Ecoforest 22kW single phase heat pumps serving large farm buildings © Nuenta Ltd
According the Committee on Climate Change’s 2019 Net zero – technical report, the UK must install an estimated 19m heat pumps by 2050 to meet its emissions targets – equivalent to 633,000 per year.
With the need to phase out fossil fuel heating systems, heat pumps are set to become a mainstream technology in the built environment over the next decade and beyond.
Due to the rapid decarbonisation of grid electricity, installing a heat pump can result in 55 – 65% less carbon dioxide than a modern gas boiler. The Carbon Trust predicts these savings to increase to more than 90% as the grid decarbonises further in coming decades.
However, heat pumps are still uncommon in existing buildings in the UK, with only around 20,000 retrofits per year since 2014. So what are the barriers, and how can these be overcome to enable the decarbonisation of the building stock? In August, the mayor of London commissioned us at the Carbon Trust to write Heat pump retrofit in London, a report looking at precisely these issues.
The mayor has set an ambitious target for the capital to be net-zero carbon by 2030. This will require a rapid transition from gas to low-carbon heating. However, London currently has the lowest rate of heat pump installation in the UK, with only 304 funded under the government’s Renewable Heat Incentive (RHI) scheme since 2014.
We found that the main barriers to installation include upfront costs, the poor energy efficiency of existing buildings, the lack of space for equipment, and the noise levels of air-source heat pumps in urban areas. Our report, therefore, sought to provide a detailed appraisal of these issues and how to overcome them.
Varied and versatile technology
To research possible solutions to the practical challenges of retrofitting heat pumps in buildings, we invited manufacturers and installers to propose schemes for 15 buildings in London.
- The noise levels of modern air-source heat pumps are often below those required for permitted development, while ground-source heat pumps can eliminate noise concerns entirely.
- Vertical boreholes can be drilled for ground-source heat pumps, even with the limited space in dense urban areas.
- Internal air-source heat pumps do not require space for external fans, with air being transferred to the unit through ducting to the external wall.
- Ambient ground-source loops can be attached to individual heat pumps in each dwelling. Fifth-generation heat networks of this kind can be retrofitted to buildings at a lower cost than traditional high-temperature heat networks, opening up new possibilities for urban heat networks.
- Large buildings and heat networks can use a huge variety of heat sources, such as rivers, groundwater, aquifers, industrial waste heat, data centres, sewers, and London Underground tunnels. This can help achieve highly efficient pump and network performance.
- Buildings with large cooling demand can benefit from passive cooling or reversible air-source heat pumps.
Lower flow temperatures
Our research found that flow temperatures are key to the efficiency and performance of heat pumps. Compared to common gas boiler flow temperatures of 55–70C, we identified that many heat pumps operate most efficiently at 35–55C. This can cause problems for retrofits in existing buildings where the thermal efficiency of the fabric may be poor and existing heat emitters, such as older single- and double-panel radiators, are designed for higher temperatures.
Our analysis discovered that most buildings will require some combination of improvements to the building fabric and upgrades to emitters if they are to achieve these lower flow temperatures. In some larger buildings and in heat networks, it may be best to replace existing heat distribution systems entirely to achieve optimal performance.
However, buildings already fitted with low-temperature heat emitters – such as fan coil or air handling units, or underfloor heating – will often be able to use these. We also found that many existing radiator systems are oversized for their current applications, meaning that they can accommodate lower flow temperatures without further upgrades.
In terms of energy efficiency, we found that an energy performance certificate (EPC) rating of C or above was a useful proxy for a building that is thermally efficient enough to accommodate lower flow temperatures. However, thermal demand needs to be calculated on a case-by-case basis, and an EPC cannot be used as a reliable guide.
A further consideration is that an increasing number of high-temperature heat pumps can provide heat in excess of 70C when necessary, modulating to lower temperatures at other times.
While reducing flow temperatures will always result in the highest efficiencies, high-temperature heat pumps are likely to have a key role to play where extensive thermal fabric improvements or emitter upgrades are not feasible.
For commercial and large-scale applications with high peak loads or high temperature requirements, it is likely that heat pumps will often be paired with gas boilers as a heat source in hybrid systems.
The key message of our research is that, with an holistic approach to design that considers the technology alongside upgrades to the building fabric and heat emitters, heat pumps are a viable option for all buildings.
Financing heat pumps
The primary barrier to heat pump retrofit is upfront expense. Our research showed that the average cost of a domestic air-source heat pump was £9,498 greater than that of a domestic gas boiler. For non-domestic buildings the gap can be proportionally larger, although this differs significantly depending on the specific building.
However, new government funding schemes including the Green Homes Grant, the Public Sector Decarbonisation Fund, and the Social Housing Decarbonisation Fund Demonstrator can support heat pump retrofit alongside thermal fabric improvements. The RHI will also continue to operate for domestic buildings until 2022.
Maintenance costs can be marginally higher for heat pump systems than for gas, although they can obviate the need to pay the gas standing charge or annual safety checks.
- The building fabric’s efficiency can be improved to reduce space heating demand.
- By optimising system design, including reducing flow temperatures, heat pump efficiency can be maximised.
- There are financial incentives for the flexible use of electricity, such as time-of-use tariffs, which offer low or negative costs for electricity when demand is low or supply is plentiful. Our research suggests that homes could reduce bills by up to 23% with active shifting of heat demand outside the peak time of 4.30–7.30pm, with larger savings possible in commercial buildings and those using dedicated thermal or battery storage.
- those that currently have electric heating systems
- those with a high demand for cooling
- ageing communal and district heating networks in need of renewal
- buildings where major refurbishments are being planned.
The primary choice
Our overall conclusion was that, with appropriate technology choice and design, heat pumps can work in all building types and heat networks. Given uncertainty around the potential role for hydrogen in heating buildings, we recommended that heat pumps should be considered the primary technology choice for decarbonising heat in London.
As building owners are increasingly committed to decarbonisation, and heat pumps are at the heart of the £3bn decarbonisation funding from the government outlined above, it is essential that surveyors consider heat pump retrofit as an option in all buildings.
“With appropriate technology choice and design, heat pumps can work in all building types and heat networks”