Guidance Note: Domestic hot water temperatures from instantaneous HIUs

The impact of domestic hot water on energy efficiency is a relatively unexplored subject. Historically, domestic hot water has represented a relatively low percentage of building heat energy demand and has been supplied largely via cheap combustion heat sources. 

In the context of the climate emergency, and a move to lower carbon building designs, we have seen space heating energy requirements fall significantly. This means that domestic hot water now represents an increasingly significant percentage of total building energy consumption. 

However, the design requirements for domestic hot water are inconsistently understood and applied, particularly in low volume instantaneous hot water systems. This is exacerbated by disparate sources of guidance providing conflicting advice, in terms of both function and safety. There is a clear opportunity to develop a consistent approach to the design of these systems. This approach should prioritise function and safety, but also support low-carbon building design.

This Guidance Note summarises current standards and legislation, clarifies where these are relevant to low volume instantaneous hot water systems and sets out a design approach that addresses all key temperature requirements. It aims to provide clear guidance to support practitioners in the delivery of lower carbon, safer and more reliable domestic hot water.

Contents:

• Executive summary
• Introduction
  • Updating guidance to suit current domestic hot water system practice
  • Why hot water temperature matters
  • Domestic hot water systems for net zero buildings
• Typical domestic hot water systems
• Summary of current guidance
• Safety requirements for domestic hot water
  • Minimising risk of burns
  • Legionnaires’ disease
  • Guidance for low volume instantaneous system risk assessments
  • Typical application of a low volume instantaneous system
  • Current practice in countries with established heat network markets
  • Current practice in the UK
  • Summary of safety requirements
• User requirements for domestic hot water
  • Cleaning temperature
  • Bath tap temperature
  • Delivery time
    • Tap temperatures in practice
    • Specifying and measuring delivery time
  • Summary of user requirements
• The benefits of lower hot water temperatures
  • Heat pumps
  • Heat losses and overheating
  • Limescale
• Conclusions and recommendations
  • Recommended approach

Principal authors

CIBSE Domestic Hot Water Working Group - Andrew Mackay, Arup (Chair); Huw Blackwell, Anthesis; Rob Boyer, Aecom; Jassim Daureeawo, Vortex; Jonathan Gaunt, Cundall; Gareth Jones, FairHeat; Phil Jones, CIBSE CHP-DH Group; Andrew Mathews, KIWA; Becci Taylor, Arup.

Contributing authors

Iona Norton, Arup; Sarah Bousquet, Arup.

Peer reviewers

Steve Harper, Galliard Homes Ltd; Paul Woods, independent consultant; Chris Parsloe, Parsloe Consulting Ltd.