F.20
FAULT CODE
Vaillant
Safety Overheat Lockout
High Severity
Engineer Required
The boiler has reached an unsafe temperature and the safety thermostat has triggered. This can be caused by poor water circulation, a seized pump, or a blocked heat exchanger.
Step-by-Step Fix Guide
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Step 1
Turn off the boiler and allow it to cool for 30 minutes
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Step 2
Check the system pressure is between 1.0 and 1.5 bar
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Step 3
Bleed radiators to release any airlocks
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Step 4 Engineer only
If the fault returns after cooling, call a Gas Safe engineer
Gas Safe Reminder
Work on gas appliances must be carried out by a Gas Safe registered engineer. The steps above cover safe checks a homeowner can perform - any internal component replacement requires a qualified professional. If in doubt, always call an engineer.