How can you accommodate thermal expansion?

All heating systems must be able to accommodate thermal expansion.

The preset water level for expansion cisterns or combined feed and expansion cisterns should accommodate at least 4% of the total volume in the primary system circuit before discharging through the overflow or warning pipe.

Did you know when heated up to 100°C the volume of water expands by around 4%, conversely when freezing this in around 9%?

Unvented systems, with the exception of some instantaneous water heaters, must incorporate specific safety features to do this.  Any discharge from a safety device should be safe and conspicuous, refer to the manufacturer’s installation instruction for further information.

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.