Quick answer: The Glow-worm F22 fault means low water pressure — the boiler has detected too little water in the system and shut down to protect itself. It’s often fixable at home by topping up via the filling loop, but if F22 keeps returning you likely have a leak that needs an engineer.
F22 is one of the most common Glow-worm faults, and happily it’s often one you can fix yourself. Here’s what it means and how to put it right. A quick note first: if your Glow-worm is more than about seven years old and F22 keeps coming back, an ageing boiler that repeatedly loses pressure can cost more in call-outs than a replacement, so it’s worth weighing up a new boiler installation.
What Does the Glow-worm F22 Error Code Mean?
F22 indicates low water pressure — there isn’t enough water in the system, so the boiler shuts down to avoid running dry.
Your heating needs to hold a set pressure to run safely. When it drops too low, F22 stops the boiler firing. It’s a protective cut-out rather than a breakdown in itself.
What Causes a Glow-worm F22 Low Pressure Fault?
The main causes are gradual pressure loss, a leak in the system, or recently bled radiators.
- Natural pressure loss — systems shed a little pressure over time
- A leak — in pipes, radiators or a boiler part
- Recently bled radiators — releasing air lowers pressure
- Faulty pressure sensor — less common but possible
How Do You Fix a Glow-worm F22 Error?
Top up the pressure using the filling loop until the gauge reads about 1–1.5 bar, then reset the boiler.
- Locate the filling loop — usually a braided silver hose under the boiler
- Open both valves slowly, watching the gauge
- Aim for 1–1.5 bar cold, then close them
- Reset and the boiler should fire
If the pressure won’t hold and F22 comes back within days, book a boiler repair — that pattern means a leak.
Why Does My Glow-worm Keep Showing F22?
A repeating F22 nearly always means a leak — constantly topping up hides the problem instead of fixing it.
If you’re refilling every few days, water is escaping somewhere, visible or hidden. Endlessly re-pressurising isn’t a solution and can cause further problems, so it’s worth having the leak traced.
Can Servicing Prevent Glow-worm F22 Faults?
Yes — an annual service checks pressure, seals and the expansion vessel, catching slow leaks and wear before they cut off your heat.
A yearly boiler service picks up pressure problems early. In Scotland, an autumn service means you enter winter with the system holding pressure as it should.
When Should You Call an Engineer for Glow-worm F22?
Call an engineer if the pressure won’t hold, you can’t find the filling loop, or F22 keeps returning despite topping up.
Topping up once or twice is a fine DIY task. Beyond that, you need someone to find where the water is going.
Local Glow-worm Boiler Repairs Across Edinburgh and Scotland
We’re Edinburgh-based and cover central Scotland, so we can reach a stubborn F22 quickly and trace any leak at source. If your Glow-worm keeps losing pressure, get in touch with our Gas Safe engineers.
Frequently Asked Questions About the Glow-worm F22 Error
What’s the difference between the Glow-worm F22 and F75 codes?
F22 is a low water pressure fault, while the Glow-worm F75 error relates to the pump or pressure sensor not detecting a pressure change. F22 is often a simple top-up; F75 usually needs an engineer.
What pressure should my Glow-worm boiler be at?
Around 1–1.5 bar when cold is the usual range. Below 1 bar and you’re likely to see F22; aim for the middle when topping up.
My Glow-worm shows F28, not F22 — is the fix the same?
No. The Glow-worm F28 error is an ignition failure, so re-pressurising won’t fix it. F28 means checking your gas supply and condensate pipe instead.
How often should I need to top up my Glow-worm’s pressure?
A small drop over several months is normal. Topping up every few days points to a leak that should be traced rather than repeatedly refilled.
