Expansion vessel failure is very common . The expansion vessel consists of a metal tank containing an elastic rubber membrane that separates air (or another inert gas) from the water in the system.

This elastic membrane has to be constantly expanding and contracting. Every time the heating is turned on, the reservoir has to absorb the expansion. That's why it's a part subjected to a lot of wear and tear.

There may be a momentary increase in pressure in the circuit that caused the expansion vessel membrane to rupture, but if it were not very weak, the safety valve "should" activate before the rupture.

A fault in the expansion vessel should be indicated by the constant evacuation of water through the system's safety valve. In systems with a wall-mounted boiler, the safety valve is routed through a transparent pipe beneath the boiler itself. In oil-fired or heat pump systems, the safety valve should be located near the expansion vessel.

ATTENTION: Junkers has changed the labeling of expansion vessels to comply with eurregulations. It is very likely that the expansion vessel you receive will have a slightly higher or lower volume indicated on the label. Please check if your boiler's serial number matches those listed here. This item is shipped directly from the factory and is non-returnable.