Retrofitting boiler heating systems with heat pumps: challenges and solutions

The latest legislation in the United Kingdom mandates that the installation of gas-powered boilers in newly constructed homes will be prohibited after 2025. Consequently, alternative heating systems such as heat pumps will be required for all new homes built after this deadline. However, there is a substantial number of existing homes in the UK, approximately 23 million, that currently rely on gas-powered boilers to provide central heating and hot water. Recent market statistics from the Heating and Hot Water Industry Council (HHIC) indicate a significant increase of nearly 41% in domestic boiler sales in the UK during the first four months of 2021, compared to the same period in 2020. This upward trend is expected to persist until the 2025 deadline. In the coming decades, engineers will face the challenge of developing straightforward and cost-effective retrofit options for these existing boiler installations, considering that the typical lifespan of boilers ranges from 15 to 25 years. However, the conventional heat pumps currently available on the market have limitations, as they can only produce water supply temperatures of 55-60°C, which is insufficient for the existing radiators designed for water supply temperatures of 70-90°C provided by boilers. Consequently, retrofit applications involving heat pump installations would necessitate either the replacement of existing radiators with larger ones or the conversion to underfloor heating options. These requirements render heat pump installations both expensive and highly disruptive. Moreover, existing heat pumps on the market necessitate an electric boost through an additional heating element to provide supplementary assistance during extremely cold outside temperatures or when the system requires extra legionella protection in the hot water cylinder. In accordance with building regulations in the UK, hot water cylinders are expected to regularly reach temperatures exceeding 60°C to safeguard against Legionella. This paper aims to present illustrative examples and a design guide for cost-effective retrofit options using newly developed high-temperature heat pumps capable of delivering water supply temperatures of 75-85°C, comparable to those produced by existing oil or gas boilers. Additionally, it explores alternative or novel design possibilities to aid practicing engineers or consultants in devising simple and energy-efficient heat pump installations for both new constructions and retrofit applications.