Glow-Worm F22 Fault Code: Low Water Pressure
The F22 fault code on a Glow-Worm boiler signals that the water pressure inside your central heating system has dropped below the minimum threshold the boiler needs to operate safely — typically below 1.3 bar. When this happens, the boiler's built-in protection shuts the appliance down to prevent damage. You may notice a pressure reading as low as 0.3 bar flashing alongside the F22 code on the display. On some older Glow-Worm appliances, the same underlying fault may be displayed as F9 instead.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Glow-worm F22 fault code mean?
The F22 fault code on a Glow-Worm boiler signals that the water pressure inside your central heating system has dropped below the minimum threshold the boiler needs to operate safely — typically below 1.3 bar. When this happens, the boiler's built-in protection shuts the appliance down to prevent damage. You may notice a pressure reading as low as 0.3 bar flashing alongside the F22 code on the display. On some older Glow-Worm appliances, the same underlying fault may be displayed as F9 instead.
Common causes
- System pressure has simply dropped over time Common
Central heating systems naturally lose a small amount of pressure over months of use. If your gauge reads below 1 bar and you have no other obvious symptoms, a straightforward top-up via the filling loop is often all that is needed. This is the most frequent trigger for F22.
- Water leak somewhere in the system Common
A slow drip from a radiator valve, compression fitting, or even internal boiler components such as the pump seals or heat exchanger can gradually drain pressure away. Leaks are not always visible — they can occur beneath floorboards or behind panels. If pressure keeps dropping within days or weeks of topping up, a leak is the most likely culprit.
- Trapped air following radiator bleeding Sometimes
Bleeding radiators releases air but also lets water out of the system, lowering the overall pressure. If F22 appears shortly after you last bled your radiators, repressurising the system should resolve it.
- Faulty expansion vessel Sometimes
The expansion vessel absorbs pressure changes as water heats and cools. If its internal bladder fails, pressure can swing wildly — often dropping too low when the system is cold. The boiler will require professional attention to recharge or replace the vessel.
- Pressure relief valve (PRV) weeping or discharging Sometimes
A PRV that is passing water — often seen as a damp patch near the discharge pipe outside — will slowly deplete system pressure. The valve may need replacing by a Gas Safe engineer.
- Sludge, limescale, or debris blockage Rare
A build-up of magnetite sludge or limescale in the pipework or heat exchanger can restrict circulation. While this is less likely to cause a single F22 event, it can contribute to ongoing pressure irregularities in older, unmaintained systems.
How to fix it
- Check the pressure gauge reading DIY safe
Find the pressure gauge on your Glow-Worm boiler — it is usually a dial or digital display on the front panel. A healthy reading is between 1.0 and 1.5 bar when the system is cold. If it shows below 1 bar, low pressure is confirmed as the issue.
- Top up system pressure using the filling loop DIY safe
Locate the filling loop — a short braided hose with one or two valves, usually underneath or near the boiler. With the boiler switched off and cold, slowly open the valve(s) to allow mains water into the system. Watch the gauge rise and stop when it reaches approximately 1.2–1.5 bar. Close the valve(s) firmly. Never overfill beyond 2 bar.
- Reset the boiler DIY safe
Once pressure is back in the correct range, press and hold the reset button (consult your boiler's handbook if you are unsure of its location) until the fault code clears. Allow the boiler a minute or two to complete its restart sequence. Do not reset more than two or three times — repeated resets without a fix can mask a deeper problem.
- Bleed your radiators if you suspect trapped air DIY safe
If radiators feel cold at the top or gurgle when heating up, trapped air may be contributing to the pressure drop. Use a radiator bleed key to open each bleed valve in turn, starting upstairs, until water (not air) escapes. Re-check and top up the system pressure afterwards, as bleeding releases water from the circuit.
- Inspect accessible pipework and radiator valves for visible leaks DIY safe
Walk around your home and look underneath radiators, around radiator valves, and at any visible pipework for damp patches, rust staining, or mineral deposits. These are signs of a slow leak. Do not attempt to repair pipework yourself — but noting the location will help the engineer.
- Call a Gas Safe registered engineer if the fault returns or a leak is suspected Gas Safe engineer
If the pressure drops again within a short time of topping up, or if you cannot locate the cause, contact a Gas Safe registered engineer. They will pressure-test the system, inspect internal boiler components such as the pump, heat exchanger, and PRV, and may carry out a power flush if sludge is contributing to the problem. Repeatedly losing pressure almost always points to a leak or a failing component that needs professional diagnosis.
Parts you may need
- Expansion vessel (replacement) · from £60
- Pressure relief valve · from £25
- Filling loop assembly · from £20
- Circulation pump (e.g. Grundfos UPS2) · from £120
- Pressure gauge / sensor · from £30
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £80–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Glow-Worm F22 fault myself?
In many cases, yes. If the cause is simply low pressure, topping up the system via the filling loop and resetting the boiler is a straightforward homeowner task. However, if the pressure keeps dropping, or if you find a visible leak, you will need a Gas Safe registered engineer to carry out the repair.
Why does my Glow-Worm boiler keep showing F22 even after I top up the pressure?
Recurring F22 faults after repressurising almost always mean water is escaping from the system somewhere. Common culprits include a weeping pressure relief valve, a slow leak from a radiator valve or pipe fitting, or a failing expansion vessel. If you are topping up more than once or twice a year, book a diagnostic with a Gas Safe engineer — the underlying issue will not go away on its own.
How much does it cost to fix a Glow-Worm F22 fault?
If a Gas Safe engineer is needed, most homeowners pay between £80 and £300 depending on the cause. A callout to repressurise the system and check for faults typically falls at the lower end. Replacing a pressure relief valve or expansion vessel sits in the middle of the range, while pump replacement can push towards the top. Heat exchanger replacement is significantly more expensive and rare — if that is the diagnosis, it is worth getting a quote for a new boiler as well for comparison.
Is it safe to keep resetting the boiler to clear the F22 code?
Resetting once or twice is fine and is part of the recommended troubleshooting process. However, repeatedly resetting without addressing the root cause is not advisable. It can mask a developing problem such as a leak or component failure, and some boilers will lock out permanently after too many resets. If the fault returns after two or three resets, stop and call an engineer.