As the UK races to meet its net-zero target by 2050, heating remains one of the biggest hurdles. Domestic hot water alone accounts for around 19% of total residential heat demand, yet 87–89% of that still comes from fossil fuels. While heat pumps and solar PV installations are increasing, uptake remains limited in retrofits, especially in space-constrained and cost-sensitive homes.
A new study led by researchers at Edinburgh Napier University in collaboration with Worcester Bosch, Okana Global and Sunamp explores phase change material (PCM) thermal energy storage (TES) technology – Sunamp’s heat batteries – as a practical and scalable low-carbon alternative.
The findings, which were presented at the International Retrofit Conference 2025 at University of Salford, suggest that, when paired with solar PV, PCM-based heat batteries can significantly cut emissions and energy bills in existing UK homes. But researchers say current policy and regulatory frameworks are failing to support this innovation.
“We’re looking at a currently missed opportunity to reduce emissions from domestic hot water heating with minimal disruption to households,” said co-author Prof. Robert Hairstans, Chair in Offsite Construction at Edinburgh Napier University and skills and EDI lead for the Centre of Net Zero Carbon High Density Buildings. “Heat batteries like Sunamp’s can be retrofitted, work passively once installed, and help households shift energy use to when it’s cheapest and greenest. But the technology is invisible to key policy mechanisms.”
The study used telemetry data from a typical four-bedroom retrofitted home equipped with a 2.5kW solar PV system, recorded using Okana Global monitoring equipment. The Sunamp Thermino heat battery was used to preheat water for a gas combi boiler. This simple adjustment delivered yearly cost savings of around £91 and a carbon emissions reduction of up to 86% during summer months.
Prof. Hairstans said: “The true performance potential is high, especially if installers follow best practices like better pipe insulation, optimised temperature settings, and optimal PV placement. The takeaway is clear: this is a scalable, viable solution for many homes.”
However, deployment is being blocked by a lack of recognition in tools such as the Standard Assessment Procedure (SAP), the current benchmark for measuring home energy performance, and its forthcoming replacement, the Home Energy Model (HEM). Sunamp heat batteries are also excluded from retrofit subsidies like ECO4 and the Warm Homes: Social Housing Fund.
Co-author Andrew Keyworth, Head of Business Development (Electrification) at Worcester Bosch, said: “The impending publication of the Future Homes Standard is likely to result in CO2 targets for new dwellings that can’t be met using a gas-fired boiler, and should any future ban be introduced for gas boiler installations in retrofits, the urgency for scalable, equitable alternatives is mounting. Yet high upfront costs remain a barrier for many households. We’ve shown, when paired together, these technologies can reduce emissions and costs, yet they’re not acknowledged in key government schemes. That creates a feedback loop where manufacturers can’t scale, installers don’t get trained, and households miss out.”
Sunamp’s compact design and passive operation give it an edge for retrofits, especially in social housing and smaller homes. But without integration into government policy and financial schemes, the technology risks being sidelined.
Catalina Dobas, Thermino product owner at Sunamp, said: “A just transition means enabling affordable, low-disruption solutions for everyone, not just new builds or wealthier households. If we want to decarbonise domestic heat, PCM heat batteries must be part of the conversation. Immediate action is required to recognise thermal energy storage solutions in national assessment tools, to integrate them into subsidy schemes, and to include them in installer training. Without that, the UK risks missing both its carbon targets and the opportunity to make the energy transition fair for all.”
View the video case study:
Editor’s notes
Information in this press release has been taken from: Dobas, C., Ernest, G., Hairstans, R., Gani, M., Livingstone, A., Stinson, J., & Keyworth, A. (2025) “Sunamp Heat Battery as a Pre-heating Solution for Domestic Hot Water Heating: A Case Study Evaluation”. [Publication URL: https://doi.org/10.17866/6xrn-cx50]
Collaboration funding support was provided by the Edinburgh and South East Scotland City and Region Deal – Housing Construction and Infrastructure Skills Gateway and UKRI Centre for Net Zero Carbon High Density Buildings.
