What the F22 code means
Glow-worm boilers work on a sealed heating system, which needs to be kept at a certain pressure to run properly. A pressure sensor inside the boiler keeps watch, and if the pressure falls below the safe minimum the boiler stops and shows F.22 on the display. Glow-worm's installation manual for the Energy range describes this as a safety switch-off for low water pressure in the boiler, and the operating manuals for the Betacom 4 and Energy7 both tell the user to fill the heating installation when F22 appears.
The boiler is protecting its heat exchanger. If it fired without enough water inside, parts could overheat and be damaged. So F22 is really a reminder that the system needs water, not a sign that the boiler has broken. Because Glow-worm is part of the Vaillant Group, the same code number means low pressure on Vaillant ecoTEC boilers too, and our Vaillant F22 page covers that side.
Common causes of F22
Gradual loss over time
Even a healthy system loses a little pressure over the months. If nobody has checked the display since last winter, a slow drift below the minimum may be the whole story.
Bleeding radiators
Letting air out of a radiator also lets out a little water. Bleed several radiators in one go and the pressure can drop sharply.
A leak
Weeping radiator valves, a leaking joint under the floor or a drip inside the boiler all let water escape. Look for damp patches, marks on ceilings or crusty green or white deposits on valves.
The pressure relief valve letting water out
If the pressure rises too high when the system is hot, the relief valve opens and releases water through a pipe that usually runs outside. The pressure then falls too low once everything cools. A flat expansion vessel is a common reason for this cycle.
A sensor or wiring fault
Glow-worm lists a defective water pressure sensor, or a loose or damaged cable to the pump or sensor, as possible causes. If the reading looks normal but F22 will not go, suspect this.
| Cause | What you might notice | Who fixes it |
|---|---|---|
| Normal slow loss | Low reading, nothing else wrong | You, using the filling loop |
| Radiators bled recently | Pressure fell right after bleeding | You, using the filling loop |
| Leak in the system | Needs topping up every few days, damp or staining | Gas Safe engineer or plumber |
| Relief valve or expansion vessel | Pressure high when hot, dripping pipe outside | Gas Safe engineer |
| Pressure sensor or cable | Pressure looks fine but F22 stays | Gas Safe engineer |
Safe checks you can do yourself
- Turn the heating off and let the system cool, then read the pressure. On most recent Glow-worm boilers it shows as a number on the display. Some older models have a dial gauge underneath.
- Compare it with your manual. The Ultimate 3 and Betacom 4 manuals recommend 1 to 1.5 bar. If yours is below that, it needs topping up.
- Find the filling loop under the boiler. It is usually a flexible braided hose with a valve at each end, though some models have a built-in filling device instead.
- Open the valves slowly and watch the reading climb. Close them when it reaches about 1 to 1.5 bar. Take care not to overfill.
- Make sure both valves are shut tight. On a removable hose, disconnect it afterwards if your manual says to.
- Reset the boiler as your manual describes. On the Energy7, for example, you press the reset button for about a second.
- Check the reading again each day for a week to see if it holds.
Our step-by-step guide on how to repressurise a boiler with the filling loop has more detail if this is your first time.
Planning to bleed the radiators? Do it with the heating off and cool, then top up the pressure straight away. That saves you a surprise F22 later.
When to call a Gas Safe engineer
A single top-up is normal. Book a Gas Safe registered engineer if:
- You find yourself topping up every few days or weeks.
- There is water dripping from the pipe outside or from under the boiler.
- The pressure shoots up towards 3 bar when the heating is on.
- The pressure reads normally but F22 will not clear.
- The code comes back after two or three resets.
Do not remove the boiler casing to hunt for a leak. The expansion vessel, pressure sensor and relief valve are all inside the boiler and must only be worked on by a Gas Safe registered engineer.
Typical repair costs in the UK
If a top-up sorts it, F22 costs nothing. Where there is an underlying fault, these are typical ranges. Prices vary by area and engineer.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Repairing a leaking radiator valve or joint | Typically £80 to £200 |
| Re-pressurising the expansion vessel | Typically £80 to £150 |
| New pressure relief valve | Typically £120 to £220 |
| New water pressure sensor | Typically £120 to £220 |
How to stop F22 coming back
Get into the habit of glancing at the pressure reading once a month, and always after bleeding radiators. If it holds steady between about 1 and 1.5 bar when cold, the system is in good shape. Check the filling loop valves are fully shut every time you use them. A yearly service is a good chance for an engineer to check the expansion vessel and look for small leaks, and our boiler service cost guide explains what is included. If the problem is the opposite, a reading that is too high, see our high boiler pressure guide.