What is a Hot Water Thermostat?

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  • Written By: Dana DeCecco
  • Edited By: A. Joseph
  • Last Modified Date: 04 October 2019
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A hot water thermostat is a mechanical device that controls the water temperature in a hot water heater. The device contains a sensor that detects water temperature and turns the heating unit on or off. Hot water heaters are equipped with thermostats installed from the factory. A hot water thermostat unit is mounted on the exterior of the water heater and can be replaced if it is defective.

Typical water heaters use gas or electricity as the power source to heat water, and both types are equipped with a hot water thermostat. Both types of thermostats perform a similar function, but they are mounted differently. The gas hot water thermostat is more complex and more difficult to replace. The electric hot water thermostat is comparatively easy to replace and inexpensive.

A hot water thermostat typically performs well throughout the life of the water heater. Replacement can be performed by the homeowner on units equipped with traditional line voltage controls. The services of a qualified contractor might be appropriate for microprocessor-equipped water heaters.

Electric water heaters can be single-element units or double-element units. Replacement thermostats can be purchased for single, upper or lower elements. These units are available online and from home improvement stores. The electric hot water thermostat is mounted to the body of the water heater, so draining the unit is not necessary. Before replacement or adjustment, the power to the unit should be turned off at the breaker box.


The water heater is typically on a dedicated breaker. To replace the thermostat, one should remove the upper and lower access panels and push aside the insulation to reveal the thermostat. The high temperature limit switch has a reset button and is usually the upper panel. The thermostat is calibrated in degrees or with high, medium and low settings.

One should disconnect the wires from the terminals on the thermostat and note the position of the wiring. The unit should be unscrewed from the mounting block. The same type of unit or a universal hot water thermostat should be used as a replacement. One should remount the wires in the same positions and pull back the insulation. The cover plates should be attached, and the breaker should be turned to the "on" position.

Replacing the thermostat on a gas water heater is a little more difficult. Replacement can be done by the homeowner, but a qualified plumber might be a better choice. For one to remove the hot water thermostat from a gas unit, the water must be drained from the heater because the thermostat protrudes into the unit. The thermostat should be turned off at the gas control unit, and the gas shutoff valve should be turned to the "off" position.

One should carefully disconnect the lines running from the control unit to the burner unit, then remove the union and gas pipe leading into the control unit. The control unit/thermostat should be unscrewed from the water heater. The new unit should be installed using the correct pipe dope designed for gas installations. The same pipe dope should be used to replace the pipes. The gas shutoff should be turned to the "on" position, and one should test for leaks with soapy water.

The water heater should then be lit according to factory specifications. Finally, the hot water thermostat should be set to the desired water temperature. The water temperature on gas and electric units should be set high enough to provide ample hot water. Energy savings might also be a consideration.


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