How can you accommodate expansion?

All hot water systems must be capable of accommodating expansion. There are various ways to do this including:

(a) Secondary hot water systems only
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.

(b) Primary feed and expansion cisterns
To prevent waste 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 (heating) system circuit before discharging through the overflow or warning pipe.

(c) Expansion vessels
To address contamination concerns associated with stagnation and particulate accumulation it is recommended expansion vessels be installed so as to avoid localised low turnover (stagnation).

Specifically, they be installed securely in the vertical so that the water fitting is:

1. bottom fed and upright

2. the connecting pipework to the fitting

   • rises continuously

   • is kept to a minimum

3. sized correctly for the system

4. designed to ensure an adequate turnover of water within the expansion vessel.

Please note: Where an expansion vessel is used an expansion valve which operates at an appropriate operating pressure must be fitted to ensure water discharges safely and conspicuously in the event of a malfunction. To prevent waste the expansion valve must automatically close after a discharge.

No intervening valves should be installed between the expansion vessel, expansion valve and hot water heater/storage.