Glow-worm F22 Fault Code: What It Means And How To Fix It

If your Glow-worm boiler is showing an F22 fault code or F.22 error code, it usually means the water pressure in the system is too low for the boiler to run safely. The boiler locks out until the pressure is brought back up.

We’re Hinckley Plumbing & Heating — Gas Safe registered engineers covering Hinckley, Nuneaton, Leicester, Tamworth, Coventry, Rugby, and Loughborough — and this guide explains what the Glow-worm F22 fault code on a boiler means, the most common causes, how to fix it, and when to call us.

What Does The Glow-worm F22 Fault Code Mean?

The Glow-worm F22 fault code is a low water pressure fault, sometimes described as dry fire protection. It appears when there isn’t enough water in the heating system for the boiler to fire safely.

Firing a boiler with too little water in it risks damaging the heat exchanger, so the boiler shuts down first. You’ll usually lose heating, hot water, or both.

What Is The Glow-worm F22 Error Code On A Boiler?

The Glow-worm F22 error code on a boiler is another way of saying the system pressure has dropped too low.

In plain English: there isn’t enough water going round your radiators for the boiler to work properly, so it’s stopped rather than risk running dry.

What Causes The Glow-worm F22 Fault Code?

The most common causes of the Glow-worm F22 fault code on a boiler are:

• Low system water pressure
• A small leak in the heating system
• Recently bled radiators
• A faulty pressure sensor
• A leaking or failed pressure relief valve
• An expansion vessel that has lost its charge
• A filling loop or filling key problem

Most of the time the pressure has simply drifted down and needs topping up. If F22 keeps returning, though, water is escaping somewhere.

What Pressure Should A Glow-worm Boiler Be?

Most Glow-worm boilers should sit around 1 to 1.5 bar when cold, rising towards 2 bar once the heating has been running.

If the pressure falls below around 0.6 to 0.8 bar you’ll typically see F22 and the boiler will stop. If the gauge already reads in the normal range and F22 is still showing, that usually points to a faulty pressure sensor rather than a genuine shortage of water.

How To Fix The Glow-worm F22 Fault Code

If you’re confident doing it yourself, the first step is to repressurise the boiler.

1. Turn the boiler off and let it cool down.
2. Locate the filling loop or filling key underneath the boiler.
3. Open the valves slowly.
4. Watch the pressure gauge or display rise.
5. Stop when the pressure reaches around 1 to 1.5 bar.
6. Close the valves fully.
7. Reset the boiler and check whether the F22 fault has cleared.

Glow-worm shares much of its design with Vaillant, so many models use an integrated filling key rather than a braided loop. The key slots into the underside of the boiler and needs pushing in and turning to engage before water will flow. If you’re not sure which setup you have, check the manual before turning anything.

If you go past around 2.5 bar, bleed a radiator slightly to bring it back down.

Why Does The Glow-worm F22 Error Code Keep Coming Back?

If the F22 error code clears after topping up but comes back, the system is losing pressure again.

Look for a small leak on pipework or a radiator valve, a pressure relief valve weeping outside, or an expansion vessel that has lost its air charge. Pressure that swings sharply — very low when cold, close to the red when hot — is a classic sign of a failed expansion vessel.

Topping up repeatedly will keep the heating on, but it won’t solve the cause. It also introduces fresh oxygenated water each time, which speeds up corrosion inside the system.

When Should You Call Us About A Glow-worm F22 Boiler Fault?

Call Hinckley Plumbing & Heating if:

• The pressure keeps dropping after you top it up
• The F22 fault code keeps returning
• The pressure gauge reads normally but F22 is still showing
• You can see water around the boiler, radiators, or pipework
• There’s a small pipe dripping outside near the boiler
• The boiler won’t repressurise at all
• The heating or hot water still isn’t working after topping up

Those signs usually mean the Glow-worm F22 boiler fault needs proper diagnosis rather than another top-up.

Book A Glow-worm Boiler Fault Check

If your Glow-worm boiler is showing an F22 fault code or F.22 error code, we can help.

We cover Hinckley, Nuneaton, Leicester, Tamworth, Coventry, Rugby, and Loughborough, and we’ll give you a clear, honest assessment of whether it’s a simple low-pressure issue or a leak that needs tracing. Call us or use our contact form to book a boiler inspection.

Glow-worm F22 Fault Code FAQs

Can I reset the Glow-worm F22 fault code without topping up the pressure?

Usually no. If the pressure is genuinely too low, it needs bringing back up before the fault will clear.

Is the Glow-worm F22 boiler fault dangerous?

Low pressure isn’t usually dangerous in itself, and the lockout exists to prevent damage. But it can point to a leak worth finding.

How often should I need to top up my Glow-worm boiler?

Once or twice a year is normal. Every few weeks isn’t, and suggests a leak or a failed expansion vessel.

Will the Glow-worm F22 error code go away on its own?

Usually no. The pressure needs restoring to the correct level before the boiler will start working properly again.