- Boilers
- Gas boiler
F22
Glow-worm F22 on a gas boiler: low water pressure
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
On Glow-worm boilers, F.22 means low water pressure in the heating system (F9 on older appliances). If the gauge shows under 1 bar, Glow-worm says a competent user can slowly top up to 1 to 1.5 bar with the filling loop; above 1 bar, call a Glow-worm engineer.
Safety first
Stop and check this first
- If you smell gas, Glow-worm says to leave the property immediately and call the National Gas Emergency Service on 0800 111 999. Do not operate switches on the way out and make the call from outside.
- Glow-worm says top ups are at your own risk and only if you are competent. If pressure is above 1 bar and F.22 remains, do not open the boiler: call an engineer.
Safe checks you can perform
- Pressure gauge reading
- Slow top up with the filling loop
- One reset
- Sensor, vessel and leak check
Leave these tasks to a qualified professional
- Do not attempt this professional task: Leak and expansion vessel check
- Do not attempt this professional task: Pressure sensor and wiring test
- Do not attempt this professional task: Annual safety check
What a technician must be able to justify
Technician visit
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Leak and expansion vessel check
What they doThe engineer checks the system for leaks and the internal expansion vessel charge.
They must show youthe leak or the vessel test.
In writingwhat was found and repaired.
Pressure sensor and wiring test
What they doIf pressure is correct but F.22 shows, the engineer checks the water pressure sensor and its wiring.
They must show youthe gauge and the boiler's own reading.
In writingthe values and the reference of any part replaced.
Annual safety check
What they doDuring the visit, the engineer can service the boiler and check the flue and combustion.
They must show youthe flue and combustion checks.
In writingthe service record.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Follow these steps
Note: Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
Pressure gauge reading
- Who:
- you.
- How:
- read the system pressure on the boiler gauge.
- Result that matters:
- whether it is below 1 bar.
- If it is below 1 bar:
- top up as in the next check.
- If it is above 1 bar with F.22 showing:
- Glow-worm says this is a case for its engineer; do not open the boiler.
- Evidence to keep:
- a photo of the gauge and display.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Slow top up with the filling loop
- Who:
- you, only if you feel competent, as Glow-worm states.
- How:
- locate the filling loop (your user manual shows it) and slowly increase the pressure to between 1 and 1.5 bar.
- Result that matters:
- whether F.22 clears and pressure holds.
- If it clears and holds for days:
- a one off loss is likely.
- If it drops again:
- there is a leak or an expansion vessel fault; call an engineer.
- Evidence to keep:
- daily gauge readings for a week.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
One reset
- Who:
- you.
- How:
- reset the boiler as your user manual describes.
- Result that matters:
- whether the boiler runs.
- If F.22 returns with pressure correct:
- call a Glow-worm engineer.
- If it runs:
- keep watching the gauge.
- Evidence to keep:
- a note of the reset and the reading.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Sensor, vessel and leak check
- Who:
- a Glow-worm or Gas Safe registered engineer.
- How:
- checks the pressure sensor and wiring, the expansion vessel and the system for leaks.
- Result that matters:
- why pressure drops or reads low.
- If the vessel or a leak is at fault:
- it is repaired.
- If pressure is fine:
- the sensor or wiring is replaced.
- Evidence to keep:
- a job sheet with findings.
Glow-worm advises against accessing the inside of the boiler; internal checks are for its engineers.
Evidence: HSE gas safety for homeowners · Glow-worm boiler fault codes: what they mean and how to fix them
Still not solved
If the error persists, stop here. See what the technician must do and show you.
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Boilers
- Brand
- Glow-worm
- Product type
- Gas boiler
Known code variants
- 22
- code erreur F22 chaudière Vaillant
- codice F22 Vaillant
- código F22 Vaillant
- error F22 caldera Vaillant
- Error F22 Saunier Duval
- errore F22 caldaia Vaillant
- F 22
- Fehlercode F22 Vaillant
- Glow worm boiler F22
- Glow worm F22
- Glow-worm F.22
- Glow-worm F22
- Glow-worm F9 older boilers
- panne F22 Vaillant
- Saunier Duval F.22
- ThemaFast Condens F.22
- Vaillant boiler F22
- Vaillant F.22
- Vaillant F22
- Vaillant Fehler F22
What it means
Glow-worm's fault code page gives F.22 as low water pressure in the system and lists as possible causes pressure below the normal range of 1 to 1.5 bar, a problem with the water pressure sensor, loose or faulty wiring for the pump or pressure sensor, a pump not working (open vented boilers only) and a faulty internal expansion vessel. Glow-worm's winter advice adds that older appliances show the same condition as F9. The code reports the pressure reading only.
Warnings and stop conditions
- Criticaleditorial warning
If you smell gas, Glow-worm says to leave the property immediately and call the National Gas Emergency Service on 0800 111 999. Do not operate switches on the way out and make the call from outside.
Evidence: HSE gas safety for homeowners · Glow-worm boiler fault codes: what they mean and how to fix them
- Mediumsource verified
Glow-worm says top ups are at your own risk and only if you are competent. If pressure is above 1 bar and F.22 remains, do not open the boiler: call an engineer.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Symptoms and causes
Probable causes · in order
- Pressure below 1 barCommon
Fits when the gauge reads under 1 bar; topping up clears it if nothing is leaking.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Pressure sensor or its wiringPossible
Fits when the gauge shows more than 1 bar but F.22 stays; Glow-worm then sends an engineer.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
- Internal expansion vesselPossible
Fits if pressure rises and falls widely or keeps dropping after top ups; Glow-worm lists a faulty vessel.
Evidence: Glow-worm boiler fault codes: what they mean and how to fix them
Virtual technician
Does F22 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Health and Safety Executive · official_doc
- 2
Glow-worm boiler fault codes: what they mean and how to fix them
Glow-worm · official_doc
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