Guidance to Schedule 2 paragraph 17

Hot Water Services

All unvented hot water heaters must be capable of accommodating expansion or fitted with adequate safety arrangement to prevent systems overheating and over pressurising.

Accommodation of expansion

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

This is however only allowed 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. As this may result in waste or impact on wholesomeness.

• 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.

Acceptable safety devices include:

• Temperature relief valves

• Temperature control arrangements such as non-self-resetting energy cut outs and cylinder thermostats (interlocked to prevent flow from the primary circuit).

• Combined temperature and pressure relief valves

• Pressure reducing valves

• Expansion vessels installed in combination with expansion relief valves.

For further information, including advice about safety device arrangements for different methods of heating, please refer to the relevant version of the Building Regulations.

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

Please note: in England, Wales and Scotland this requirement is met by safety devices which satisfy the relevant Building Regulations.