02-09-2010, 12:24 AM
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Hot water rises. The thermostat in your heater is near the top of the heater. If you keep it vertical, the water immediately around the heater will heat up and rise and tell the thermostat that the water's warm enough when it's really only the water right around the heater that's warm enough. If you tilt it sideways, the hot water will still rise but not right past the thermostat, so the thermostat will more accurately measure the tank temperature and do a better job of heating the tank evenly.