Vaillant F22 fault code explained
Vaillant’s current UK fault-code guidance defines F.22 as low water pressure in the system. The boiler has made a safety shutdown because it cannot confirm the water conditions needed for normal operation. On applicable models, heating and hot water may stop until the cause is dealt with.
If the pressure display is below 1 bar, Vaillant says a competent user may use the model instructions to raise it slowly to the normal 1 to 1.5 bar range. If F.22 appears while the pressure is already above 1 bar, Vaillant says the fault needs an engineer and warns against opening the boiler casing.
The code does not prove that the boiler simply needs water. Vaillant also lists possible pressure-sensor, wiring, pump and expansion-vessel issues. Those are internal diagnostic possibilities, not homeowner repair instructions.
What F.22 means on a Vaillant boiler
A sealed central-heating circuit needs enough water pressure for reliable circulation. When pressure becomes too low, a Vaillant boiler can stop rather than continue firing without the expected water conditions. Older ecoTEC instructions describe F.22 as a dry-fire or lack-of-water fault; the current public code list calls it a low-water-pressure safety shutdown.
Different Vaillant models use different controls, gauges and filling arrangements. Some show a numerical pressure value, some use a bar display, and some systems may be open vented rather than user-pressurised. Confirm the full model and consult its operating instructions before touching a filling control.
First check: what pressure is the boiler showing?
Read the gauge or digital display while the heating is off and the system is cool. Vaillant’s F.22 page says the normal range is 1 to 1.5 bar, and its radiator-bleeding guidance gives the same cold range. The exact target in your model’s manual takes priority.
If the display is below 1 bar, low pressure is a credible reason for F.22. If it is already above 1 bar, do not add more water just because the code contains the word pressure. Vaillant directs that situation to an engineer because the displayed pressure and the boiler’s internal reading or circulation may not agree.
Why might the pressure have dropped?
Pressure can fall after radiators are bled because air and a small amount of water leave the system. It can also fall because water is escaping from a radiator valve, pipe joint, concealed pipe, boiler component or the pressure-relief discharge outside. A small leak may leave only staining or may evaporate on a warm surface.
If pressure keeps falling after a correct top-up, the recurring loss is the problem to investigate. Repressurising again and again is not a repair. Record how quickly it drops and whether the fall happens when the system heats, cools or remains off.
Can you repressurise a Vaillant boiler yourself?
Vaillant says that where the system pressure is below 1 bar, a competent user can locate the filling loop and slowly bring the pressure to between 1 and 1.5 bar, following the correct user instructions. Only do this if you can positively identify the model’s filling control and understand the procedure in its manual.
Filling arrangements vary. Some boilers use an external braided loop, while others use an integrated control or a different system design. Do not copy a video for a different model, turn unidentified valves beneath the boiler or use tools on controls that should operate by hand.
A safe high-level approach
- Turn the heating demand off and allow the system to cool.
- Confirm the exact model and open its official operating instructions.
- Check that the pressure is genuinely below the manual’s cold range.
- Use only the named user filling control and open it slowly.
- Watch the pressure continuously and stop at the stated cold target.
- Close the filling control fully, check for drips and follow the manual for one restart.
If you cannot identify the controls, the pressure does not rise, the valve will not close, water leaks, or the reading climbs too far, stop and arrange help. Do not remove the casing or try to reach an internal filling component.
What if F.22 clears and then comes back?
A temporary recovery tells you that restoring pressure changed the condition, but it does not explain why pressure was lost. Look for visible moisture around radiator valves and accessible pipe joints, check beneath radiators, and observe the outside copper discharge pipe from ground level. Do not remove floors, panels or the boiler case.
Write down the cold pressure at the same time on successive days. If it continues to fall, arrange diagnosis. Tell the engineer about any recent radiator bleeding, plumbing work or visible drip. A consistent record is more useful than repeated top-ups.
What if the pressure is already normal?
Vaillant’s public advice is clear: if F.22 is displayed while system pressure is more than 1 bar, it is an engineer case. The manufacturer lists possible issues involving the pressure sensor, electrical connections, pump or expansion vessel. Testing those items requires model-specific procedures and access inside the appliance.
Do not assume the gauge is wrong, replace a sensor based on the code, tap the pump or bridge any connection. A fault code narrows the area for diagnosis; it is not a parts-ordering instruction.
Can bleeding radiators trigger F.22?
Yes. Vaillant’s radiator-bleeding guide specifically tells users to check the boiler pressure before turning the heating back on and notes that F.22 may appear. On a sealed system, releasing trapped air reduces the pressure and the system may need to be restored to its cold target.
Bleed radiators only with the heating off and cool. Close the bleed screw gently when water appears, dry the area and check it for seepage. If radiators need frequent bleeding or the pressure repeatedly falls afterwards, arrange an investigation rather than treating each event separately.
Should you reset the boiler?
A reset does not add water or repair a leak. If the cold pressure was low, first use the correct model procedure to restore it. Then follow the user guide for one reset if it instructs you to do so. If F.22 returns, stop. Repeated resetting can delay the diagnosis of a continuing pressure or component fault.
What not to do
- Do not add water when the cold pressure is already in the correct range.
- Do not leave a filling loop or filling control open.
- Do not keep topping up a system that repeatedly loses pressure.
- Do not loosen pipe joints or radiator valves to “release pressure”.
- Do not remove the boiler casing.
- Do not test, tap or replace the pump, sensor, wiring or expansion vessel yourself.
- Do not keep resetting the boiler when F.22 returns.
When should you call an engineer?
Arrange a qualified inspection if F.22 appears above 1 bar, returns after a correct top-up, the pressure falls repeatedly, the system will not fill, the pressure gauge behaves oddly, water is leaking, or you cannot identify the proper user controls. Also seek help if you are not comfortable following the official instructions.
Because Vaillant is not one of the boiler brands included in Incredaboil’s fixed-price repair scope, check the service information before booking and use a suitably qualified Gas Safe registered engineer for any gas-boiler work. The article’s guidance remains useful for deciding whether a simple model-approved pressure check is appropriate or professional diagnosis is needed.
Gas and carbon monoxide safety
F.22 is a water-pressure fault, but a separate gas or combustion warning takes priority. If you smell gas or suspect an appliance is unsafe, HSE says to turn it off if safe, open doors and windows, and call the National Gas Emergency Service on 0800 111 999. Do not use an appliance you believe is unsafe.
If a carbon monoxide alarm sounds or anyone develops headache, dizziness, nausea, weakness, confusion, chest pain or shortness of breath, go outside and seek medical advice. The NHS says to call 999 or go to A&E for severe symptoms including breathing difficulty, sudden confusion, loss of consciousness, marked weakness or chest pain.
What to tell the engineer
- The full Vaillant model and the exact F.22 display.
- The cold pressure before and after any top-up.
- How quickly pressure falls again.
- Whether radiators were recently bled or plumbing work was carried out.
- Any visible water, staining or drip from an outside discharge pipe.
- Whether heating, hot water or both stopped.
Frequently asked questions
What pressure should a Vaillant boiler show when cold?
Vaillant’s F.22 and radiator guidance gives a normal cold range of 1 to 1.5 bar. Check the exact operating instructions because system design and model controls vary.
Will topping up always clear F.22?
No. It may clear a genuine low-pressure shutdown, but F.22 can also involve sensing, circulation or internal component issues. If pressure is above 1 bar or the code returns, arrange professional diagnosis.
Can I keep using the boiler after one top-up?
If the code clears after following the manual, monitor the cold pressure. A recurring fall, leak or returning F.22 needs investigation.
Can I remove the case to look for a leak?
No. Gas Safe Register explains that an integral boiler case forms part of the appliance’s combustion seal and should not be removed by someone who is not appropriately registered.
Related boiler help
- Why does my boiler keep losing pressure?
- What should boiler pressure be?
- Boiler pressure drops after bleeding radiators
Need help with a recurring boiler fault?
Check the repair service details and arrange the right next step.