As the UK edges closer to the Future Homes Standard 2025, air source heat pumps (ASHPs) are fast becoming the go-to heating solution for new homes. Yet, despite this shift, one critical part of system design remains stuck in the past: the way we size domestic hot water cylinders.

Our latest report  Accounting for WWHRS and IES in Hot Water Cylinder Sizing, reveals how guidance is leading to oversized, inefficient systems and what needs to change to deliver truly sustainable homes.

The hidden problem in Cylinder-Sizing

Current industry methods for cylinder-sizing overlook two technologies that are now increasingly common in UK homes: Waste Water Heat Recovery Systems (WWHRS) and Instantaneous Electric Showers (IES). Their impact means designers often specify tanks that are larger than necessary — wasting energy, space and money.

What the research shows

To dig into this issue, Showersave partnered with Chris Martin (formerly of the Energy Monitoring Company) and Triton Showers to analyse how ASHPs interact with WWHRS, IES and mixer showers. The findings are striking:

  1. WWHRS and IES reduce hot water demand by nearly half – from over 200 litres per day to just over 100 litres. Smaller tanks, faster reheating and reduced reliance on costly electric backup all follow.
  2. Electricity use drops significantly – daily consumption for hot water falls from over 4 kWh to around 3 kWh, cutting both carbon emissions and bills.
  3. A better model exists – traditional sizing standards like BS 6700 overestimate tank needs, while BS EN 15316 provides a more realistic and efficient approach based on actual user behaviour.

The recommendations

Our report recommends key industry bodies like NHBC, MCS and CIBSE — update their guidance and incorporate WWHRS and IES into design standards. It also recommends that SAP factors these technologies into its calculations and that BS EN 15316 becomes the go-to sizing method.

By embracing these updates, housebuilders, designers and policymakers can help create homes that are more efficient, space-saving and future-ready. If we don’t, we risk undermining the environmental goals of the Future Homes Standard before they even take hold.

Read the full report: Accounting for WWHRS and IES in Hot Water Cylinder Sizing here.