Water Heater Repair Tutorial: Temperature Sensors, Erratic Combustion and Thermal Switches

The author provides some more troubleshooting and repair tips in the final installment of this four-part series

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One of the most basic skills a plumber should have is repairing a tank-type water heater. So far I have covered initial troubleshooting steps, specifics on diagnosing whether something is a water tank problem or a gas control issue, and the testing of thermocouples, piezos and pilot light issues.

In this final article, I’ll cover temperature sensors, erratic combustion and thermal switches.

Testing the temperature sensor

If the fault code reads that the gas control is locked out due to high tank temperature, you can test the temperature sensor to see if it is working. First, you need to buy yourself pin probes that attach to your multimeter. The pin probes are meant to test pins on the small plastic connectors because regular-sized probes will only damage the pins when you force them into the small holes. Before you get into how to test the temperature sensor, try power cycling the unit. Put the gas control knob to the “off” position, wait a few minutes, and then try to light the pilot as normal. If the unit doesn’t come on and still codes, then you need to take the gas valve apart.

Most newer water heaters use Honeywell gas valves. The Honeywell gas valve has a front, middle and back with temperature sensor and piezo ignitor connections in between. Do what you need to do to disassemble the valve but leave the sensor plugged into the back plate. Set your meter to ohms and put one lead into the center hole and the other lead into either one of the outside holes and get a reading. Write down the resistance reading.

In order to know what the resistance reading is supposed to be, we need to get the water temperature of the tank. Make sure there is no flow of water through the water heater and turn the cold water supply off to the water heater. Drain about 4 to 5 gallons of water out of the drain valve at the bottom of the water heater and throw it out. Now drain 1 gallon of water from the bottom of the tank as quickly as you can and get an accurate temperature reading.

Look up the manufacturer’s chart for sensor resistance at various temperatures and figure out what your resistance reading should be. The temperature reading is very critical and detail is of the utmost importance. Don’t think that if it says 82 degrees that 80 degrees is close enough. That chart requires exact numbers to get exact resistance readings.

Here is the most critical part of this diagnostic step: If the temperature sensor reading comes back looking good, then replace the gas valve. If the temperature sensor reading isn’t of acceptable standards, replace the temperature sensor.

Cleaning the burners

There are two types of burners — steel and cast iron. When you take the burner assembly out, remember that some of the nuts may be reverse thread. Now that you have the burner in your hand, look for and remove any loose soot buildup, especially in the burner ports. Clean everything. Clean the burner, the burner assembly, the orifices, the burner ports, the air shutter (if it has one), feed line, venturi (if it has one), pilot assembly, etc. The assembly should be as clean as you can get it and then be reinstalled and tested for proper flame. 

How to test the thermal switch

Remove both wires from the thermal switch, set your meter to ohms, and put one of the leads on one of the connection points and the other lead on the other connection point. If you get a continuity reading, then it is good. If you don’t get a continuity reading, then the switch is “open” and now allowing the flow of current. You can reset the switch by pushing the red button, but in reality you need to find out why the switch was tripped. This switch trips and opens when the water heater burner gets too hot. Don’t hit the reset switch and head out to your next call. Think through why it could overheat.

The unit could overheat for a few reasons, most of which have to do with incomplete combustion.

Smells, odors, condensation and erratic combustion

Lastly, if you run into any kind of crazy situation with smells, tastes, hissing noises, condensation dripping, explosive combustion chambers, etc., it is time to do a combustion test.

About the Author

Anthony Pacilla is a registered master plumber for McVehil Plumbing in Washington, Pennsylvania. He has 22 years of experience in the plumbing and HVAC trades, and has a bachelor’s in business and economics from Thiel College. 


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