Well here is a good reason to hire only licensed Plumbers! Watch what can happen with an improperly installed or maintained hot water heater turns into a bomb. Click link below.
This hot water heater failed because the thermostat was disabled and the elements just kept cooking. Also they sealed up the temperature pressure relief valve which is meant to blow off if the temperature or pressure get to high. Generally a TPR valve has Preset Setting at around 150 Psi - Temp 210 F.
Here is a picture of a Potentially dangerous water heater I am getting ready to replace.