Glow-worm F33 Fault Code: Causes, Fixes & Repair Costs
What does the Glow-worm F33 fault code mean?
The F33 code on a Glow Worm boiler signals that the fan has malfunctioned or that there is an airflow problem preventing safe operation. When this fault appears, the boiler locks out and will not attempt to fire until the underlying issue is resolved. The fan's job is to push combustion gases safely out through the flue — if it cannot reach the correct operating speed, the boiler's control board cuts power as a safety precaution to prevent potentially harmful gases, including carbon monoxide, from entering your home. On UK models specifically, F33 relates to a fan or airflow fault rather than the air pressure switch definition that appears in some international reference guides. You may also see the closely related F3 code on some Glow Worm models, which is officially described as the fan being unable to reach ignition speed — the cause and remedy are essentially the same.
General guidance only — not a substitute for professional advice. 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
- Worn or failed fan motor Common
The fan motor is the most common culprit. Over time the motor windings can burn out or the bearings can deteriorate, preventing the fan from spinning at the speed required for ignition. You may hear a humming noise if the motor is trying but failing to turn, or total silence if it has failed completely.
- Wiring fault or loose electrical connection Common
The fan relies on a secure electrical supply from the PCB. Vibration over years of use can loosen connectors, and wiring can chafe or corrode. A poor connection may cause the fan to cut out intermittently, producing the F33 code even though the fan itself is physically intact.
- Blockage in the flue or air intake Sometimes
Debris such as leaves, bird nesting material, or a collapsed flue terminal can restrict airflow. The boiler detects that exhaust gases cannot escape at the correct rate and triggers a lockout. Checking the visible section of the flue outlet outside is a sensible first step.
- Frozen condensate pipe Sometimes
During cold snaps the condensate pipe — which drains acidic water away from the boiler — can freeze solid. A blockage here creates back-pressure that can interfere with airflow through the heat exchanger, potentially triggering an F33 code alongside or instead of a condensate fault.
- Faulty tachometer or speed-sensing circuit Rare
A small sensor (tachometer) monitors the fan's rotational speed and feeds that data back to the PCB. If the sensor or its wiring fails, the board receives no speed signal and assumes the fan is not running, logging F33 even when the fan is physically spinning normally.
How to fix it
- Check the gas supply to the property DIY safe
Make sure other gas appliances such as a hob or gas fire are working. If there is no gas supply at all, contact your gas supplier rather than resetting the boiler.
- Inspect the flue terminal outside DIY safe
Go outside and look at the point where the flue exits the wall or roof. Check for obvious obstructions like debris, a bird's nest, or a damaged terminal guard. Do not attempt to dismantle any flue components — just note what you can see from a safe distance.
- Check for a frozen condensate pipe DIY safe
In cold weather, locate the plastic condensate pipe (usually a white or grey 21mm pipe running from the boiler to an external drain or soakaway). If it feels solid or looks iced over, you can carefully thaw it using warm — not boiling — water poured over the pipe, or a heat wrap. Once thawed, attempt a reset.
- Reset the boiler once or twice DIY safe
On most Glow Worm models, turn the central heating control knob to the reset position (marked with a power symbol), hold for a few seconds, then return it to your desired temperature setting. Wait a minute to see if the boiler fires. If the F33 code returns within a short time, do not keep resetting — repeated lockouts without a fix indicate a genuine fault that needs professional diagnosis.
- Call a Gas Safe registered engineer to diagnose and repair the fan system Gas Safe engineer
A qualified engineer will use diagnostic tools to test the fan motor speed, check the tachometer signal, inspect wiring and connectors for damage, and examine the ignition electrodes and flame sensor while they are in the boiler. If the fan motor hums but will not spin, or shows no output at all, it will need replacing. All work on the fan assembly, flue system, and internal boiler components must be carried out by a Gas Safe registered engineer.
Parts you may need
- Boiler fan / flue fan motor assembly · from £95
- Fan wiring harness and connector set · from £25
- Fan tachometer (speed sensor) · from £30
- PCB (printed circuit board) — if fan fault is misdiagnosed · from £220
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 £225–£375, depending on the underlying cause.
Frequently asked questions
Is the Glow Worm F33 the same as the F3 fault code?
They are closely related. On UK Glow Worm boilers, F3 is officially defined as the fan being unable to reach ignition speed, while F33 points to a broader fan malfunction or airflow problem. In practice both codes lead to the same diagnosis — a problem with the fan, its wiring, or the airflow path — and both require a Gas Safe engineer to investigate and fix.
Can I fix the Glow Worm F33 fault myself?
There are a couple of checks a homeowner can safely carry out: confirming the gas supply is on, looking for obvious obstructions at the flue terminal, thawing a frozen condensate pipe if the weather is freezing, and attempting one or two resets. Beyond that, the fan assembly, internal wiring, and any flue components are not safe or legal for an unqualified person to work on. You need a Gas Safe registered engineer for the actual repair.
How much does it cost to fix an F33 fault on a Glow Worm boiler?
For most households this means a fan replacement, which typically costs between £225 and £375 all in, covering parts and labour. The fan unit itself usually accounts for roughly £100–£200, with the remainder being engineer time. If only a wiring connection needs reattaching the bill will be lower. In rare cases where the PCB has also been damaged, costs can rise further — your engineer will advise after diagnosis. Emergency or out-of-hours callouts will attract a premium on top of standard rates.
My Glow Worm boiler is still under warranty — do I need to pay for F33 repairs?
Glow Worm offers a standard warranty of up to 7 years on their boilers, extendable to 15 years if the boiler has been correctly installed and serviced annually. If your boiler is within the warranty period and has been regularly serviced, contact Glow Worm directly before booking an independent engineer, as the fan repair may be covered at no cost or at a reduced fixed call-out charge. Keep your service records handy when you call.