Reductions in maximum flow temperatures in Scottish domestic heating

Decarbonisation of domestic heating systems is crucial for achieving the Scottish Government’s ambitious climate change targets of net zero emissions by 2045. The transition to clean heating (e.g., heat pumps, district heating) will require changes to the Scottish housing stock, including preparing it to operate at lower flow temperatures than the current majority of 70-80°C. Flow temperature is the temperature a wet heating system warms water to before sending it to radiators in different areas of a building.

This study summarises the current evidence for flow temperature reduction in hot water (wet) systems and considers how this might be applied to the Scottish housing stock. Suitability is defined as a dwelling’s ability to reach thermal comfort for a range of external temperature test criteria. The researchers assessed the suitability of the present housing stock at present and with two different cost levels of retrofit.

Findings

Most of the Scottish housing stock is currently unsuitable for flow temperature reduction to 55°C or below on a winter peak day. However, many dwellings in Scotland could reach suitability for 55°C flow temperatures after the inclusion of retrofit(s). Effective retrofit measures include efficiency measures such as wall and/or loft insulation, upgrading radiators or a combination of smaller efficiency measures such as hot water tank insulation, draughtproofing and reduced infiltration measures.

In the higher cost retrofit scenario, 76% of homes become suitable for a flow temperature of 55°C on a winter peak day. This could prepare the housing stock to be ready for zero direct emissions systems without requiring gas boilers to be removed from homes immediately.

The research found that 30% of the overall housing stock is unsuitable for a flow temperature below 75°C, and 20% require a flow temperature above 75°C. This suggests these dwellings are either running at temperatures higher than 75°C or are currently unable to reach thermal comfort during periods of peak demand.

Fuel bill savings from reducing flow temperature are significant and range from £151m to £501m in the stringent external temperature test cases. The associated greenhouse gas emission savings are estimated to be 6.17–10.18 MtCO2 equivalent per year, depending on external temperature cases and retrofit scenarios. Exploring the potential for varying flow temperatures throughout the year could be one way to increase savings.

The most important factor when assessing suitability for flow temperature reduction is in setting temperature criteria that captures the needs of occupants. The research used stringent criteria in the assessments, requiring a dwelling to be heated to 20°C during the coldest hour of an average or 20-year winter peak. It was selected to ensure the heating needs of the most vulnerable households were considered. This may not reflect the reality of how heating systems should be expected to perform.

If you require the report in an alternative format, such as a Word document, please contact info@climatexchange.org.uk or 0131 651 4783.