40 Rear flue duct 180-239
The P-811 error code on Intergas boilers indicates 40 rear flue duct 180-239.
Intergas P-811 means: 40 rear flue duct 180-239. On this component the documented failure modes are flue or air intake blocked or restricted. A Gas Safe registered engineer should diagnose the root cause.
What Does P-811 Mean
The P-811 code is shown by the boiler’s control board when it detects 40 rear flue duct 180-239. The boiler then stops instead of continuing to run in that condition: the code is the appliance reporting a problem, not proof that the boiler is broken.
| What you see on the boiler |
|---|
| The display shows P-811 and the boiler stops until the fault is cleared |
Common Causes
The manufacturer does not publish a cause list for this specific code. The causes below are the failure modes documented for the same component on other boilers, confirmed by the service literature of 7 different manufacturers.
| Cause |
|---|
| Flue or air intake blocked or restricted |
Safety Warning
P-811 is a gas-related fault. Never attempt to repair or adjust the gas valve, burner assembly, or flue system yourself. All gas work must be carried out by a Gas Safe registered engineer (UK) or licensed HVAC technician (US). Repeated resets after ignition failure can cause unburnt gas to accumulate inside the combustion chamber — this is a serious safety hazard. If you smell gas at any point, turn off the gas supply at the meter, open windows, leave the property, and call your gas emergency service immediately. Warning: this fault code may relate to gas supply, flue or combustion issues. Do not attempt repair — contact a Gas Safe registered engineer immediately.
Parts & Prices
Prices shown are indicative. Actual prices and availability vary by region.
How to Fix It
Clear any flue blockage
correct the flue installation if it exceeds maximum length or bends
clear ice from the flue terminal (insulate external condensate pipe)
clear the condensate drain
if combustion analysis shows incorrect values, adjust the gas valve or service the burner
Inspect the flue and air intake along their whole length, including the outside terminal, and clear any obstruction or debris.
Refer to the Intergas service documentation for the detailed procedure. Only a qualified engineer should carry out these repairs.
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Affected Models
Intergas models that can display fault code P-811:
| Model Family | Output Range | Frequency |
|---|---|---|
| All Intergas models | — | Possible |
Frequently Asked Questions
Clear any flue blockage. According to the technical documentation, this is the first check on the list for P-811.
P-811 is classified as a high-severity fault: it involves gas, combustion or flue components. Do not keep resetting the boiler and do not attempt the repair yourself — call a Gas Safe registered engineer.