The waste water heat recovery (WWHR) sector has issued a formal response to the proposed Building Regulations Approved Document Part L (England) 2026 and the implementation of the Future Homes Standard (FHS), warning that current SAP 10.3 assumptions risk “designing out” one of the most effective technologies for reducing domestic hot water energy demand.
Supported by independent modelling from AES Sustainability Consultants, the response highlights a growing disconnect between the Future Homes Standard ambitions and the way homes are currently assessed for compliance.
Why Hot Water Matters
As homes become increasingly efficient through improved fabric performance and low-carbon heating systems, domestic hot water (DHW) is becoming one of the largest remaining energy demands in UK homes. In some house types, AES modelling found that hot water accounts for more than 50% of regulated energy demand.
Waste water heat recovery systems address this challenge directly by recovering heat from shower wastewater and using it to pre-heat incoming cold water. The technology is passive, low maintenance, and designed to work continuously without requiring any behavioural change from occupants.
Since the introduction of interim Part L 2021 standards, around 160,000 new-build homes have incorporated WWHR systems, helping developers reduce carbon emissions, improve EPC performance, and support net-zero housing delivery.
The SAP 10.3 Challenge
While WWHR remains included within the Government’s “notional” dwelling specification, the sector argues that SAP 10.3 effectively removes the incentive to specify it in many new homes.
The concern centres around outdated assumptions used within SAP 10.3 for heat pump performance, including:
- A notional heat pump CoP of 2.5 based on historic data that does not reflect modern system efficiencies.
- An “in-use” factor of 0.6, reducing the target efficiency threshold significantly below real-world heat pump performance.
- Assumptions originally intended to support apartment compliance now being applied across standard housing types.
As a result, many dwellings can achieve compliance using fabric measures, standard air source heat pumps, and photovoltaic panels alone — without reducing hot water demand through WWHR.
Independent Modelling Confirms the Risk
Independent SAP 10.3 modelling by AES Sustainability Consultants demonstrated that all tested dwelling types achieved compliance without WWHR systems installed.
However, when WWHR was included, the benefits were substantial:
- Annual running cost reductions of £50–£83 per dwelling in SAP calculations.
- Separate independent studies indicating savings of up to £158 per year for typical households.
- Total regulated energy demand reductions of up to 10.8%.
- Improved heat pump performance through reduced domestic hot water demand.
The findings reinforce the growing importance of hot water efficiency as homes continue to decarbonise.
Supporting Heat Pumps and Reducing Grid Stress
WWHR also plays an important role in supporting heat pump efficiency and reducing pressure on the electricity grid.
Without WWHR, heat pumps must work harder to raise cold mains water temperatures, particularly during winter months when inlet water temperatures are lowest. This can increase reliance on direct electric immersion heaters, leading to higher household energy bills and increased peak electricity demand.
The sector warns that widespread heat pump adoption could add between 6 GW and 7.5 GW to UK grid peak demand. By reducing hot water demand by 5–10%, WWHR can help alleviate strain on local infrastructure and support a more resilient transition to electrified heating.
Unlike photovoltaic systems, which generate less electricity during winter, WWHR systems are most effective precisely when heating and hot water demand are highest.
A Need for Regulatory Alignment
The response also highlights potential inconsistencies between SAP 10.3 and the forthcoming Home Energy Model (HEM), which uses more detailed event-based modelling for showers and hot water use.
This creates the risk of a “dual approach” to compliance, where homes built under different methodologies could have noticeably different operational costs despite meeting the same Future Homes Standard objectives.
Proposed Solutions
To ensure domestic hot water efficiency is properly recognised within Future Homes Standard compliance, the WWHR sector is proposing several key adjustments:
- Update heat pump CoP assumptions for dwellings under three storeys to better reflect modern system performance.
- Increase the notional dwelling in-use factor from 0.6 to 0.95.
- Introduce a functional requirement to reduce domestic hot water demand by at least 25%.
Building Net-Zero Ready Homes
Waste water heat recovery remains one of the simplest and most cost-effective ways to reduce domestic hot water energy demand in modern homes. Manufactured primarily from recyclable copper and designed to last more than 20 years, WWHR systems support circular construction principles while delivering long-term operational savings.
As the Future Homes Standard moves closer to implementation, the sector is calling for regulations that fully recognise the role of hot water efficiency in delivering genuinely low-carbon, affordable, and future-ready housing.
