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Water Heating Systems

Seisco Tankless Heaters

Electric Water Heaters

Temperature sensor replacement

Required Tools:

  • Small opened ended box wrench
  • deep socket wrench or crescent wrench
  • needle nose pliers
  • battery powered volt meter
  • bucket.

Warning:To avoid electrical shock, make sure that all of the power is off to the heater before attempting to remove the old thermistor and while installing the new thermistor.

Seisco Tankless Water Heater

Thermistor #4 or TH-4 shown with ohm meter probes attached to measure resistance.

The TH-IN thermistor is located under the right side of the heater directly below the inlet fill tube. TH1 and TH3 are located near the top of chambers 2 and 4. TH2 and TH4 are located near the bottom of chambers 2 and 4.

  1. Turn off all the power to the heater. Confirm the power is off with a voltmeter, even if the status light (or LED) on the control board is off. There may be multiple breakers powering your model heater.
  2. Open a hot water faucet nearby. While the water is running, turn off the water supply to the heater. This will help siphon some of the water out of the heater.
  3. For TH1, TH2, TH3 and TH4, remove the green wire with needle nose pliers and pull the red capped wire from the stud on the end of the thermistor.
  4. For TH-IN (the inlet thermistor), remove the green wire with needle nose pliers and the red wire by loosening the hex nut with a wrench while holding the back nut with pliers or another wrench.
  5. Next, while removing any of the thermistors with a socket wrench or box wrench, have a bucket ready to collect water that will drain from the hole that the thermistor occupied in the chamber.
  6. Install the new thermistor making sure the new seal is positioned around the collar of the thermistor.
  7. Turn the thermistor clockwise by hand. Caution: Only use the wrench to make the thermistor snug and only tightened to 14 in.-lbs., which is a little past finger tight. The seal will do the work to prevent leaking not the tightness.
  8. Reconnect the green and red wires to the thermistor.
  9. If any water dripped onto the control board, use a hair dryer to dry the board and the rest of the heater. Make sure to dry behind the board as well. The heat from the hair dryer will not harm the control board.
  10. Fill up the heater with water. If there are no water leaks, turn the power back on.

Note: If the heater sounds an alarm and a 126 diagnostic code appears, turn the power off, check for leaks again and dry the entire heater thoroughly. The alarm means that moisture was detected and the control disabled the heater.

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