I know this question has been asked many times, but the solutions haven't helped me as of yet.
Culprit: 2001 AO Smith gas 40 gallon water heater.
Problem: Hot water only lasts about 10 gallons. Luke warm after 11 and getting cold at 12.
I've cleaned the tank.
I've replaced the dip tube (which was fine).
I buttoned up the air so I have no leaks, have a nice blue flame.
Any advise appreciated.