Sime
Over‑Pressurization / Expansion Vessel Failure
Pressure gauge on the furnace rises rapidly when heating and may trigger the pressure relief valve.
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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.
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Possible Causes
Failed expansion vessel diaphragm, Incorrect pre‑charge, Overfilled system, Blocked expansion connection
How to Fix / Troubleshooting
Safety first: Allow the system to cool before working on the expansion vessel. Water may be hot and under pressure.
- Check cold pressure: When cold, the gauge should be around the recommended value (often 1.0–1.5 bar). If higher, bleed water from a radiator to reduce pressure.
- Test expansion vessel: A technician can check the air charge at the Schrader valve on the Sime expansion vessel. If water comes out of the valve, the diaphragm has failed and the vessel must be replaced.
- Inspect connection: Ensure the pipe between the system and expansion vessel is not blocked.
- Do not cap relief valve: Never block the pressure relief discharge; it is a safety device.
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Repair Difficulty
Hard
4/5
Required Part
Expansion Vessel
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