Alpha combi boiler and water meter.

[Keith G. Powell]

In figure 5 of the installation manual, it states

"Note: if the mains is fitted with water meter, check valves or loose jumper stopcock, then a DHW expansion device must be fitted"

Does anyone know why? Oh, and what is a loose jumper stopcock and a DHW expansion device?

Keith G. Powell

[John]

The cold water in the dhw system expands as it heats up. If there is a non-return valve in the supply path to the boiler it cannot expand back into the supply pipework so the pressure within the trapped volume will rise and could in the extreme cause a rupture of the pipework or fittings since fluids are relatively incompressible . To avoid this an expansion device has a chamber with a diaphragm and a charge of compressed gas which "is" compressible and so cushions the system As the actual volume of expansion is "small" this may or may not be realistic, particularly if some of the system contains plastic pipework such as HEP2O. Water meters are often installed with a non return valve included in the kit. Loose jumper top taps can act as non return valves since the jumper can fall back onto the seat when no flow is passing. HTH