Gas water heater poor temperature control
I would not have expected this problem for a heater only 5 years old but sometimes the burner does not kick on after the tankful of water has sat unused for several hours and cooled down somewhat. Then when someone uses the not so hot water and colder water enters the bottom of the tank, the burner finally kicks on.
If you manually turn the knob and the heater kics on after just one or two notches, then the water inside was not so hot. Turn the knob back to where it was whether or not the burner came on.
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