ClimateXChange published a study on 3 April 2025 examining whether Scottish homes can run at lower flow temperatures, the temperature a wet heating system warms water to before sending it to radiators1. The research, carried out by Element Energy (part of the ERM Group), was completed in February 20232. It assesses the current housing stock and two cost levels of retrofit against a range of external temperature test criteria1.
The study found that most of the Scottish housing stock is currently unsuitable for flow temperature reduction to 55°C or below on a winter peak day1. The current majority of wet systems operate at 70-80°C, and around 85% of Scottish dwellings are heated by water based boiler systems2. Suitability is defined as a dwelling's ability to reach thermal comfort for a range of external temperature test criteria1.
"In the higher cost retrofit scenario, 76% of homes become suitable for a flow temperature of 55°C on a winter peak day"
Effective retrofit measures identified include wall and loft insulation, upgrading radiators, or a combination of smaller measures such as hot water tank insulation, draughtproofing and reduced infiltration measures1. The study states this could prepare the housing stock for zero direct emissions systems without requiring gas boilers to be removed from homes immediately1.
The research also 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, suggesting these dwellings either run at higher temperatures or cannot reach thermal comfort at peak demand1.
| Measure | Finding |
|---|---|
| Homes suitable at 55°C today | Most of the stock unsuitable on a winter peak day1 |
| Homes suitable at 55°C after higher cost retrofit | 76%1 |
| Stock unsuitable below 75°C | 30%1 |
| Stock requiring above 75°C | 20%1 |
| Fuel bill savings from flow temperature reduction | £151m to £501m in stringent test cases1 |
| Greenhouse gas emission savings | 6.17 to 10.18 MtCO2 equivalent per year1 |
The study used stringent criteria, requiring a dwelling to be heated to 20°C during the coldest hour of an average or 20-year winter peak, selected to ensure the heating needs of the most vulnerable households were considered1. ClimateXChange notes this may not reflect the reality of how heating systems should be expected to perform1. The report also states that exploring varying flow temperatures throughout the year could be one way to increase savings1.
Why it matters for households
Flow temperature determines how much energy a wet heating system uses to reach the same room temperature. A home that cannot reach thermal comfort at 55°C on a winter peak day may need larger radiators, better insulation or both before it can run efficiently at the lower temperatures that heat pumps operate at2. The study links lower flow temperatures to reduced fuel bills and emissions in gas boiler and zero direct emissions systems alike2. For a household, suitability depends on the building envelope, radiator and pipework capacity, and how much heat the property loses2. The research does not set out costs per home, eligibility for support, or a timetable for any requirement to change heating systems; none of these have been reported in the study.
What happens next
The study sets out no dated next steps. It notes that the transition to zero direct emissions heating will require changes to the Scottish housing stock, in the context of the Scottish Government's target of net zero emissions by 20451.
Sources2 cited
- Reductions in maximum flow temperatures in Scottish domestic heating | ClimateXChange, climatexchange.org.uk
- Reductions in maximum flow temperatures in Scottish domestic heating | ClimateXChange, climatexchange.org.uk