Grant Expansion Vessel Failure

Boiler pressure rises significantly when heating is on and drops back when off, with occasional discharge from PRV.

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

Loss of air charge in expansion vessel, Ruptured vessel diaphragm, Incorrect vessel sizing, Blocked connection between vessel and system

Safety: Hot water may discharge from the PRV; avoid contact. Do not cap or block the discharge pipe.

User actions:

Monitor the pressure gauge during heating cycles and note the range for the engineer.

Engineer actions:

Isolate and depressurise the system, then check the expansion vessel pre-charge with a tyre gauge.
Recharge the vessel to the specified pressure using a pump, or replace if the diaphragm has failed (water from Schrader valve).
Ensure the connection pipe between vessel and system is clear.

Medium 3/5

Expansion vessel

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