As implied in these posts, the water heater has TWO separate systems to heat the water. They work INDEPENDANTLY (Gas or Electric) or both at the same time.
With either system, you MUST have water in the tank to avoid damage. Make sure by flipping the leaver on the pipe pressure relief valve ( COLD ONLY! ); hot water will spurt out if the water is hot.
If air escapes the tank was not full. If nothing comes out, damage has already occurred. If water comes out it is full.
The outside switch only works the 120 volt AC side and the inside switch works gas.
The explosion you heard was most likely gas collecting in the combustion tube that finally got to a mixture that will burn.
You most likely have a bug nest in the combustion tube. Shut off the inside switch; clean it out with a water hose and let it dry.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW