Gas Absorption and Other Alternative Heat Pumps

A heat pump, in the words of Electricity North West, is "a device which uses a small amount of electricity to absorb the natural heat from the air or the ground and pump it around the heating system of your property"¹. Nearly every heat pump sold in the UK today follows that pattern: official methodology guidance states plainly that "most heat pumps use an electrically driven compressor"². A gas absorption heat pump is the main exception. Instead of using electricity to squeeze a refrigerant vapour, it uses heat itself, from burning gas or from another thermal source, to drive the cycle that lifts warmth from outside into your home.

The European Heat Pump Association describes this distinction directly: the energy that drives a heat pump's compression "can come from electricity, gas or thermal energy"³. Units driven by gas or thermal energy rather than an electric motor are known as gas sorption heat pumps, and they are recognised as a distinct product family under the European standard EN 12309, which covers "gas sorption heat pump (GAHP) and thermal compression heat pump (TCHP)" together⁴. For a householder, the appeal is simple: a heating system that behaves like a heat pump, extracting free ambient warmth, without drawing most of its energy as electricity.

That appeal has to be weighed carefully, because almost all of the UK's published heat pump evidence, efficiency figures, cost data and grant support, relates to electrically driven units. This page sets out what is known about absorption and other thermally driven heat pumps, what they can run on, how their efficiency and costs compare, and where they fit in a country that is planning to move millions of homes off the gas grid.

Understanding gas absorption heat pumps and how they compare with electric models

Every heat pump moves heat rather than generating it from scratch. The conventional design uses an electrically driven compressor to raise the pressure and temperature of a refrigerant vapour; the Planning Portal describes the sequence as a heat pump working "via an evaporator, a compressor and a condenser" to transfer heat to a hot water tank which feeds the heating system⁹. That compressor is the component that consumes the electricity, and it is the component a gas absorption heat pump replaces.

In a thermally driven unit, the compression of the refrigerant is achieved by heat instead of an electric motor. The European Heat Pump Association's general description of heat pump technology allows for exactly this: the drive energy "can come from electricity, gas or thermal energy"³. A gas absorption heat pump burns gas, or takes in heat from another source, and uses that thermal energy to circulate and compress the refrigerant. A small amount of electricity is still needed for controls and fans, but the main energy input is heat.

These products are not informal experiments. They are standardised: EN 12309 covers both the gas sorption heat pump (GAHP) and the thermal compression heat pump (TCHP)⁴. The term "sorption" is the umbrella that covers two variants, absorption, where a fluid takes up the refrigerant, and adsorption, where a solid surface holds it. UK guidance for householders does not yet describe the internal workings of either variant in detail, so a buyer's practical reference points are the standard itself and the manufacturer's documentation.

The two designs side by side:

The difference matters for energy independence in two opposite ways. On one hand, a gas absorption unit reduces a home's dependence on electricity, which is useful where the electricity supply to a property is limited. On the other hand, it keeps the home tied to a combustible fuel supply, usually the gas grid or delivered gas, which is precisely the dependency the UK's heat policy is trying to unwind. An electric heat pump paired with solar panels and a battery can move a home towards self-sufficiency; a gas absorption unit cannot, because its driving fuel must keep arriving.

Fuel options: mains gas, LPG, the sun and recovered heat

Two separate "sources" are at work in any heat pump, and it is worth keeping them apart. The first is the ambient heat the unit harvests. Approved Document L lists the options for dwellings as "capturing heat from ambient air, ventilation exhaust air, or a water or ground heat source using a heat pump"¹⁰, and CIBSE summarises the same point: a heat pump "extracts energy from the air, ground, or water and converts it into heat"⁵. Scottish building standards add geothermal sources to the list¹². A gas absorption heat pump harvests from the same ambient sources as an electric one: outside air, the ground, or water.

The second source is the driving energy. For most heat pumps this is electricity; for a sorption unit it is heat, which can come from:

• burning natural gas or propane³
• any sufficiently hot waste heat stream³
• solar thermal collection³

The European Heat Pump Association's framing, compression energy from "electricity, gas or thermal energy", is what makes solar and waste heat legitimate driving fuels for these units in principle³.

The closest product with published UK guidance is the solar assisted heat pump. Energy Saving Trust describes it as a unit that "heats water by absorbing heat from direct sunlight and from the air", using "a large, flat evaporator panel that absorbs the heat from sunlight falling directly onto" it, and notes that these "can also work without direct sunlight"¹³. That is a solar-assisted electric heat pump rather than a sorption unit, but it shows how solar energy already enters the heat pump family in the UK market.

Water-source variants add another ambient option. The permitted development legislation defines a water source heat pump as "a heat pump where the collecting medium is water"¹⁴, and the European Heat Pump Association notes that "water source heat pumps use the energy stored in ground water, surface, or sea or sewage water"⁴. A thermally driven unit drawing on a water source, with its compression powered by waste heat or solar thermal energy, would be among the most independent heating arrangements available, needing no electricity for its main cycle and no fuel deliveries. The practical catch is that UK guidance documents no such domestic product in normal service, so this remains a design possibility rather than an off-the-shelf choice.

When absorption heat pumps outperform electric ones, and when they do not

The headline efficiency figures for heat pumps in the UK are striking and consistent. The Climate Change Committee states that "heat pumps are around three-to-four times more efficient than gas boilers, which should lead to lower household energy bills"¹⁵. Bristol City Council's Warm Homes Plan makes a similar statement, that heat pumps are "typically over three times more efficient than the most efficient gas boilers or electric heaters"¹⁶. Nesta's assessment of individual ground source heat pumps finds they "provide 3 to 4 times the amount of heat than the electricity they use"¹⁷.

Two further comparisons frame the size of the advantage. Electricity North West notes that a heat pump uses "only a quarter of the electricity used by conventional electric heaters to produce the same amount of heat"¹. And Nesta makes the same point from the other direction: direct electric options such as infrared panels, storage heaters and electric boilers "use three to four times more electricity than a heat pump"¹⁸.

Where does absorption fit in these numbers? The honest answer is that the published UK figures are built on electrically driven units. The three-to-four-times comparisons measure heat output against electricity input, which is the right measure for a compressor-driven machine. A gas absorption heat pump's efficiency is measured differently, as heat output against the gas or heat burned to drive it, and no official or independent UK figure for domestic gas absorption units appears in the current guidance base. What can be said is structural: because the unit harvests ambient heat as well as burning fuel, it delivers more heat than the fuel alone could provide, which is why it is classified as a heat pump at all.

Where absorption can beat an electric heat pump is in electricity consumption. An electric unit's entire drive is electricity; a sorption unit's drive is heat, so its electricity draw is limited to auxiliary components. In a home with a constrained electricity supply, or where the aim is to minimise electricity purchases, that is a genuine advantage. Where it falls short is in carbon and fuel dependence: burning gas to drive the cycle keeps the home on the gas grid, whereas the European Heat Pump Association notes that "if green electricity is used, for example from solar or wind energy, then the heat pump uses 100% renewables and is carbon neutral"³, a condition an electric heat pump can meet and a gas-driven absorption unit cannot.

Pricier to install but cheaper to run

Across the heat pump family, the cost pattern is consistent: more to install than a fossil boiler, less to run. Energy Saving Trust's response to Scotland's draft Climate Change Plan states that "at current prices, the upfront costs of heat pumps are higher than fossil fuel boilers"¹⁹. Government analysis of running costs finds that "running costs for heat pumps are typically lower when compared to those of traditional gas boilers", provided the system is set up well²⁰.

Within the family, the type of unit drives the installation cost. Air source heat pumps are "usually less expensive than ground source heat pumps"²¹, while ground source systems "give you lower running costs and lower CO2 emissions, but will cost more to install"²¹. Air-to-air units have "lower upfront costs than other heat pump types"¹⁹, with Nesta noting their "low upfront cost in smaller homes"²². Government cost analysis treats the investment figure as covering "all capital upfront costs needed for heat pumps including installation and ancillary work"²³, so the quoted price should include everything needed to make the system work.

For a sense of the whole-life picture, Nesta's modelling of an air source heat pump with subsidy puts total cost at £960, broken down as "upfront costs: £300, maintenance costs: £80, running costs: £580"¹⁸. That figure is for an electric air source unit, not an absorption one, but it shows how subsidy and running costs shape the total.

For gas absorption heat pumps specifically, there is no published UK price. These units are installer-quoted, and the cost will depend on the unit, the ambient source and the gas connection work needed. Two cost cautions follow from the official position. First, the main grant routes do not name them: the Boiler Upgrade Scheme's installer guidance lists "air-source, ground-source and water-source heat pumps" as the eligible types¹¹, and gas sorption units are not on that list. Second, running costs depend on the gas price as well as the unit's efficiency, so a home considering one is really choosing between exposure to electricity prices and continued exposure to gas prices. A time-of-use tariff can cut an electric heat pump's running costs significantly²⁴, an option that does nothing for a gas-driven unit.

Where they are used: homes, industry and cooling

Heat pumps as a class are not a narrow domestic technology. The European Heat Pump Association defines a heat pump as "a device that can provide heating, cooling and hot water for buildings, districts and even for industry"³, and CIBSE describes the same breadth: "a heat pump is a versatile system capable of providing heating, cooling and hot water for homes, commercial buildings and industrial applications"⁵. Absorption technology has a long association with cooling and refrigeration, and the thermally driven cycle is well suited to any setting where heat is cheap or waste heat is available, which is why industrial and district applications are its natural territory.

In UK homes, the deployed base is electric. Northern Ireland's official guidance lists where the common types fit: air source heat pumps are "suitable for: residential homes, apartments, small commercial buildings", while ground source heat pumps suit "single-family homes, larger residential buildings, commercial properties"²⁶. Air source units are "the most common type of domestic heat pump in the UK and are suitable for most types of homes"²⁴, and they can also "heat water stored in a hot water cylinder for your hot taps, showers, and baths"²⁴. Air-to-air units "transport and compress heat from the air outside, but transfer it to a home as blown air"²².

Grant policy reflects the same electric focus. The Warm Homes: Local Grant, which supports low carbon heat in England and Wales, lists eligible measures including:

• air source heat pumps
• ground source heat pumps
• hybrid heat pumps in mains gas heated homes
• shared ground loops
• high heat retention storage heaters
• biomass boilers²⁵

Gas absorption units are not among them. Official deployment statistics likewise count "all types of heat pumps including air source heat pumps (ASHP) and ground source heat pumps (GSHP)"⁸, with no separate category for sorption units, which tells you how small the domestic installed base is.

For a householder weighing an absorption unit, the practical reading is this: the technology is real, standardised and suited to heating, cooling and hot water across buildings, districts and industry, but in the UK it is being bought and supported as a niche product, while the residential market, the installer base, the grants and the statistics all point to electric air and ground source units.

Looking after the system and its refrigerant

A heat pump is a serviced appliance, not a fit-and-forget one. Energy Saving Trust's advice is to get "an engineer to service your heat pump according to the manufacturer's advice, usually once a year"⁷. A typical service is broader than a boiler service, involving:

• an electrical safety check
• checking the thermostat operation
• a general visual inspection of the unit
• refilling refrigerant (if needed)
• checking for leaks and pressures
• cleaning filters and coils
• checking all large components such as the compressor, fan and pump
• lubricating moving parts
• replacing any small parts such as seals and gaskets, per the manufacturer's guidelines⁷

A gas absorption unit adds a gas-burning side to this schedule, so the servicing obligation sits somewhere between a heat pump's and a boiler's.

On refrigerants, the UK's official descriptions of heat pump systems reference refrigerant-filled circuits: Northern Ireland's guidance describes a ground source heat pump as one that "extracts heat from the ground via buried pipes filled with a refrigerant"²⁶, and the legacy Renewable Heat Incentive eligibility rules required a ground source system to "use a liquid to provide space heating"²⁷. For sorption units specifically, the relevant reference is EN 12309, the standard covering gas sorption and thermal compression heat pumps⁴; a buyer checking a product's refrigerant and sorbent credentials should expect the manufacturer's documentation to reference conformance with that standard. Guidance on how to read product claims and standards is covered in technology readiness and energy product claims.

Sizing is the other thing that protects both performance and the appliance. Building regulations guidance is unambiguous: "heat pumps should be selected to meet the full space heating requirement at the design condition chosen for heat loss calculations", with no heat from additional electric heaters within the design external temperature range²⁸. An undersized unit of any type leaves a home cold at the worst moment, and an undersized absorption unit adds a fuel bill to that failure.

Do absorption heat pumps suit UK households phasing out gas?

The Parliamentary Office of Science and Technology's bottom line on heat pumps is that they "are technically suitable for most UK homes if installed appropriately"²⁹. The Scottish Government's Heat Pump Sector Deal report set out the scale of the intended transition: "up to 10 million homes on the gas grid in the UK transitioning to heat pumps and hybrid heat pumps by 2035"³⁰. That is the policy current a gas absorption heat pump swims against: the national direction is towards electric heat pumps, in homes that are also moving off the gas grid.

Against that current, the case for absorption is real but narrow. It suits homes where electricity is constrained and a gas supply exists, and it can in principle run on solar thermal or waste heat, which would make it one of the few heat pumps needing neither grid electricity nor purchased fuel for its main cycle. The case against is that it keeps a home on gas, excludes the household from the main grant routes that name only air, ground and water source units¹¹, and lacks the UK-specific efficiency and cost evidence that electric units now have in depth.

For a household whose goal is energy independence, the honest summary is this: an electric heat pump with solar generation and storage can run on energy the home produces or buys as cheap green electricity, and the European Heat Pump Association's position is that with green electricity the system is carbon neutral³. A gas absorption heat pump reduces electricity dependence but deepens fuel dependence, and its future in a UK moving off gas depends on either green gas supplies arriving at scale or a shift to solar and waste heat as the driving source. Neither is yet reflected in UK grants, statistics or guidance, so anyone considering one is an early adopter, and should read it in that light, alongside our page on what emerging energy technology costs early adopters.