My Vallaint Turbo-Pro is about 3 years old. I have been having intermittent hot water and heating problems -- it keeps shutting itself off. So far the Corgi plumber has been out 5 times. Initially the error was s.33, then s.13, now f.22. Gas has been checked for consistent flow. So far the diverter valve, plate to plate heat exchanger, and fan have all been replaced. Any thoughts?