What does F22 mean on a Vaillant boiler?
F22 is Vaillant’s code for a safety switch-off due to low water pressure. Vaillant’s ecoTEC plus installation manual sets out the thresholds: below 0.08 MPa (0.8 bar) the boiler indicates low pressure, and below 0.05 MPa (0.5 bar) it switches off and the display shows F.22, alternating with the current pressure. The instruction for the user is to top up the heating water to start the boiler again.
The same code is used across many Vaillant ecoTEC models, and Glow-worm, which is part of the same group, uses F22 for water pressure on its current boilers too.
Why has the pressure dropped?
A sealed heating system should hold its pressure. If it falls, water has left the system. The usual causes are:
- A small leak at a radiator valve, a towel rail or a joint under the floor.
- The pressure relief valve releasing water. Check the copper pipe that runs from under the boiler to the outside wall: if its end is wet or dripping, the valve has been opening.
- A flat expansion vessel, which lets the pressure rise sharply as the system heats, forcing the relief valve open, then drop as it cools.
- Bleeding radiators, which lets out some water along with the air. One top-up afterwards is normal.
- Low water in the boiler itself, which Vaillant’s manual also lists among the causes of related pump and pressure faults.
What can you safely do?
- Read the pressure on the display or gauge with the heating cold.
- Top up the system using the filling loop or filling device, exactly as your Vaillant user manual shows. Aim for the range in the manual, usually 1 to 1.5 bar cold, and close the valves fully afterwards.
- Reset once if the boiler does not restart by itself.
- Watch the gauge over the next few days and note how quickly it falls.
- Look for leaks you can see without removing any covers: radiator valves, visible pipework and the relief pipe outside.
If you are not confident using the filling loop, or cannot find it, leave it to an engineer. Overfilling can push the pressure high enough to open the relief valve.
When should you get an engineer?
If F22 comes back within days or weeks, if you need to top up regularly, or if you can see water inside or under the boiler, the cause needs fixing. Repeatedly topping up a leaking system draws in fresh oxygenated water, which speeds up corrosion and sludge. The engineer will check the expansion vessel charge, test the relief valve and pressure-test the system to find the leak. Many of these are single-part repairs.
If you also see F75 on the same boiler, the pump and pressure sensor need checking too; see Vaillant F75.