Ruud water heaters that have thermocouples are gas water heaters. The gas pilot flame heats the thermocouple, which in turn allows the gas valve to open. The gas valve will not open if the pilot flame is out and the thermocouple is cool. This is so that gas is not allowed to leave the gas pipe if it's not going to be burned by the flame nozzles. Gas that accumulates can create an explosive environment. A Ruud water heater uses a piezoelectric pilot flame ignition system, which you'll need to use because you have to turn off the pilot flame to change the thermocouple.
Things You'll Need
Open-end wrench set
Find the Ruud control box with the knob on top for gas control. It will be near the access panel for the water heater. Turn the knob clockwise to the off position. Wait 10 minutes before continuing so you can smell for gas. If you smell gas, stop immediately and call a professional--you may have a gas leak, which is very dangerous.
Use a screwdriver to remove the access panel screws. Once all of the screws are removed, take off the access panel, which will expose the thermocouple, pilot flame nozzle and gas valve.
Position an open-end wrench on the nut holding the thermocouple to the thermocouple bracket just above the pilot flame nozzle. Loosen and remove the nut. Note the position of the thermocouple in relation to the pilot flame nozzle because you'll want to install the replacement in the same location.
Locate the other end of the thermocouple cable, where it connects to the gas valve. Use an open-end wrench to remove the nut connection and fully slide the entire thermocouple assembly out of the Ruud water heater.
Turn the gas knob on the control box counterclockwise to the pilot position. Press the red Ruud starter button beside the pilot flame assembly to ignite the pilot flame.
Continue turning the gas knob on the control box to the on position.
Place the access panel back over the water heater opening and tighten it into place with a screwdriver by driving in the access panel screws.
Keep a plastic bowl handy to hold your tools so you can keep them together and won't lose them.
Never use an open flame in an area in which you smell gas. Gas can settle in enclosed spaces and can create explosive conditions.