BoilerFaultCodesIdeal H9 Fault Code
The Ideal H9 fault code means: your boiler has sensed that the exhaust gases are getting too hot and has shut down as a safety precaution to prevent internal damage.. It appears on 9 Ideal models in our database.
Technical description: FLUE GAS OVERHEAT
What causes the Ideal H9 fault?
This fault occurs when the internal temperature of the exhaust fumes exceeds a safe limit, triggering an emergency shutdown to protect the boiler's components. The most common reason is a build-up of limescale or debris inside the heat exchanger, which prevents heat from being transferred to your water and instead sends it out through the flue. It can also be caused by a faulty sensor giving an incorrect reading or a blockage in the system's circulation.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Ideal models that show the H9 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Ideal H9 fault code information on this page is sourced and cross-checked against the official Ideal service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026