Glow-worm F.22 Fault Code
The F.22 fault is the most homeowner-resolvable Glow-worm error — system pressure has fallen below the minimum operating threshold. In most cases, repressurising via the filling loop and resetting the boiler resolves it without an engineer.
What is the Glow-worm F.22 fault code?
The Glow-worm F.22 fault is triggered when the pressure sensor detects the central heating system has fallen below the minimum operating threshold — typically 0.5 bar. The boiler locks out to prevent running dry or overheating on low water volume. This fault code is safe for a homeowner to address provided there is no visible leak. Repressurise to 1.2–1.5 bar using the filling loop (two valves beneath or beside the boiler), then reset. If F.22 returns within days, a leak or failed expansion vessel is the cause and an engineer is needed.
What causes the F.22 fault?
All sealed central heating systems lose a small amount of pressure over time — a drop of 0.1–0.3 bar per year is normal. If the system has not been topped up in years, this alone can trigger F.22.
Bleeding radiators releases water alongside air, reducing system pressure. Always repressurise after any bleed.
A slow drip from a radiator valve, pump gland, or compression joint can cause pressure to drop weekly or monthly. Check all visible pipework and radiator tails for damp.
If the vessel loses its air charge, pressure fluctuates dramatically — it may read correct when cold but spike and vent when hot. The relief valve dripping is a sign of this.
A stuck-open automatic air vent slowly allows water to escape over weeks, causing gradual pressure loss without obvious signs.
What you can safely check yourself
The gauge is usually on the front of the boiler. If below 0.8 bar cold, repressurise is needed. If at zero or if you can see a damp patch, call an engineer rather than topping up.
Open the filling loop valves (two small valves under the boiler connected by a flexible hose) until the gauge reads 1.2–1.5 bar. Close both valves fully before resetting.
Reset the boiler and monitor the pressure gauge over the next week. If pressure drops by more than 0.3 bar within a few days, there is a leak — call a Gas Safe engineer.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Pressure test the system to locate hidden leaks
- Inspect and test the expansion vessel — recharge or replace if flat
- Check automatic air vents for sticking
- Inspect pump gland seals for weeping
- Check radiator valve gland nuts and tail connections
- Replace leaking components and retest system pressure
Expansion vessel failures are common across South London. Thames Water's hard water accelerates corrosion inside the vessel membrane, causing the air charge to gradually escape. Across Wandsworth, Merton, Lewisham, Lambeth, Southwark, Greenwich, and Bromley, 10–15 year old Glow-worm boilers frequently have flat expansion vessels — causing recurring F.22 faults that cannot be solved by repressurising alone.
How much does a F.22 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Glow-worm boiler showing F.22
Check the pressure gauge — repressurise to 1.2–1.5 bar if below 1 bar.
Ensure both filling loop valves are fully closed after topping up.
Hold the reset button for 3–5 seconds.
Monitor the pressure gauge — it should remain stable at 1.2–1.5 bar cold.
If pressure drops again within days, call a Gas Safe engineer.
F.22 alone is almost never a reason to replace a Glow-worm boiler. The only exception is if a cracked heat exchanger is discovered as the source of the pressure loss — which is rare and would present with other symptoms first.
Common questions about the Glow-worm F.22 fault
Can I repressurise a Glow-worm boiler myself?
Why does my Glow-worm boiler keep losing pressure?
What pressure should a Glow-worm boiler be at?
Is a Glow-worm F.22 fault dangerous?
How do I know if my Glow-worm boiler needs a new expansion vessel?
Other Glow-worm fault codes
Need a Glow-worm engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
