Bad thermostat? gas or electric heater
Stacking? gas or electric
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

http://waterheatertimer.org/How-to-replace-thermostat-on-electric-water-heater.html

http://waterheatertimer.org/Stacking.html

Gene

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