Baxi Expansion Vessel Failure / PRV Discharge

Boiler frequently loses pressure without visible leaks, pressure rises when hot then drops when cold

Safety Warning

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

Failed or flat Baxi expansion vessel, Pressure relief valve passing water, Hidden leak in pipework under floors, Overfilling the system repeatedly causing PRV damage

How to Fix / Troubleshooting

Safety first: Allow the system to cool before checking pressure.

User checks:

Note the pressure when cold and when the system is hot. If it rises above 3 bar and then drops very low when cold, the expansion vessel may be faulty.
Check outside for water dripping from the PRV discharge pipe when the boiler is running.
Avoid repeatedly topping up the system; this introduces fresh oxygen and can cause corrosion.

Professional actions:

An engineer will test and recharge or replace the expansion vessel, and replace the pressure relief valve if it has been passing.

Repair Difficulty

Medium 3/5

Required Part

Expansion Vessel

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