain on the grid as more and more heat pumps are deployed.

As for the installation training itself, the skills challenge is surmountable and the skills of gas heating installers are certainly transferable to heat pumps. This is particularly true once you separate out the ground array district infrastructure elements from what happens in the house, in the same way as we currently do for gas infrastructure and boiler installation. All in all, both options present a challenge and change to the way we heat our homes, but it’s certainly worth the momentary disruption that is inevitable in the stretch to net zero.

Carbon emissions of hydrogen vs. ground source


Although hydrogen won’t emit carbon when used by a boiler, the production of hydrogen itself can emit carbon depending on the production method. For example, blue hydrogen relies on Carbon Capture and Storage (CCS) to capture and store the CO2 produced; the production of grey hydrogen involves releasing CO2 into the atmosphere, and pink relies on electrolysis with nuclear energy as the source.

Meanwhile, the method that produces green hydrogen – electrolysis – must be powered by renewable electricity from wind or solar farms rather than electricity produced by burning fossil fuels. The same goes for yellow, which solely relies on solar power. This shows the choice of hydrogen production is crucial, as it will define the amount of carbon emissions produced.

Even if the issues are solved, blue hydrogen is still a fossil fuel. There is a finite supply, meaning it’s a finite non-renewable form of energy. Therefore, only green hydrogen can be considered a truly renewable, low-carbon, sustainable and environmentally-friendly option for an energy transition.

Ground source

When placed in a district scheme with ambient-temperature pipework and waste heat capture, ground source heat pumps can be 600% efficient. High efficiency plays a part in carbon emissions because it means the majority of energy is absorbed locally from the ground, reducing how much energy is coming ‘down the wires’ from the electricity grid. The majority of the energy that comes from the heat pump is harvested locally from the ground.

Consequently, the only form of carbon produced is from electricity generation. However, as electricity continues to decarbonise through the increased generation of renewable energy – from sources such as wind and solar – the energy it does consume will also be renewable.

This is even more possible now through clever innovations such as load shifting. This means that, when coupled with smart controls, a ground source heat pump is capable of shifting or moving its energy consumption to times when the electricity grid is cheap and low in carbon thanks to renewable energy generation.

If blue hydrogen remains the gas supply industry's proposed end state, there is a significant risk that repurposing the gas grid using unproven technology at scale could prove less feasible than previously thought [...] This would leave little time to implement alternatives, and in the midst of a climate emergency, we should be leaving little to chance.

Carbon Offsetting – Friend or Foe? Max Fordham, as cited in Hydrogen: A decarbonisation route for heat in buildings?

Hybrid heat pumps vs. ground source heat pumps

A hybrid heat pump system, also known as a dual fuel system, couples renewable energy with fossil fuels to provide heating and hot water to a home. A hybrid can be made up of a heat pump and a fossil fuel heating system, but in this context, we’ll look at how a hybrid – made up of an air source heat pump component and a gas boiler component – compares to a single ground source heat pump system.5

Firstly, how exactly does a hybrid heat pump work? Hybrid systems can have flexible heating modes, which include:

  • Switch hybrid mode – where the entire heat demand for a certain period is met by the boiler if the heat pump component cannot meet the demand
  • Parallel hybrid mode – where the heat pump can contribute some heat during that period.6

A hybrid heat pump system is designed to meet the building’s heating and hot water requirements, whilst reducing carbon emissions through the use of renewables. It involves the installation of both the fossil fuel heating system component and the heat pump component.

Cost of hybrid heat pumps vs. ground source

Hybrid heat pump

It’s important to consider all of the costs associated with a hybrid heat pump, including initial, operating and replacement costs. This is important when comparing the technology to a singular ground source heat pump, as ground source tends to be cheaper in the long run.

Initial costs

The initial cost of a hybrid system all depends on the scenario, such as if you already have a system. For example, if you would like a hybrid system and you already have a gas boiler installed, that could mean you only have to account for the cost of the air source component.

If you were to install both components together from scratch, the total cost of a hybrid domestic heating boiler with its associated air source heat pump can be between £7,500 to £15,000. However, this will vary depending on the type of project and heating requirements.7

Operating costs

Fuel costs will be somewhere between the costs of a gas boiler and an air source heat pump (depending on the operating split). However, some of the other operating costs are doubled up. For example, you end up paying two standing charges – one for electricity and one for gas. You also need annual servicing and regular maintenance to check that the gas boiler is combusting fuel safely.

Lifetime or replacement costs

One of the main issues with a hybrid system is that it will incur twice the replacement costs of relatively short-lived appliances typically every 10-15 years for both appliances.

Ground source

The initial cost of a pure ground source heat pump conversion from mains gas is currently more expensive than an air source heat pump/gas hybrid. However, the ground array infrastructure has lifetimes in excess of 100 years and because ground source heat pumps are typically located indoors they have longer lifetimes of 20-25 years.

Once you model the costs over longer periods and account for fuel consumption, standing charges, servicing, maintenance and replacement costs then ground source heat pumps become the lowest cost option.

The long-lifetime, long-term savings and lack of maintenance that comes with a ground source heat pump soon make up for the initial cost, which is a significant advantage in the context of mass deployment.

Carbon emissions of hybrid heat pumps vs. ground source

Hybrid heat pump

In terms of emitting carbon emissions, a hybrid heat pump still has the disadvantage of burning fossil fuels. Although this is reduced by the heat pump component, it will still produce a higher percentage of emissions versus a single ground source heat pump system.

A study published in 2018 by the UK government into the effectiveness of hybrid heat pumps suggests that, on behalf of the heat pump component of the system, the carbon emissions intensity will improve over time. This aligns with the rapid decarbonisation of the electricity grid, as that’s where a heat pump gets a proportion of its power from. The scenario is based on a typical semi-detached house, where the hot water demand is met by the boiler component of the hybrid heat pump.

However, most hybrid systems have to choose between saving carbon and reducing bills and it is not clear how the controls are to prioritise a mixture of these benefits. Air source heat pumps are far lower carbon but have higher running costs. If the system prioritises carbon, the gas boiler will only run for 5% of the year when the electricity system is high carbon. If they prioritise running costs, the split will be more like 50/50 when electricity is cheap – if on a time-of-use tariff.

Ground source

This projection of decreased carbon intensity over time is even better reflected in ground source heat pumps. To put this into perspective, the study suggests that by 2050, heat pumps could be left with a carbon intensity of around 00.01kgCO2/kWh.8

In fact, ground source heat pumps coupled with smart meters and flexible tariffs can produce bill savings of 25% and carbon savings of 77% compared to mains gas.

Oil & LPG boilers vs. ground source heat pumps

Oil and liquefied petroleum gas (LPG) is traditionally the go-to alternative fuel for buildings that are not connected to mains gas. However, prices of oil continue to fluctuate against a backdrop of growing global concerns – particularly energy security.

Meanwhile, heat pump technology has evolved, along with accessible funding streams and affordable running costs. Ground source heat pumps have therefore become a more viable and preferred option for off-gas areas.

Add to this the pressure of government plans to ban fossil-fuel heating systems in new builds from 2025, and it looks like oil and LPG boilers could be phased out entirely by 2050.

Disadvantages of oil & LPG boilers

  • Vulnerable to energy price hikes and fluctuations
  • Bulky and unsightly fuel storage tank
  • Highly flammable
  • Reliance on fuel deliveries
  • Possibility of fuel theft
  • Pay for the energy upfront all in one go
  • Complicated installation
  • Contributes to air pollution through the emission of particulates, Nitrogen Oxide (NOx) & Sulphur Oxide (SOx)
  • High carbon content.

Unsightly tank

Running costs of oil boilers vs. ground source

Oil boilers

There are a number of factors that make oil prices volatile. The price of crude oil is particularly sensitive to the laws of supply and demand, which can vary worldwide with fluctuations in the economy, political unrest and extreme weather events.

Supply can be influenced by the Organisation of Petroleum Exporting Countries (OPEC), who will cut output in order to strengthen prices by reducing supply.

Ground source

Ground source heat pumps can save occupants around 30 – 50% in running costs compared to oil boilers. Electricity prices, as used to drive a ground source heat pump, do not suffer from the same volatile prices.

Aside from a ground source heat pump’s inherent efficiency, delivering typically 3 to 4kW of free heat energy for 1kW of electrical energy and making the use of off-peak tariffs in well-insulated buildings, the running costs of ground source heat pumps are significantly lower than oil boilers.

That is a benefit you wouldn’t see from oil.

Combating fuel poverty & oil in off-gas areas

The savings on our energy bills compared with oil is fantastic and our house value has increased as a result of the installation - we’re over the moon!

Stephen Chidgey, Homeowner

Maintenance of oil boilers vs. ground source

Oil boilers

Oil boilers use combustion in the heating process, so they must be serviced and maintained correctly. Failure to do this can lead to major health risks, such as the production of poisonous gases like carbon monoxide.

Ground source

As ground source heat pumps do not use combustion, there is no risk of poisonous gases being emitted.

Moving parts are kept to a minimum and are non-serviceable, so maintenance is minimal. This makes ground source heat pumps significantly cheaper to maintain, saving occupants or ground source owners money by eliminating the need for annual servicing.

Lifetime of oil boilers vs. ground source

Oil boilers

These days most oil boilers are condensing boilers, as they provide a much higher efficiency than non-condensing boilers. Oil-fired condensing boilers typically have a shorter life (7 to 10 years) than traditional non-condensing boilers, as the heat exchangers can rot out quickly due to the acidic condensate.

Ground source

Ground source heat pumps generally have a design life of approximately 20 years due to their lack of moving parts and secure installation inside the home.

Storage of oil vs. ground source


Oil supply for oil boilers must be stored on site. It’s generally stored in a bunded oil tank, which is basically a small tank within a large tank. The larger tank is able to hold 110% of the capacity of the smaller tank. This is used to protect the surroundings of the tank from any oil spills of leakages. When storing oil, there needs to be sufficient space and access to the tank.

With OFTEC health and safety regulations stating that it must be located within 1.8m of the building, the unsightly oil tank can take away from the aesthetics of a property. As well as that, security measures against theft need to be considered if large volumes of oil are being stored on site.

Ground source

As ground source heat pumps run on electricity, no fuel storage or deliveries are required on site – allowing for additional space, peace of mind, and preventing the need for storage that many people feel is an eye sore.

Carbon emissions of oil boilers vs. ground source

Using the carbon intensity factors proposed for SAP 10, an oil boiler emits almost five times as much carbon as a ground source heat pump.

In terms of carbon emissions, there is no justification for the use of oil or LPG boilers when far more appealing and renewable heating options are available. This is why the government is phasing out oil boilers in favour of lower carbon renewable heating systems such as ground source heat pumps.

Can a heat pump replace an oil boiler?

Yes. For older properties that are not especially well insulated, Kensa will help you to establish whether the existing heating system and existing insulation levels will be compatible with a ground source heat pump, or if your heating distribution system and energy efficiency measures require any upgrades.

If you are undecided on whether you should upgrade your radiators or insulation levels, you can run a trial over a heating season. To do this, we recommend you turn down the flow temperature of your existing boiler to 50°C and run the system through the winter heating season to mimic the output of a ground source heat pump system.

If you find you can maintain your required comfort conditions at this lower flow temperature, then you have proved that your existing heating distribution system and the current heat loss of your property is compatible with a ground source heat pump installation – without any upgrades required. If you were not satisfied with the comfort levels then we advise you to upgrade your radiators, and improve the insulation levels in the property prior to installing a ground source heat pump.

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Geothermal heating vs. ground source heat pumps

What is the difference between ground source heat pumps & geothermal heating?

There is often much confusion when it comes to comparing ground source heat pumps to geothermal heating. Some people use the terms interchangeably, but there are key differences:

  • Geothermal heating uses heat directly from the earth’s core, such as hot springs, geysers and volcanic hot spots.
  • Ground source heat pumps absorb the sun’s energy stored in relatively shallow ground and upgrade this for use in domestic and commercial applications.

Ground source heat pumps, like those manufactured by Kensa Heat Pumps, generally take heat from 1.2m – 200m depths. In this zone, there is a large amount of low-grade energy available, which needs a heat pump to upgrade this to more useful temperatures.

The reason geothermal heating is often confused with ground source heating is that both ultimately harness heat energy from the ground to provide heating and hot water to buildings.

However, the main differences are where the heat originates and the application the heat will be used for.

What is geothermal heating & how does it work?

Geothermal heat comes from the earth’s core; ‘geothermal’ literally means ‘earth’s heat’. It uses heat directly from drilling deep or shallow sources emanating from the earth’s core, such as hot springs, geysers and volcanic hot spots.

Borehole drilling for geothermal heating can often be drilled to depths many kilometers below the ground. In the UK, you have to go down 500m – 2500m before there is any appreciable input from the earth’s core. Geothermal typically needs to be deployed at large scales to make it financially viable.

Geothermal systems operate at temperatures of 120°C – 300°C and therefore do not necessarily need a heat pump to upgrade the heat, nor a ground loop system.

Large-scale plants and communities tend to use geothermal energy over ground source heat pumps.

Ground source heat pumps vs geothermal heating - A geothermal power plant

Why does the difference between geothermal & ground source matter?

When you drill a deep geothermal hole, it’s not always clear what the conditions will be like further down. Inevitably, this could determine how well the system will work.

However, ground source heat pumps absorb energy at shallower depths, which means there is a much higher level of certainty around the conditions you will find.

What are the other common names for ground source heat pumps?

The following terms are sometimes used when referring to ground source heat pump installations, and can be misleading and technically inaccurate due to their ‘geothermal’ references:

  • Geothermal heat pumps
  • GeoExchange heat pumps
  • Earth-coupled heat pumps.

Other less commonly used terms for geothermal heating include:

  • Deep geothermal
  • Enhanced geothermal systems
  • Geothermal, direct use
  • Hydrothermal
  • Hot rocks
  • Hot dry rocks
  • Geothermal power.

Biomass vs. ground source heat pumps

Disadvantages of biomass heating compared to ground source heat pumps:

  • Space required for fuel storage
  • Reliance on regular fuel deliveries
  • High maintenance compared to ground source heat pumps
  • Planning permission required due to emissions
  • Contributes to air pollution through the emission of particulates, Nitrogen Oxide (NOx) & Sulphur Oxide (SOx).


  1. UK housing: Fit for the future? Committee on Climate Change, 2019
  2. Hydrogen for Heating? Prof. David Cebon, University of Cambridge, as cited in Hydrogen: a decarbonisation route for heat in buildings? LETI, 2021
  3. Is hydrogen the solution to net-zero home heating? The Guardian, 2020
  4. Hydrogen: a decarbonisation route for heat in buildings? LETI, 2021
  5. What is a Hybrid Heating System? Boiler guide
  6. Hybrid heat pumps final report, BEIS, published 2018
  7. What is a Hybrid (Dual Fuel) Heat Pump System? IMS Heat Pumps
  8. Hybrid heat pumps final report, BEIS, published 2018