Intergas F018 Fault Code: Causes, Fixes & Repair Costs
What does the Intergas F018 fault code mean?
The F018 code tells you that your Intergas boiler has detected a restriction or blockage in either the flue outlet or the air intake duct. Unable to safely expel combustion gases or draw in enough fresh air for burning, the boiler locks itself out as a protective measure. Left unresolved, a restricted flue can allow dangerous gases to accumulate — so the boiler is right to stop. The fault is most commonly traced to something obstructing the external flue terminal, though damaged flue pipework or an incorrectly specified flue run are also possibilities.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Blocked external flue terminal Common
Debris such as leaves, cobwebs, a bird's nest, or a build-up of ice and frost in cold weather can partially or fully block the flue outlet on the outside wall or roof. This is the single most frequent trigger for F018, particularly in autumn and winter.
- Damaged or poorly fitted flue pipework Sometimes
A kinked, cracked, or incorrectly installed flue pipe restricts the flow of exhaust gases. This may have been present since installation or could result from physical damage — for example, a ladder strike or settling of the building.
- Flue run too long or too many bends Sometimes
Intergas boilers have maximum flue length specifications. If the original installation pushed beyond these limits, or if extra bends were added, airflow resistance may exceed what the boiler can tolerate, causing intermittent F018 faults — especially under heavy demand.
- Restricted air intake Sometimes
On room-sealed (balanced flue) systems, the combustion air intake and the exhaust share the same terminal assembly. Furniture pushed against an internal air duct, accumulated debris near the intake, or insulation lagging partially covering the duct can starve the boiler of combustion air and trigger F018.
- Wind or adverse weather at the flue terminal Rare
In exposed locations, driving rain, heavy snow, or persistent strong winds can temporarily disrupt the pressure balance at the flue terminal and cause a nuisance lockout. If the fault clears after a reset and fine weather returns, this is the likely culprit.
How to fix it
- Check your gas supply is working DIY safe
Try a gas hob ring or another gas appliance. If there is no gas supply to the property at all, contact your gas supplier rather than resetting the boiler.
- Inspect the external flue terminal DIY safe
Go outside and visually examine the flue outlet — typically a white or grey plastic terminal on an external wall or through a roof. Look for obvious blockages: a bird's nest, compacted leaves, cobwebs, or a build-up of ice. Clear any surface debris carefully. Do not attempt to reach inside the flue pipe itself or remove the terminal.
- Clear ice from the flue terminal if frozen DIY safe
In freezing weather, condensate or moisture can ice over the flue opening. You can carefully remove external ice with warm (not boiling) water poured gently over the terminal. Never use tools to chip at the flue itself.
- Check the area around the air intake DIY safe
On balanced flue systems the air intake is part of the same terminal assembly as the exhaust. Make sure nothing is positioned directly in front of it — bins, garden furniture, stored items, or overgrown vegetation can all restrict airflow.
- Reset the boiler (up to twice) DIY safe
Once you have cleared any visible external obstruction, follow the Intergas reset procedure for your model — usually holding the reset button for a few seconds until the display changes. Allow the boiler to run through its ignition sequence. If it fires up and stays running, monitor it over the next day or two. Do not reset more than two or three times in a row; repeated resets without fixing the root cause will not help and may mask a more serious problem.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If the boiler returns to F018 after a reset, or if you noticed any visible damage to the flue pipework, you need a Gas Safe registered engineer. They will carry out a full flue integrity check, inspect internal components such as the air pressure differential switch, test flue gas spillage, and carry out any necessary repairs or replacements. Any physical work on the flue — even replacing a terminal — is legally classified as gas work and must be done by a registered professional.
Parts you may need
- Intergas flue terminal (horizontal kit) · from £95
- Flue extension pipe (per metre, compatible) · from £35
- Air pressure differential switch · from £45
- Flue elbow/bend (45° or 90°) · from £25
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£400, depending on the underlying cause.
Frequently asked questions
Is the Intergas F018 fault dangerous?
It can be if ignored. The boiler locks out precisely to prevent combustion gases from accumulating in the property. While the boiler itself is not actively putting you at risk once it has shut down, you should fit a carbon monoxide alarm near the appliance if you do not already have one, and do not attempt to bypass the lockout. Get the fault diagnosed and fixed before using the boiler normally.
Can I fix the F018 fault myself?
A small number of checks are safe for a homeowner: inspecting and clearing the external flue terminal of debris or ice, making sure nothing is blocking the air intake, and resetting the boiler once or twice. Beyond that, all work on the flue itself — including removing or replacing the terminal, rejointing pipework, or inspecting internal components — is legally classed as gas work and must be carried out by a Gas Safe registered engineer.
How much will it cost to fix an Intergas F018 fault?
If the problem is simply a blocked flue terminal that an engineer clears during a call-out, you are typically looking at £100–£180 including labour. If the flue pipe or terminal needs replacing, a standard horizontal flue job usually costs £180–£350 parts and labour. More complex situations — a vertical flue through a roof, or one requiring scaffolding — can push costs to £400 or beyond. These higher-end scenarios are relatively uncommon; most F018 repairs fall in the £100–£400 range.
Why does my Intergas boiler keep showing F018 even though the flue looks clear?
If the external terminal is visibly unobstructed but F018 keeps returning, the fault is likely inside: a damaged section of flue pipe, a flue run that exceeds Intergas's maximum length or bend allowance, or a faulty air pressure differential switch that monitors flue flow. A Gas Safe engineer will carry out a full flue integrity test and check the switch and associated wiring to pinpoint the cause.