Overview
E07 on a Keston C36 combi means the boiler has detected an overheat condition in the flue. The flue temperature is higher than the boiler's safe limit, indicating that combustion gases are leaving the heat exchanger hotter than they should. This can be caused by poor heat transfer inside the heat exchanger (scale, blockages, or a crack), incorrect or restricted flue flow, failed sensors/thermostats, or poor system circulation so the return water is too cool compared with the flue. Severity: This is a serious fault because it indicates the boiler is not exchanging heat correctly and can force the appliance into lockout to protect components. High flue temperatures reduce efficiency and, in extreme cases, may indicate a failing heat exchanger or other component that a qualified engineer must inspect. There is also potential risk of unsafe combustion behaviour if the flue or heat exchanger is compromised. DIY vs professional: Some basic, non-intrusive checks can be done by a competent homeowner (reset the boiler, check the external flue terminal is clear, check system pressure and that the pump is running, bleed radiators). However, dismantling the flue, heat exchanger, or performing gas/combustion checks, replacing sensors or components must be done by a Gas Safe / qualified heating engineer. If the fault persists after basic checks, call a professional immediately and do not attempt internal repairs yourself.
Possible Cause: Flue overheat – check combustion and flue temperature vs. return temperature – the flue temperature should be no more than 10C above return temperature. Possible heat exchanger fault.
Troubleshooting Steps
Safety precautions:
1) If you smell gas or suspect a carbon monoxide issue, leave the property immediately and call the gas emergency number. Do not operate electrical switches or the boiler.
2) Turn off the boiler at the mains isolator and the gas supply if you will be working in the appliance vicinity. Only perform visual and external checks unless you are a qualified engineer.
3) Ventilate the area around the boiler and keep children/pets away. Do not remove the boiler casing or covers unless you are Gas Safe registered.
Initial homeowner checks (non-intrusive):
1) Note the exact error code and when it appeared. Attempt a standard reset following the boiler user manual and observe whether the code returns.
2) Check the external flue terminal: visually inspect that it is not blocked by bird nests, leaves, snow or other debris. Clear any safe-to-reach obstruction from the outside terminal only. Do not force or probe the flue internally.
3) Check heating system pressure on the boiler gauge. If very low, repressurise to the manufacturer recommended level (usually ~1–1.5 bar) following the manual, then reset and test. Low pressure can cause circulation issues.
4) Verify radiators and thermostatic valves are open and that central heating demand is present. Bleed a radiator to remove airlocks that might reduce circulation.
5) Listen for the pump when the boiler runs; feel the flow and return pipes at the boiler (careful, they may be hot). The pump should be running and pipes should be warm. If the pump appears not to run or pipes are not warming, this indicates circulation problems.
Specific diagnostic steps you can perform safely:
1) After the simple checks above, run the heating and observe temperatures if the boiler displays flow, return and flue temperatures. Compare flue temperature to return temperature: the flue temperature should normally be no more than about 10°C above the return temperature. If the flue is much higher, note the values and how quickly they rise.
2) If the flow temperature is excessively high or the difference between flow and return is very large (more than ~20°C), this suggests poor circulation. Continue by bleeding more radiators, checking any accessible zone valves are open, and ensuring there are no frozen/closed radiator valves.
3) If the external flue terminal was blocked and you cleared it, retest the boiler. If the fault clears, monitor for recurrence; a permanent blockage or damaged terminal may need an engineer to replace or reposition it.
4) If the pump is suspected faulty (no vibration/heat, or system not circulating despite pump power), you can try a brief manual override if your boiler manual describes it and you are comfortable. Otherwise, stop and call an engineer.
When to stop and call a professional:
1) If the E07 code returns after you have done the basic checks (reset, cleared flue terminal, checked pressure and circulation), stop further attempts. Persistent flue overheat commonly points to a failing heat exchanger, blocked internal passages, a faulty flue thermostat or sensor, or other gas-side/component faults that require dismantling and testing.
2) Any work that requires removing the boiler front cover, burner, flue components, heat exchanger, sensors, or any gas or combustion adjustments must only be carried out by a Gas Safe registered engineer. DIY gas appliance repairs are unsafe and illegal.
3) Inform the engineer of the checks you performed and the temperature readings (flow, return, flue) and whether the pump was running. This information helps them diagnose faster.
Summary and important notes:
1) Temporary resets and clearing an external flue obstruction or restoring proper circulation may clear E07, but this code can indicate a major component fault. Do not ignore repeated occurrences.
2) For safety and correct repair, call a qualified Gas Safe / competent heating engineer to inspect combustion, flue temperatures, sensors/thermostats and the heat exchanger if the fault persists. Avoid any intrusive repairs if you are not qualified.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.