Worcester Bosch Failed Expansion Vessel / Over-Pressurisation

Boiler pressure rises rapidly when heating is on and may discharge water from the pressure relief pipe.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

Flat or ruptured expansion vessel diaphragm, Incorrect pre-charge pressure, Blocked connection to expansion vessel, Overfilled system

Safety first: Hot water may discharge from the PRV; keep clear of the discharge pipe.

Check cold pressure: When the system is cold, ensure pressure is around 1.0–1.2 bar. If higher, safely drain some water from a drain-off point or radiator to reduce pressure.
Observe pressure rise: If pressure climbs above 2.5–3.0 bar when hot, the expansion vessel likely needs attention.
Engineer tasks: An engineer should isolate and check the expansion vessel pre-charge with a gauge, recharge or replace the vessel, and replace the PRV if it has been passing.

Hard 4/5

Expansion Vessel

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