An F22 fault on a Glow-worm boiler means system water pressure has dropped too low and the boiler has shut down to protect itself.

This is one you can usually fix yourself in about five minutes.

What F22 actually means

The boiler needs a minimum amount of water in the system to run safely. Below roughly 0.5 bar it stops firing rather than risk running the heat exchanger dry.

So it is not really a fault. It is the boiler telling you the system has lost water.

What you can safely check yourself

Check the pressure gauge. It should read 1 to 1.5 bar when cold. If it is near zero, repressurise using the filling loop until it reaches 1.5 bar, then reset.

If it drops again within days you have a leak and it needs finding rather than topping up.

When to stop, and why

What causes it beyond the simple checks

A leaking radiator valve, a pressure relief valve discharging outside, corroded pipework under the floor, or a failed expansion vessel. We can trace a hidden loss with leak detection without lifting your floors.

How Vantage can help

We are Gas Safe registered and cover Hertfordshire, Bedfordshire and Cambridgeshire, with an emergency line answered 24 hours a day.

Frequently asked questions

How often should I top up the pressure?

Once or twice a year is normal. More than that means a leak.

What pressure should it be?

1 to 1.5 bar when the system is cold.

Is it expensive?

If it just needs repressurising it costs nothing. A leak or a failed expansion vessel needs an engineer.

Related fault codes

Other Glow-worm codes: Glow-worm F28

The same problem on other makes (low water pressure): Vaillant F22 · Ideal F1 · Baxi and Potterton E119

See also our guide to boiler keeps losing pressure, the full boiler advice section, and boiler repair.

When to call an engineer

Anything past the free checks above needs a Gas Safe engineer with the right test kit. Ring Vantage on 01462 506374, any hour, any day of the year. You will get an honest arrival time rather than a comforting one, and the price before the work.