Hybrid heat pumps: a step in the right direction

With 23 million homes still using gas boilers, Liv Stokes argues that hybrid systems featuring heat pumps and top-up gas boilers could provide a stepping stone for the decarbonisation of heat

Liv Stokes

The route to the decarbonisation of heat remains uncertain. The Future Homes Standard, which will ban gas boilers in new builds from 2025, is looming. 

Many developers are looking to heat networks and heat pumps to replace gas, but what should be done about the 23 million where gas boilers are still being used? Hydrogen is currently being trialled but its feasibility and relevance in decarbonising heat won’t be known for years.

Hybrid heat pump systems may enable existing homes to decarbonise in the short term. These systems have an air or ground source heat pump working with another heating source such as a gas or LPG boiler or immersion. Heat pumps are installed to work alongside the existing heat source, or both systems can be installed simultaneously. 

Hybrids typically use the heat pump at higher ambient temperatures and the boiler below a certain ambient temperature, optimising efficiency and minimising energy cost. Domestic hybrids don’t require a hot-water cylinder, as the gas boiler will usually provide hot water. However, if a cylinder is included with an appropriately sized coil, it enables the heat pump to contribute to delivery of hot water.  

Hybrids are included in the National Grid’s Future Energy Scenarios 2022 (ASHP + Hydrogen boiler), and were in the Renewable Heat Incentive, but are not in the recently introduced Boiler Upgrade Scheme. This suggests the government does not consider hybrids to be a low carbon heating technology and it is uncertain what support will be given in the future. 

The electricity Grid upgrades needed for widespread deployment of heat pumps poses a challenge. However, hybrids have smaller heat pump units with lower peak power demands than standalone heat pumps and could be rolled out without major Grid upgrades. 

If used with smart controls, hybrid systems could help the National Grid balance supply and demand by using renewable electricity when available.

Hybrids are not the only option for flexibility. Standalone heat pumps – for example, ground source with thermal storage – coupled with smart controls, remote grid balancing, and time-of-use tariffs may also assist the National Grid by turning on when there is excess generation and turning off when the Grid is under strain.

Currently, hybrids don’t present a long-term solution for decarbonisation because of the carbon emissions from the gas boiler. However, if the UK had contributions from hydrogen, hybrids have long-term potential. 

Hybrids might introduce the concept of heat pumps to those members of the public who are sceptical or reluctant

Compared with standalone heat pumps, hybrids are easier to install. They avoid the cost and disruption of changing radiators or improving building fabric through insulation or window upgrades if these are undesired or challenging. Hybrids work with or without a hot water cylinder, offering additional convenience and choice to homeowners.

Poorly insulated homes have greater heat loads, requiring higher operating temperatures meaning the heat pump runs less efficiently. Considering the urgency of the climate emergency, efficiency is arguably less crucial if utilising renewable electricity.

However, given the UK’s poor levels of insulation, adopting hybrids shouldn’t be at the expense of improving the energy efficiency of homes. More insulation will drive down heating demand, cut the size and cost of equipment, optimise system efficiency, and minimise the gas needed on cold days. .

The future costs for installing a hybrid in individual dwellings are uncertain, as they are not currently included in the Boiler Upgrade Scheme. However, hybrids (GSHP + electric; ASHP + electric; GSHP + gas; or ASHP + gas) – if used with carbon offset payments – could be the most cost-effective option for decarbonising heat networks, compared to standalone heat pumps1.

Hybrids need additional maintenance expertise and yearly service regimes. Renting a heating system, as opposed to purchasing – for example, through BeWarm’s rent-a-boiler scheme – could make hybrids more accessible and affordable, with maintenance and service costs factored into customers’ bills.

Hybrids aren’t a perfect solution but, with correct support, they could quickly contribute to the decarbonisation of the 23 million homes currently on gas. 

Regardless of what the decarbonisation of heat entails, the UK desperately needs a national retrofit strategy addressing poor insulation and widespread use of fossil fuels in existing buildings to meet 2050 net zero carbon targets.

About the author

Liv Stokes, mechanical and sustainability engineer at GLJ Design Services