A.O. Smith Signature / ProLine and State Select electric tank water heaters: quick sheet

The heater is wired for non-simultaneous operation: only one element heats at a time. On a cold tank the upper thermostat sends power to the upper element first, so the top of the tank recovers quickly. When the top reaches setpoint the upper thermostat switches over and passes power down to the lower thermostat, which then runs the lower element to heat the bulk of the water. Because the upper and lower elements never run together, the circuit only has to carry one element's current.

Each thermostat is a surface-mounted snap-disc device clamped against the tank wall behind its own access panel and insulation. Both legs of the 240 V supply come into the upper thermostat/ECO block. One side of each element is fed directly; the thermostat switches the other side. That is why, with power on, you can read about 120 V from an idle element terminal to ground and roughly 240 V across an element only when it is being called to heat.

The ECO is a manual-reset high-limit built into the upper thermostat assembly (red button). If the tank water overheats, typically because a thermostat contact welded shut or an element is grounded and heating continuously, the ECO opens both legs and stays open until someone presses the button. Published trip values for compatible upper thermostats vary (listings show figures between about 170 and 190 F); read the label on the installed part rather than assuming a number.

Cold water enters through the dip tube at the top and is delivered near the bottom; the DynaClean-style inlet is designed to swirl incoming water to keep sediment from settling. The anode protects the steel tank; these lines use a stainless-core anode intended to last longer than a plain magnesium rod, but it still needs inspection. Recovery on a 4500 W element is slow compared with gas: A.O. Smith and other sources put full reheat of a typical residential tank at about 60-90 minutes.