Specific Installation Guidance


Accommodation of expansion

The water fittings regulations/byelaws permit the accommodation of thermal expansion from unvented water heaters (with the exception of instantaneous water heaters with a capacity less than 15 litres) within the secondary hot water system.

This is however only permitted if:

  • The expanded water does not increase the temperature of the water in the supply pipe to in excess of 25°C in England and Wales 20°C in Scotland and Northern Ireland.
  • Both the expanded water plus any displacement can be accommodated within the supply pipe. It is not permissible for water displaced as a result of accommodating expansion to backflow into the pipework owned by the local water undertaker.

For further information contact the local water undertaker.


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Uncontrolled if downloaded. This is informative, non-statutory guidance and intended for general guidance purposes only; it is subject to change.

Compliance with this information should not be relied upon as guaranteeing no enforcement action will be taken by water undertakers. Water Regs UK accepts no liability for loss, indirect or consequential loss arising from or in connection with this guidance document.