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Boiler Overheat thermostat and safety cut-outs: What It Does and Signs It's Failing

These are the last line of protection if the normal controls fail. A heat exchanger overheat thermostat is mounted on the heat exchanger itself, and many boilers also have a flue overheat thermostat or flue gas sensor on the exhaust side. Worcester's Greenstar i, for example, has both.

How it works

Each one is a temperature switch wired in the safety circuit. If water or flue gas passes its limit, it breaks the circuit, the gas valve closes and the boiler locks out until someone resets it. Software on newer boilers tries to shut down before the thermostat ever has to trip.

Signs it is failing

• Overheat or flue gas temperature lockout
• Boiler locking out every time it has run for a while
• Reset brings it back briefly before the same code returns
• Lockout that won't clear even when the boiler is cold

Overheat and flue gas temperature codes come from these devices. Most of the time the switch is doing its job and the cause is low pressure, a stuck pump or diverter, or scale.

Can you replace it yourself?

Never bypass, link out or disconnect a safety cut-out. Check the pressure, let the boiler cool and reset it once as your manual shows; Ideal's fault-finding steps also cover refilling and venting the system and opening the isolation valves. If it trips again, or you notice fumes or soot, switch the boiler off and have a Gas Safe engineer (or an RGI in Ireland) find the cause.

Buying the right part

These are safety parts, so only the manufacturer's listed part for your model should go in.

Error codes linked to the overheat thermostat and safety cut-outs

• E110 / E130Overheat lockout (primary water or flue)Baxi
• F.25Flue gas temperature too highVaillant
• E9 / E9 224Overheat limiter trippedWorcester Bosch

Last reviewed 2026-10-21.