Challenges of retrofitting air source heat pumps to old heating systems

Air source heat pumps (ASHPs) are being touted as a cornerstone of the UK's transition to low-carbon heating, replacing fossil fuel boilers with systems that extract heat from the outside air. However, while the technology offers significant environmental benefits in new builds, retrofitting ASHPs to existing homes with older pipework and radiators can present a unique set of challenges, sometimes negating the expected efficiency and comfort gains.

The problem with low flow temperatures

ASHPs operate most efficiently at lower flow temperatures (typically 35–55°C) compared to traditional gas boilers (which often operate at 70–80°C). This difference is crucial. Older heating systems, prevalent in many UK homes, were designed for these higher temperatures, relying on smaller diameter pipes and radiators with a smaller surface area.

1. Inadequate heat output

• Radiator sizing: Older radiators are often undersized for the lower flow temperatures of an ASHP. With cooler water circulating, they simply can't emit enough heat to adequately warm the rooms, leading to cold spots and a generally uncomfortable living environment. This often necessitates replacing radiators with larger, higher-output models, adding significant cost and disruption to the retrofit process.
• Undersized pipework: Older houses commonly use microbore pipework (8–10mm) which is not appropriate for ASHP systems. This smaller diameter pipework will cause increased resistance to the flow of water, reducing the system's ability to transport heat efficiently. The pumps will need to work harder, and the temperature drop between the flow and return will be higher.

2. Increased running costs and reduced efficiency

• Heat pump cycling: When an ASHP struggles to deliver sufficient heat due to inadequate radiators, it can lead to "short cycling". The heat pump constantly turns on and off to try and maintain the desired temperature, placing extra strain on the components and significantly reducing its efficiency and lifespan. This also leads to higher electricity bills, negating one of the key benefits of ASHPs.
• Higher flow temperatures (and the efficiency penalty): To compensate for undersized radiators, homeowners or installers might be tempted to increase the ASHP’s flow temperature. While this can improve heat output, it drastically reduces the system's coefficient of performance (CoP) – the measure of its efficiency. The higher the flow temperature, the more electricity the ASHP consumes to generate heat, eroding the cost savings and environmental benefits.

3. System balancing issues

Uneven heat distribution: Older heating systems may not be properly balanced, meaning some radiators receive more hot water than others. This problem is exacerbated with an ASHP's lower flow temperatures, leading to some rooms being adequately heated while others remain cold. Thorough system balancing, including potentially upgrading thermostatic radiator valves (TRVs), is crucial but adds to the installation complexity.

Sludge build-up: Older systems often contain a build-up of sludge, made from corroded metal and other debris. This can block waterways and cause issues with pumps and valves. A thorough system flush is usually required before installation.

4. Potential for noise and vibration

• Increased water flow: While ASHPs generally operate quietly, the increased water flow required to compensate for lower temperatures can sometimes generate noise within older, narrow pipework, particularly if there are restrictions or air pockets.

5. System flushing and water quality

• The ASHP is far more sensitive to the water quality running through it than a traditional gas boiler. Because of this, older systems require extensive cleaning, often needing a ‘power flush’ – a more intensive version of a standard chemical flush. This can be a problem in older systems as the process of flushing can expose leaks in old pipes and joints.

Mitigation and solutions

While the challenges are real, they are not insurmountable. A successful ASHP retrofit to an older heating system requires careful planning and often necessitates upgrades:

• Thorough system assessment: A qualified heating engineer should conduct a comprehensive assessment of the existing system, including radiator sizing, pipework diameter, insulation levels, and overall heat loss calculations.
• Radiator upgrades: Replacing undersized radiators with larger models, potentially including double-panel or low-surface-temperature radiators, is often necessary.
• Pipework modifications: In some cases, upgrading sections of pipework to a larger diameter might be required, especially if the existing pipework is severely undersized or corroded.
• Proper system balancing: Ensuring all radiators receive the correct flow of water is essential for even heat distribution.
• System flushing and ongoing maintenance: The installer should flush the system to remove any debris, and it is important to maintain the cleanliness of the system by adding inhibitor and potentially fitting a filter.
• Insulation and airtightness: The key to good heat pump performance is a well-insulated and airtight house. The lower the heat demand of the house, the less the issues with pipe and radiator sizes become relevant.

Conclusion

Retrofitting ASHPs to older homes is not a simple "plug-and-play" solution. While the long-term benefits of lower carbon emissions and potential cost savings are attractive, homeowners must be aware of the potential pitfalls associated with using existing pipework and radiators.

A well-planned installation, based on a thorough assessment of the existing system and incorporating necessary upgrades, is crucial to ensure the ASHP operates efficiently and effectively, delivering the desired comfort and environmental benefits. Failing to address these issues can lead to an underperforming, inefficient system that ultimately costs more to run and fails to deliver on its promise.