I have a rheem 42v40 water heater which seems to have a combustion air supply problem. If you wait a period,(don't know exact time) pilot lights just fine but will go out after maybe 5 min. and will not relight until a period has passed. If thermostat is set for heating, after pilot is lit, burner will light but extinguish itself and the pilot within seconds and then the time has to be observed again before the same thing can be done again with the same results.