My Prima F50 has been giving me a lot of grief of late. In Jan it stopped igniting about half an hour after start up but continued to run, you could smell gas fumes outside. An engineer said it was either the fan or the PCB, he replaced the fan but this did not solve the problem so I replaced the PCB (with a second hand unit) and it worked for about a month. Now it trips its reset switch once or twice a day, I think it does this before it's timed to go on. I see that one engineer suggests that this could be an airflow switch problem on another site, is this likely to be the case?