Vaillant F22: low water content and what it is really telling you
F22 stops the burner because the boiler believes there is not enough water in the primary circuit. Sometimes that is true; often the boiler is being told the wrong thing by a sensor or a stalled pump.
What matters most with a Vaillant F22 fault
- Refill slowly to 1.2 bar with the boiler off and cold
- F22 with healthy pressure means a sensor or pump problem
- Air in the pump head is common after radiator work
- Repeated F22 after top-up means you also have a leak to find
- Never wedge or bypass the sensor to keep the boiler running
Who should undertake the work
- Gas appliance work must be completed by a suitably qualified Gas Safe engineer
- Oil appliance work should be completed by a suitably qualified OFTEC technician
- Jamie Crawford Heating states both Gas Safe and OFTEC registration
Where a Vaillant F22 fault usually comes from
The appliance decides water content from the pressure sensor and the pump's feedback. A genuinely low system clears the moment you refill via the filling loop. If pressure reads normal on the gauge and F22 persists, the pressure sensor, its wiring, or a seized pump is misreporting. Air locked in the pump head produces the same behaviour after any drain-down.
Where we deal with this
Need help with a Vaillant F22 fault?
In practice: Top up once. If F22 returns, stop refilling and get the pressure sensor, pump and leak path checked properly.
Jamie Crawford Heating is based at 24 Bristol Road, Radstock, BA3 3EE.
Questions we get asked
- Why does F22 come back after refilling?
- Because the water is escaping again. The refill is a symptom fix; the leak or vessel fault still needs finding.
- Can I fill the system myself?
- Yes, if you have a filling loop and know the target pressure. Stop at the manufacturer's cold figure and re-check when hot.
Related help pages
- Temperature fluctuationHot water running hot then suddenly cold mid-shower
- Lockout guidanceBoiler lockout and safe resets: the two-attempt rule explained
- No responseBoiler will not fire up at all: power, demand and safety chain
- ControlsThermostat not calling for heat: wiring, batteries and placement
- Buying decisionHow to actually choose between boiler brands
- Noise diagnosisWater hammer: the sharp bang that follows a valve closing
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