This error code appears in Vaillant boilers when the pressure sensor detects a pressure drop below 0.3 bars. When it happens, the boiler stops working and displays the code. In this situation, the system will not be able to pump water around the central heating system.
As a temporary measure, you can add water in the system and thus increase the pressure. However, this will not solve the problem. There are quite a number of possible causes of pressure drops, so you can hardly do it without a proper diagnostics.
In most cases, pressure droops are caused by leaks. These may occur at pipe joints, due to holes in the piping (resulting from physical impact, corrosion, etc), or due to a poor radiator valve.
Although most leaks are small, they can do some bad things like ruining a plastered partition wall. It does not take a big leak to cause you a lot of trouble. Check the pressure gauge on the front of the boiler. If the needle shows a serious pressure drop, you may be dealing with something bigger than a drip. Cut off water supply and use professional help. Failure to eliminate leaks can potentially inflict serious material damage and unexpected costs.
Call a technician. He/she will locate and fix the leak. Do not do it yourself, because next thing you do is repressurize the boiler. If you have done something wrong, you may end up with even greater probleems.
To repressurize the boiler, open the filling loop and wait until the system is filled with water and regains pressure. Most advanced Vaillant boilers operate quite well at 1.3 bar. Some specialists advise 1.5 bar just for easier remembering. Once it reaches the appropriate level, close the valve. The F22 error should disappear once the pressure reaches 0.6 bar. The 1.3 bar level is the most optimal.