Hello, It sounds like you may not be getting circulation thru the heater. Make sure the valves are open to allow water to pass thru the heating system. Some systems use a bypass on the heater so there is not a lot of wear on the coil system in the heater. If it is not circulating it will get too hot and the safety shut down will occure. If you are getting water circulating thru the heater, then the thermocouple or auto start igniter system is not working correctly. It is trying to start and can't then recycles. If the burners are igniting on the startup and then shutting back down you may have a bad control unit or thermal sensor unit. Since the unit is gas have a professional service it for you. Good Luck, RAC