Hot Water Heater Thermostats

How to replace hot water heater thermostat on gas and electric residential appliances

Hot water heater thermostats are adjustable thermostats used to control the temperature of the outgoing hot water. We will review two of the most typical types of water heating appliances; gas and electric.

Both residential appliances will contain at least one thermostat to operate the heater and one high limit switch to protect the unit from an excessive temperature.

Different hot water temperature is needed for different applications. Hot water is needed for shower, dish washing, clothes washing... and this is why you use hot water heater thermostat, to set or adjust the temperature.

Temperature range on both gas and electric water heaters differs but the common is that both have thermostats that are factory pre-set, usually at 120 F, in order to protect users from scalding water.

It is important to know that if exposed to hot water of 120 F or 49 C it takes more than 5 min to produce burns, for 130 F less than 30 sec and for temperature of 160 F less than a second.

Note: There are few ways how to protect you and others from accidental burns. Install temperature limiting valves that attaches to faucets to limit hot water or install mixing valve to reduce the hot water temperature, by mixing cold and hot water.

Hot water heater thermostats on electric appliances

Electric water heaters are usually equipped with two adjustable thermostats, one on each heating element (if water heater has two).

Temperature range on residential electric units is usually from 90 F to 150 F with the high limit around 170 F. The high limit switch (ECO) is located in the same area as the upper thermostat, has a manually reset (AO Smith taken as the example) and it is used to shut the unit down when the extremely high water temperature is present (above 170 F in this case).

Electric water heater thermostats are surface mounted, installed into a bracket that holds the element against the side of the tank. Back surface of the thermostat is in full contact with the tank and therefore it responds to tank surface temperature change.

When adjusting thermostat on hot water heater recommendation is to set both thermostats to about the same temperature or set the top element to slightly lower temperature so the bottom element activates first.


Thermostat Adjustment - How to set Water Heater Temperature

To change the temperature setting on electric water heaters follow the steps:

  • Adjusting thermostat on hot water heater is easy and you should do it only when the power is off. Disconnect all the power to the water heater by opening the switch on the fuse box or the main circuit breaker.

  • Remove the access panel, cover and fold the insulation away from the controls, to clear the way to the controls.

  • Using the screwdriver rotate the adjustment knob to the desired position (temperature).

  • Bring the cover, access panel and insulation back and turn on the electrical supply and the water heater.


emergency plumber

Replacing a Thermostat on Electric units

Thermostat is not expensive and that is why fixing thermostat on hot water heater is not recommended.

  • Turn off the electric power to the water heater.

  • Remove the front panel door and fold up the insulation pad.

  • Unclip the thermostat from the bracket.

  • Disconnect wires from the thermostat. Label wires for proper re-connection to the new element.

  • Remove the water heater thermostat.

  • Install the new thermostat into the bracket.

  • Attach wires to the thermostat.

  • Set thermostat to the original temperature setting.

  • Bring removed elements back.

  • Restore power to water heater.

It is recommended to check, clean or replace heating elements as well. When bringing the thermostat back, make sure it remains firmly against the surface of the tank. Inside the access cover there is a wire diagram; use it to properly re-connect wires to the thermostat.

electric water heater thermostatsIf your electric water heater is equipped with two heating elements and two thermostats, the lower ones will operate more frequently, have exposure to the lime build-up and fail more often than the upper ones.

When looking for water heater thermostats, it is important to know the voltage of the heating element or water heater. It can be found on the back of the element or on the main energy guide label.


Hot water heater thermostats on gas fired units

On gas water heaters, when the incoming cold water activates the thermostat, gas flows to the main burner. Once the water inside the tank reaches the set temperature, thermostat interrupts the gas flow to the main burner.

Residential gas water heaters are usually operating within temperature range from 80 F to 160 F where high limit safety device opens at 195 F (AO Smith for example).

Keep in mind that even with the thermostat set to 120 F the heater might shut off at 110 or 130 F. This is because the unit depends on the accuracy (+/-10 F tolerance) of the metal rod immersed inside the tank to measure the temperature.

gas hot water heater thermostatWater heaters thermostats on gas appliances are part of the gas control valve assembly. The thermostat knob is usually located at the front of the gas valve. It is used to control the water temperature and during the process of lighting the pilot and main burner. The temperature dial has markings and turning it counterclockwise you will increase the temperature setting.

Before performing any maintenance on gas water heater, turn the temperature dial on the lowest setting.

Note: Lower thermostat setting is recommended to use during low demand periods as the way to reduce energy losses during cold days and to prevent the tank from freezing.

One of the main problems related to the water heater thermostats are when the hot water is too low, too high temperature and when there is no hot water at all.

Reasons behind are simple; thermostat is set either too low or too high. If there is no hot water, either thermostat is not working or high temperature limit switch turned the unit off. If thermostat doesn't shut off, or you have are running water heating with high operation costs, simply replace the water heater thermostat.


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(All pictures are courtesy of Amazon.com)