Glow-worm F22 fault codewhat it means and how to fix it
The boiler has locked out because system pressure is too low to run safely. Topping up to 1.2 bar and resetting normally fixes it.
Take care
What does the gauge read cold?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Below 0.8 bar
Confirmed low pressure. Top up to 1.2 bar and reset.
Between 1.0 and 1.5 bar but F22 still shows
Pressure is correct, so the sensor reading it is not, or something is stopping water reaching it.
Stop resetting and book an engineer — the boiler believes it has no water.
You have topped it up more than twice this year
A leak. Repeated refilling corrodes the system from the inside.
Ask for a pressure test and an expansion vessel check.
What F22 means
F22 is Glow-worm signalling low water pressure, and it behaves the same way as the Vaillant code of the same number because the two share a control platform. Below roughly 0.5 bar the boiler stops the burner rather than risk running a heat exchanger without enough water in it. That protection is doing its job. The pressure loss underneath it is the actual fault, and since a sealed heating system does not consume water, a falling gauge always means water is going somewhere — a weeping valve, a buried joint, a failed expansion vessel, or a safety valve passing on a piece of debris.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Seepage nobody has spotted | Pipework in a screeded floor or under kitchen units. It goes on for months at a rate that never announces itself, and only the gauge records it. | Common |
| Recent bleeding without a top-up afterwards | The most benign explanation and worth ruling out first. If somebody in the house has been round the radiators lately, that is very likely all this is. | Common |
| Diaphragm split in the expansion vessel | Once it goes, every heating cycle over-pressurises and vents. The system then reads low when cold, which sends people looking for a leak that is not there. | Occasional |
| Filling loop passing in reverse | Less familiar but genuinely seen: a failed non-return valve on the loop lets system water push back towards the mains when pressure spikes. | Rare |
What to try yourself
Read it cold. Glow-worm appliances usually have an analogue gauge low on the front panel or on the pipework beneath.
Identify the filling arrangement before you open anything. Glow-worm fit both external braided loops and internal keyed links depending on the range and the installer.
Fill slowly to 1.2 bar. If you are using an internal filling key, turn it gradually — the flow rate through those is higher than you expect.
Never leave a filling key in place after use. Left engaged, the system slowly equalises with mains pressure and you will be looking up a high-pressure fault next.Close everything, remove the key if there is one, and hold reset for three seconds.
Note the date somewhere you will find it again. The interval between refills is the single fact an engineer will want.
Get a Gas Safe registered engineer to look at your Glow-worm F22
Gas Safe Register engineers only. No obligation.
What it costs to fix
Typical repair cost
£0–£450
Time on site
10 minutes
Free to top up. Leak trace £90–£160. Expansion vessel £150–£280.
Typical UK prices including parts and labour, updated August 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Expansion vessel (8 litre, sealed system)
EV-8L£35–£85Plumbing merchant
Price checked 5 August 2026
Pressure relief valve, 3 bar
PRV-3B£18–£42Plumbing merchant
Price checked 5 August 2026
What other owners found
Nobody has shared what worked for them on this code yet. If you have had it, you would be the first — and it genuinely helps the next person.
Related Glow-worm fault codes
- F75Get it checkedThe boiler saw no pressure rise when the pump started, so it locked out. Almost always the pressure sensor or the pump, not low pressure.Related: water pressure
- F.9Get it checkedA circulation or pressure sensing fault. Check the pressure and open all radiator valves before assuming the worst.Related: water pressure
- F.22Get it checkedThe boiler has shut down because there is not enough water in the system. Pressure has dropped below roughly 0.5 bar and it will not fire until you top it up.Related: water pressure
- F.75Get it checkedThe boiler cannot detect a rise in pressure when the pump starts, so it has locked out. Usually a failed pressure sensor or a pump that is not moving water.Related: water pressure
- F1Get it checkedSystem pressure has dropped below the minimum the boiler will run at. Topping it back up to 1.2 bar usually clears it straight away.Related: water pressure
- CE 207Get it checkedSystem water pressure has dropped too low for the boiler to run safely. Topping the system back up to 1.2 bar usually clears it in five minutes.Related: water pressure
Related guides
Common questions about Glow-worm F22
Is Glow-worm F22 the same as Vaillant F.22?
Effectively yes — the two brands share a control platform, so the code means the same thing. The display format differs slightly and the filling arrangements vary by model, but the diagnosis and the fix are the same.
What pressure should it be?
1.0 to 1.5 bar cold, aiming for 1.2. It reads higher when the heating is running, which is expected. Set it cold.
How often is too often to top up?
Once or twice a year is unremarkable. Monthly is a leak. The reason it matters is not the water but the oxygen that comes with it, which is what produces the black sludge that kills pumps and radiators.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
Checked by Technical reviewer (Gas Safe registration to be displayed on appointment) on 5 August 2026.