Suburban SW6D

Maybe I didn't explain properly. It has always taken 30 to 40 mins to heat up this 6 gallon tank from start. When I run the water after the premature shut down it runs hot for maybe 15 seconds then turns cold. That is not normal. Thought there was an air bubble and emptied the tank but it changed nothing. I have run the system with 110 degrees temps outside and yes it takes less time maybe but always over 1/2 hour. Plus after it shuts down the inside switch will not light it up until the next day and by that time the water in the tank is basically cold.