Performed inspection of existing 75 gallon water heater. Inspected venting, draft for venting, gas supply as well as wires and everything appears to be working properly. Re lit water heater and waited approximately 45 min and everything was running properly until at about 55 min the water heater made a loud pop and then a flame rollout occurred. This usually is an indicator that the water heater needs to be replaced . Previous plumber had already replaced gas control valve and thermocouple which leaves burner assembly the only thing left to be replaced. If customer does not want to replace water heater we can attempt to replace burner assembly and cleaning out debris on bottom of water heater to see if that addresses the issue. But cannot guarantee .