I have a problem with the ignition. It works automatically sometimes, as it should, but other times the supply comes on the fan starts up but the burner fails to light. Now as I see it there are two options either the gas valve is not opening or the igniter is not firing. I am not sure whether the igniter would fire if the gas is not passing through to the burner.I will try testing the supplies tomorrow ie the electrical supply to the gas valve. Can anyone explain simply as in a flow chart what happens when?