Advice
The Expansion Vessel: What It Does and Why It Fails
A hidden component behind many pressure faults.
The expansion vessel absorbs the increase in water volume as your heating system heats up, keeping pressure stable. When it loses its air charge or its internal diaphragm fails, you'll see pressure swinging high when hot and dropping when cold — and the relief valve may discharge.
Re-charging or replacing the vessel is a technician's job.
Signs of expansion vessel failure
- Pressure rises sharply when heating, drops when cold
- Relief valve discharging water outside
- Frequent need to top up or bleed pressure
- Pressure gauge needle swinging widely
This is not a DIY repair
Combi boilers are gas-fired appliances regulated under the BC Safety Standards Act. Opening the unit, touching the gas valve, or working on the flue without a valid BC gas-fitter licence is both unsafe and illegal. Diagnosing the fault is helpful — fixing it should always be left to a certified technician.
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