F-28 is a common error for my vaillant. I have had this problem serviced at least 10 times (all under warranty). It is an ignition problem and can be connected to a number of things. I have had the starter electrodes cleaned only to later have them replaced(4 times). I just had the "panel" replaced yesterday (looks like a motherboard or electronics panel). This is the second time to have this replaced. All of these things can give you an F-28 error message. I have an atmotech pro and have had ongoing F-28 issues for two years. They are looking at venting/air issues as well to see if this is the problem. douglas