The boiler stopped working and showed fault F22 dry fire. I changed the 3 amp fuse in the electric switch and pressed the test button in the consumer unit. A switch dropped down which I then pushed up. The boiler then fired up and the fault F22 was no longer showing. The boiler then worked as normal the rest of the day. Is it safe or should I turn it off and get a heating engineer to look at it?

Sylvia, March 2020

The F22 on a Vaillant boiler normally means that your system water pressure is low and would not suggest a dangerous fault nor anything to do with the electrics.
Have a look at the pressure guage and my guess is that it will be reading 0.3bar or lower, its needs to be reading 1.5bar.
If this is the case, and you are not familiar with topping up the system pressure, reply on this thread and I will provide a guide on the process to help you.

Rupert, March 2020