BoilerFaultCodesTechnical description
Safety thermostat (STB) exceeded
What causes this fault?
This fault occurs when the water inside the boiler gets too hot, causing the safety sensor to trip and shut the system down to prevent damage. The most common reasons are a broken circulation pump that isn't moving the water away from the burner quickly enough, or a build-up of sludge and debris creating a blockage in the heat exchanger.
This issue often comes to light during the first cold snap of autumn when the heating is turned back on after being idle all summer, which can cause internal pump parts to seize.
Engineer required
This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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