Alternative to cold-water expansion valves

The NZ Building Code G12 Amendment 13 details an alternative to cold water expansion valves (CWE), utilising an expansion vessel for mains pressure hot water cylinder installations.

The use of an expansion vessel has been used in many countries as a means of allowing for thermal expansion for various reasons. These include the elimination of the CWE relief drain pipework being exposed at its termination outside where freezing conditions may prevent the safe operation of these important safety devices and systems.

There are several considerations when this option is being explored as an alternative to a cold-water expansion valve. It must:

• Comply with BS EN 13831 and be suitable for use with potable (drinking) water
• Be sized to ensure that the maximum system pressure does not exceed the working pressure of the hot water storage vessel and the working pressure of expansion vessel itself
• Be pre-charged to a pressure matching the water supply pressure to the mains pressure storage water heater
• Be installed with a heat trap on the cold-water supply pipe between the expansion vessel and the storage water heater when the expansion vessel is located within 500 mm of a storage water heater inlet
• Be installed in a manner which provides for easy access for replacement, servicing and maintenance
• Be adequately supported or restrained to prevent damage at the point of connection of the vessel to the pipework if the vessel is subject to external forces

The sizing of the expansion vessel is critical to its correct operation.

Calculating the minimum capacity

G12/AS1 paragraph 6.6.8 gives the formula for calculating the minimum capacity.

The minimum capacity of an expansion vessel shall be calculated from the formula:

Ve = Vs x η / AF

Where:

Ve = minimum capacity of expansion vessel (litre)

Vs = volume of hot water storage (litre)

η = expansion factor (from Table 7)

AF = (P2 – P1)/(P2 + 101)

P1 = water supply pressure (kPa, typically the setting of the pressure limiting or pressure reducing valve)

P2 = 0.85 x TPR valve setting (kPa)

Tables

The attached picture Table 7: Expansion Factors shows common sizes of hot water cylinders and the thermostat setting is also provided to assist in the selection of the correct sized expansion vessel.

If the cylinder size or temperature setting of the thermostat is not listed in the Ve Minimum Expansion Vessel Capacity (litre) picture (attached), use the formula to calculate the vessel size.

Tech help

The Board's technical helpline can assist with any situation that is outside of these parameters. Call 0800 743 262 during business hours to speak to a technical advisor.