Rheem manufactures a variety of hot water heaters, the most common of which are the gas and electric models. While these water heaters are relatively simple in the way they work, they still have parts that will fail occasionally. On the gas model, the most common failure is the thermostat. On the electrical model the thermostat or the heating elements will fail. Regardless of which model water heater you have, the thermostat can be replaced easily by the average do-it-yourselfer.
Gas Water Heaters
Shut off the gas supply to the water heater. Turn off the water supply to the water tank by turning the valve in the line just above the tank. Open the water tank relief valve on top of the heater. Connect a garden hose to the drain valve and drain the water from the hot water tank.
Disconnect the gas line and thermocouple from the front of the water heater at the thermostat using a pipe wrench. Remove the thermostat from the water tank using a pipe wrench to turn it counterclockwise. Wrap Teflon tape around the threads of the new thermostat.
Install the new thermostat on the water heater and tighten it with the pipe wrench, making sure to line it up with the burner fittings. Connect the gas line and thermocouple to the new thermostat and tighten them with a pipe wrench. Close the relief valve at the top of the water heater.
Close the drain valve at the bottom of the heater and remove the hose. Turn the water inlet valve on at the top of the water heater to allow the tank to fill. Open the hot water faucet at the kitchen sink or in the bathtub to allow the air to escape. Once the water is running uninterrupted, turn the faucet off.
Open the gas line to the water heater. Set the thermostat at the lighting position. Depress the button and light the pilot light using a match. Do not allow gas to build up. Light the pilot immediately after depressing the pilot light button.
Electric Water Heaters
Turn off the circuit breaker for the water heater. Label the breaker so that someone does not accidentally turn it on while you are working. Remove the cover on the front of the thermostat using a screwdriver. Remove the insulation covering from the face of the thermostat.
Disconnect the electrical wires from the old thermostat using a screwdriver. Remove the thermostat from the water heater. Put the new thermostat in place and connect the wires using a screwdriver. Place the insulation on the face of the thermostat.
Install the cover on the thermostat and tighten the retaining screw using the screwdriver. Note that the procedure is exactly the same if you have to change the lower thermostat and not the upper unit.