Glow-worm F.33 fault codewhat it means and how to fix it
F.33 is an air supply fault on the two ranges that carry it: a Compact's flue losing too much pressure, or a Betacom3's air pressure switch not proving. Not in the Energy or Ultimate tables.
Take care
Get a Gas Safe registered engineer to look at your Glow-worm F.33
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What F.33 means
Only two current Glow-worm manuals list F.33, and they describe it from different sides. The Compact installation manual defines it as "excessive pressure losses from the air/flue system" and gives three causes with their measures: "air/flue pipe blocked or partially blocked — check the entire air/flue pipe", "fan defective — check that the fan works correctly", and "PCB defective — replace the PCB".
The Betacom3 manual titles it "Fan frost protection function active: air pressure switch fault" — the "pressure monitor has not switched or has switched at a fan speed value that is too high (insufficient air supply)" — and blames "pressure monitor plug not correctly plugged in, pressure monitor defective, line removed from between the pressure monitor and silencer, air/flue pipe blocked and silencer not installed properly". The Betacom3 has an air pressure switch that most of its siblings do not, which is why the code exists for it and not for the Betacom4, and why the two-digit Ultracom2 and Betacom tables and every older Glow-worm list omit it.
In both versions the board is watching whether the air it needs is actually moving; when the flue resists more than Glow-worm allow, or the switch cannot prove the airflow, it stops rather than run combustion it cannot guarantee. A flue terminal that has picked up leaves, a bird guard clogged with debris, a joint that has slipped and condensate pooling in a run with the wrong fall are the usual reasons the resistance climbs; on a Betacom3 a tube knocked off the pressure switch during a service is the cheap one. Neither user guide gives a householder step for F.33, and there is none that would be safe: the flue is the engineer's to inspect.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Flue or air pipe blocked or partly blocked | Glow-worm's first cause on the Compact and one of four on the Betacom3. Debris at the terminal, a bird guard clogged, a flue run that has sagged and filled with condensate, or an air inlet obstructed. The whole pipe is checked, not just the terminal. | Common |
| Air pressure switch plug, tube or switch — Betacom3 | Glow-worm name the plug not correctly plugged in, the switch itself, and the "line removed from between the pressure monitor and silencer". A sensing tube pulled off during a service is the commonest of the three and the cheapest to put back. | Common |
| Fan losing performance — Compact | "Fan defective — check that the fan works correctly". A fan with worn bearings cannot develop the pressure the board expects even in a clear flue, and the board reads the shortfall as flue resistance. | Occasional |
| Silencer not installed properly — Betacom3, or PCB — Compact | The Betacom3 manual's last cause is the intake silencer fitted wrongly, which the pressure switch senses across. The Compact manual's last measure is to replace the PCB, reached only when the flue is proven clear and the fan proven good. | Rare |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£100–£400
Time on site
1h 30m
A flue inspected and cleared £100–£200. A pressure switch tube refitted on a Betacom3 is a call-out fee. A fan on a Compact £250–£350 fitted. A PCB is the top of the range.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Glow-worm fan assembly
£110–£220Authorised Glow-worm parts distributor
Price checked 28 September 2026
Glow-worm main PCB
£150–£320Authorised Glow-worm parts distributor
Price checked 28 September 2026
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Related Glow-worm fault codes
- F.32Get it checkedF.32 is a fan fault — the fan speed is outside what the board will accept, or the fan cannot be seen at all. Older ranges show F2 or F3. An engineer's code throughout.Related: fan and flue
- F.77Get it checkedF.77 on current ranges is the flue non-return flap or condensate pump not answering. On a Betacom2 or Ultracom2 it is the gas valve motor; on an Ultracom or Flexicom the optional condensate pump.Related: fan and flue
- F.25Get it checkedOn current ranges F.25 is the flue gas safety cut-out — a break in its plug or harness. On an Ultracom, Flexicom or cxi, F25 is a circulation fault. Different generations, different fault.Related: fan and flue
- F.28Get it checkedThree ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Related: same brand
- F.22Get it checkedGlow-worm's low-water-pressure shutdown, and the one code every Glow-worm user guide lets you clear yourself by topping the system up. The threshold differs by range.Related: same brand
- F.75Get it checkedGlow-worm's own documents give this code three different cause lists — pressure switch only, or sensor, pump or lack of water — and the fullest one is the Compact's eight-way diagnostic.Related: same brand
Own a Glow-worm boiler? Save it and its codes come first next time.
Related guides
Common questions about Glow-worm F.33
I have an Energy or Ultimate and the display shows F.33. Is that possible?
It is not in Glow-worm's tables for those ranges. Read the display again — F.32 and F.83 are the likelier codes — and quote what you see to the engineer.
Why does the Betacom3 manual talk about frost protection?
For the same reason its F.32 does: the board's frost routine runs the fan, and that is often the moment the air pressure switch fails to prove. The fault is the airflow, not the frost function.
Can I check the flue terminal myself?
You can look at it from the ground if it is visible and tell the engineer what you see. Do not climb to it or remove anything from it; the flue is part of the gas installation.
How is F.33 different from F.32?
F.32 is the fan not reaching the speed asked of it. F.33 is the fan reaching speed but the air not moving as it should — the board blames the flue first.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.