I got my gas licence less than a year ago, so I'm quite new to the field. Anyway I work as a service tech. mainly fixing water heaters and some furnaces. Quite often i have problems with CV water heaters. Usually they require thermocouple replacement, but sometimes I get called back, the pilot is off again. I have my routine by now: Check TCO, than Voltage on THC, check if pilot is not to weak or yellow flame, check if the venting is correct, if there is no back-spill. Even if everything works OK, still in some locations there are repeated calls for no hot water. Am I missing something?