Decarbonisation of heat: Hybrid heat pumps

In our first blog we covered the concept of heat pumps, why they’re needed to both decarbonise the UK’s heating and potentially reduce heating costs for households. Technical Project Development Coordinators Bryony Holroyd and Paul Rogers also covered which technologies could be best suited to differing situations.

We mentioned previously that air-source heat pumps (ASHPs) have lower seasonal performance (efficiency) than water or ground-source heat pumps due to the lower energy density of air compared to the ground.  One way to counteract this is by using hybrid heat pumps. These heat pumps combine a traditional fossil fuel boiler with an air-source heat pump, and control systems determine whether it’s cheaper to heat using the heat pump, the boiler or combination of both to meet heating demands in any given situation.  Such systems can also provide flexible demand for the grid as shown in the Freedom Project. During periods of excess demand on the local electricity network, hybrids could switch to the fossil fuel boiler, while at times of excess renewable generation the hybrid could operate primarily with the heat pump.

A hybrid heat pump is a fossil-fuelled boiler (left) combined with an air-source heat pump (right), with a control system which determines which heat source to use based on cost and heating need.

Fuel bill savings from switching to a hybrid are greatest for off-gas properties (homes not connected to the mains gas network) using LPG or oil. While carbon emissions will be reduced by replacing a pure fossil fuel boiler with a hybrid, a greater reduction in emissions would be achieved by using a heat pump on its own.

A hybrid system is eligible for the Domestic RHI, but the payments are only made on the metered heat produced by the heat pump, and so requires more advanced metering arrangements to satisfy Ofgem RHI eligibility requirements.  An approved MCS (Microgeneration Certification Scheme) installer should understand the requirements.

NEA studied the performance of hybrid systems in socially rented homes during several Technical Innovation Fund (TIF) projects. In a project where Daikin Altherma Hybrid heat pumps replaced existing gas boilers, half of the installations retained the existing radiators, while the others had new “over-sized” radiators fitted, designed to work at a lower temperature. The seasonal coefficient of performance was higher for the systems which had the new radiators, and the heat pump operated for a higher proportion of the time. We also found there was a small reduction in energy bills for the households which had the new radiators. See the full report here.

In another project, homes on the west coast of Cumbria were compared against similar ones on the East coast  (South Shields) that had electric storage or panel heaters were connected to the gas network and had new central heating systems with a 5kW Daikin Altherma Hybrid heat pump. Before installation, residents were neither satisfied nor dissatisfied with how warm their home got in their homes when it was cold outside. After the installation, they were very satisfied. The output temperatures in Cumbria were set at 50°C, while those in South Shields were at 45°C. On average, the properties in South Shields used gas for 30% of their energy requirements, while in Gosforth, the average usage was 50%. The average seasonal coefficient of performance for the properties in South Shields was higher. On average, the residents in Gosforth saved 20% on their annual fuel cost while those in South Shields saved 31% after the hybrid system replaced their electric storage and panel heaters. The system will operate with older traditional pressed steel radiators, but is less efficient when doing this. See full report

A project in North Lincolnshire fitted 5kW Mitsubishi Ecodan Hybrid ASHPs alongside existing oil-fired combi boilers. This showed some of the challenges of installing combined systems and the need for training of both installers and households on how best to set up and use such an installation. Among the homes monitored, only one correctly used the oil boiler and new hybrid ASHP in hybrid mode! Some stopped using the oil-fired boiler, or turned it on manually when it was particularly cold. Another household turned the ASHP off while they had a stock of oil and then switched to the ASHP once the oil was depleted. This shows the need for better engagement with the customer to ensure they understand the operation of the equipment. We have found through the Technical Innovation Fund that it is likely that more effective use of the systems would have been achieved if the residents were properly made aware of the operation principles of the system, that their electricity bill would rise, but oil costs would fall.

An interesting investment model, with reduced upfront costs to householders can be seen with the new B-Snug offering (a Shell collaboration with PassivSystems Ltd). They offer installation of their smart hybrid heating system at a similar cost to that of a gas boiler – whereas a hybrid ASHP system would normally cost £5-10,000. The ASHP would be fitted alongside the current boiler, combined with smart controls to switch between the two heating sources. Payments from the Domestic RHI would be assigned to B-Snug. The householder can also apply for an Ofgem Meter, Monitoring and Service Package (MMSP) which provides payments over 7 years which can reportedly offset a required upfront household contribution. N.B. These types of finance models are likely to become more common in the future, combining revenues from one or more sources, possibly demand side response and other mechanisms.  We have not monitored these systems to date.

The Committee on Climate Change believe hybrid heat pumps will play an important role in decarbonising heat in some properties, including larger, hard-to-insulate homes where a standard heat pump would be inefficient at certain times. The aim is for the majority of heating to be supplied by electricity (through the heat pump), with high demand in cold weather being supplemented by the boiler. This guide to the pros, cons & costs of hybrid heating systems covers a wide range of issues and the government incentives available for their installation.

NEA has a wide experience in the deployment of non-standard heating technologies, and could enhance existing or planned projects, particularly in situations where fuel poverty is a risk.  Find out more about NEA’s technical team and the services that we can offer. If you’d like (us) to design a study to test, compare or evaluate the difference that heating systems make, or provide extra support to residents during a heating upgrade programme, please contact us on

  • Next – Part 3: Advanced heat pump systems which use heat pumps with energy storage


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