How to Test & Repair the Electric Water Heater High Temperature Cutoff & Reset Switch
InspectAPedia^®  -    

• Electric Water heaters: how to inspect, test, adjust, repair a water heater
• How to find & identify the parts & controls on an electric water heater
• How to test and repair an electric hot water heater
• Test procedure for electric water heater high limit cutoff thermostat switch
• Test procedures for electric water heater heating elements
• How to replace the heating element on an electric water heater
• Guide to inspection of electric water heaters
• No Hot water? how to get more hot water flow, quantity, performance from your water heater

Before testing the electic water heater elements we should check out the high temperature cutoff switch since a problem there will simply turn off power to both heating elements.

On an electric water heater the upper heating element may have two, three, or four electrical terminals, and above the upper thermostat is a high temperature cutoff switch with four terminals. You should be able to identify these parts by looking at the water heater's wiring diagram found on the tank or inside the panel cover.

The electric water heater high-temperature cutoff swtich will turn off the entire water heater if temperatures are too high. If this cutoff is not working the water heater will simply not heat at all. You can use this switch to test to confirm that the incoming electrical power circuit is indeed bringing power to the water heater.

Turn off electrical power to the heater at the electrical panel: WARNING: possible fatal electrical shock. See Using DMMs & VOMs Safely and see Voltage Measurement & Detection. General electrical inspection safety advice is at SAFETY FOR ELECTRICAL INSPECTORS.

Set the multitester or VOM to its 250 VAC scale, and attach the tester probes or alligator clips to the two uppermost terminals on the control (L1 and L3 on the top left and top right in our photo below). These are normally the incoming electrical wires from the power circuit to the heater. "L" refers to "Line voltage" on most controls.

Test for line voltage: Now you can turn on electrical power to the heater. Your VOM should show voltage close to the voltage on the water heater's identification tag: typically this will be 208V or 240V for a 240V water heater, or it might be just 115V or 120V for a 120V water heater. If you do not see voltage close to the rated voltage you need to call an electrician to diagnose and repair the problem.

Turn off Electrical Power to the Water Heater again. Don't skip this step or you may be killed or you may ruin your test equipment.

Test the High Temperature Cutoff Switch itself: set the VOM to its Ohm or resistance scale using the most sensitive setting (R x 1). Touch one probe to the upper left terminal on the control (L1 in our photo at left). Touch the other probe to the lower left hand terminal on the control (T2 in our photo at left).

The ohm meter needle should move (or your digital multimeter DMM should show some number other than infinity). If no continuity and infinite resistance is what you read, try pressing the reset button on this control (the red "RESET" button in our photo at left).

If you cannot reset the control then this device is not working and it needs to be replaced.

FYI we do not usually find a problem with this electric water heater high limit cutoff switch. The most common problem on an electric water heater that is not heating is a burned-up water heating element - tests that we describe next at How to Test Electric Water Heating Elements.

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