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Smart hybrid heat pumps provide the UK’s pathway of choice to decarbonise heat

“Most exciting of all is the prospect of producing low-carbon heat; using smart hybrid heat pumps in combination with natural gas in the short-term, with the potential for hydrogen in the long-term.”


Lord Deben, chairman of the Committee on Climate Change (CCC) commenting on the potential for smart hybrid heat pumps in the CCC’s November 2018 report ‘Hydrogen in a low-carbon economy’.


  1. Freedom project inspires UK to revise government policy on low-carbon heat
  2. Passiv is the first company to market with a hybrid heat pump solution that uses smart predictive controls, which is ready to deploy at scale today
  3. CCC targets hybrid heat in 10 million homes by 2035

The Freedom project measured the consumer, network and energy system implications of hybrid heating system deployments, where domestic heating systems had the option of operating using a standard gas boiler, an air source heat pump (ASHP), or both.

The collaborative project, managed by Passiv, proved beyond doubt how multi-fuel, hybrid home heating systems can play a pivotal role in reducing carbon emissions to the 2050 levels, as mandated by the Climate Change Act.

Freedom also demonstrated how hybrid heat systems can significantly lower running costs for consumers – both on and off the gas grid – while improving comfort levels.

The project further showed how hybrid heating can help to decarbonise domestic heating with no increase in peak load, if operating within the context of a demand ‘flexibility’ solution that is designed specifically for householders.

Project scope

The Freedom project’s hybrid heating system comprises an exterior ASHP, a reliable, high-efficiency boiler inside the home, and a hybrid control panel. The hybrid heating solution was installed in 75 homes – a mix of social and private housing, including some that are off the gas grid – in and around Bridgend, South Wales.

For the first time, this project brought together the gas and electricity network operators in the field trial region and provided robust, field-tested data to enable long-term network investment planning. The cross-sector scope made this a unique project, which set the benchmark for holistic, ‘whole systems’ projects.

Designing heating systems that combine gas boilers with ASHPs, while employing smart switching between the gas and electric load, enables the choice of fuels to match consumer demand for heat. This highly flexible approach delivers multiple benefits.

Decarbonising heat

Recent developments in the CCC’s modelling of future energy system scenarios has helped inform its assessment of the most feasible approach to decarbonising heat for buildings.According to the CCC, the path to near-full decarbonisation by 2050 now entails near-term deployment at scale of hybrid heat pumps in buildings on the gas grid, alongside substantial improvements in energy efficiency, low-carbon new-build and other ‘low-regrets’ heat decarbonisation deployment.CCC acknowledges that retrofitting smart hybrid heat pumps “would lead to greater reductions in near-term emissions from buildings and provide greater confidence that very low levels of emissions can be reached by 2050. This would keep open the option of switching the remaining gas supply to hydrogen at a later date…”

Reducing costs

Hybrid heating systems can help householders save money on heating and hot water bills while supporting the shift towards the decarbonisation of heat. Avoiding the use of electricity during times of peak demand will help reduce the need for further investment in generation capacity. It also enables the heating system to take advantage of time-of-use price differences between the two fuels – so-called ‘fuel arbitrage’.

The heating system is especially cost effective in homes off the gas grid. One family, who live in a rural former hill-farmer’s cottage in the Welsh Valleys, saved £736 on their heating bill between October 2018 and April 2019. This significant saving was achieved without the need to improve the thermal efficiency of the cottage or replace any radiators.

According to the homeowner: “Before having our new heating system installed, we used LPG gas, which was very expensive. We wanted to find a solution that kept us warm, was cheaper and greener too.”

The systems optimise fuel use for a range of fuel price scenarios. For example, gas boiler use is strongly favoured due to the very low cost of gas compared with electricity. However, for homes running on liquified petroleum gas (LPG) achieved cost savings by switching around 80% of their heating load to the ASHP.

Supporting flexibility

As the UK replaces its traditional, centralised power stations with distributed renewables on the grid, the lack of flexibility inherent in renewable sources of power leads to increasing difficulty in balancing supply and demand.

Demand side response (DSR) – the ability to turn down, turn off or time-shift electrical loads in response to a request from the National Grid – is an established approach to grid balancing in industrial and commercial contexts. DSR provides a valuable source of flexibility back to the grid.

The Freedom hybrid heating system trials show that smart switching between the gas and electric load can be effective in delivering flexible DSR in a domestic setting.

PassivEnergy, Passiv’s smart energy management platform, considers the heat storage of the home (including the thermal mass of the building, multi-vector heat, and hot water storage). It switches to gas or oil when the carbon intensity of the grid is at its highest, or when the heat pump needs additional energy to provide the heat needed, such as on very cold days when the coefficient of performance is low, or the heat pump can’t provide enough heat.

A typical scenario would see the heat pump warm the house using cheap electricity overnight ready for the morning. Come mid-afternoon, the smart controls call on the gas boiler to quickly reheat the property. During early evening, the smart control system can switch between the gas boiler and electric heat pump to avoid adding to peak electricity demands on an overloaded grid.

The system uses smart controls to manage network load using two strategies. The heating controls use predictive optimisation of running costs so that the heat pump can pre-heat the building ahead of an occupancy period. This spreads the heating load, timing the demand ahead of current system peaks, and enabling the ASHP to operate at a low flow temperature for efficiency. In addition, the aggregated load of all homes was forecast by the half hour for the coming 24-hour period.

The demand forecast uses weather data, learned building thermal properties and schedules for each home to predict the expected demand shape. Demand can be constrained individually for each home or at a portfolio level.

Key results

According to Freedom project partner Imperial College, using gas in conjunction with an air source heat pump (ASHP) and conventional boiler could achieve savings up to £15bn per year – compared to full electrification – in a 25g per kWh energy system.

The Freedom project demonstrates that the benefits of moving towards smart hybrid heating systems are significant. Smart, multi-fuel heat pumps can:

  1. Enable near-full decarbonisation of the UK’s heating system by 2050
  2. Reduce heating costs for consumers
  3. Provide fully flexible demand that is able to respond dynamically to network, price and carbon signals and constraints
  4. Deliver a ‘no-regrets’ transition to low-carbon heat at a lower cost than full electrification
  5. Use green gas technology to support a long-term solution for low-carbon heat provision
  6. Save billions in avoided generation and electricity network capacity enforcement costs.

With smart hybrid heating technology, the UK has an opportunity to transform the domestic heating market to deliver carbon reduction obligations whilst creating jobs and export opportunities in markets targeted by the UK’s industrial strategy.

Download the Committee on Climate Change ‘Hydrogen in a low-carbon economy’ report here.

Download the Freedom project report here.

Watch our Freedom Project video – the future of home heating here.