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I am trying to repair a small water distiller. The machine is fairly simple, heater boils water, the vapor is cooled by a fan and the water is collected.

More information: How to use

  1. Connect power cord to power outlet.
  2. Press ‘Starting Button”. The fan will come on automatically and the tank will begin to heat, thus starting the distilling process.
  3. Water will begin to drip into the storage bottle after approximately 30 minutes. It will take 4 hours to distill full tank. The unit will automatically shut down when the cycle has finished.

Circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

Thermostat1 is NC (KSD301-G) 170 degree C.

I don’t know if Thermostat2 is NC or NO since it is not working. The part number is KSD301-R 115 degree C with manual reset button.

I cannot understand why there are 2 thermal switches in series with different temperature rating. One will control on/off (115) and the other one will sit there and do nothing (170.)

EDIT: I couldn't find a replacement for thermostat2. Using NO sensor rated for 115 degree doesn't make sense right? So I used a NC thermostat with 120 degree rating to test the machine operation. Now it works but instead of running for a complete cycle (4 hours) it works for 5-10 minutes then turn off 1-2 minute then again repeats until it is done. This means the new sensor is controlling the circuit and thermostat1 is doing nothing.

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  • \$\begingroup\$ Some photos of the thermostats in question with markings? Did you replace both or just #2? \$\endgroup\$
    – Spehro Pefhany
    Commented Sep 4, 2023 at 12:24
  • \$\begingroup\$ I replaced both just to be sure \$\endgroup\$
    – Eng Sam
    Commented Sep 5, 2023 at 10:47
  • \$\begingroup\$ You might want to put the one that was working back in just to compare. There are a few subtleties. \$\endgroup\$
    – Spehro Pefhany
    Commented Sep 5, 2023 at 13:28
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    \$\begingroup\$ For what it's worth, I use my water distiller with an additional clock timer so that it stops the boiling a bit before all the water is gone. This makes clean-up easier and reduces bad odors from burning minerals. \$\endgroup\$
    – Nayuki
    Commented Sep 5, 2023 at 18:35

2 Answers 2

Reset to default
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The KSD301-R is there to initiate a heating cycle. It opens when hot, that is, when a heating cycle completes, and stays open until reset manually.

The KSD301-G is a thermostat that opens when hot, and closes again when the temperature drops. It opens at a much higher temperature than the KSD301-R. It is there for safety in case there is a failure in the circuit (such as a failure in the KSD301-R) that might allow the temperature to rise to an unsafe level.

(Some devices, but not yours, use a thermal fuse that opens, and cannot be reset, when there is excessive temperature.)

Datasheet for KSD301 series thermal switches.

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    \$\begingroup\$ And because it is boiling water, 115°C will be reached only after all water has boiled off. \$\endgroup\$
    – jpa
    Commented Sep 5, 2023 at 5:46
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    \$\begingroup\$ So the 115 is the one for controlling the circuit , but the 170 is for safety and should not activate in normal conditions \$\endgroup\$
    – Eng Sam
    Commented Sep 5, 2023 at 10:45
  • \$\begingroup\$ if that is the case why is the 120 degree sensor keeps turning on and off ? could it be something mechanical , like mounting and thermal interface ? \$\endgroup\$
    – Eng Sam
    Commented Sep 5, 2023 at 10:47
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    \$\begingroup\$ @EngSam "So the 115 is the one for controlling the circuit , but the 170 is for safety and should not activate in normal conditions" Correct. "if that is the case why is the 120 degree sensor keeps turning on and off ? could it be something mechanical , like mounting and thermal interface ?" Could be, or maybe a short in the heating element? You would have to troubleshoot on your own, because repair of commercial products is off-topic here. :-(. But you can always ask specific questions, like the one here about why 2 thermal switches. \$\endgroup\$
    – Math Keeps Me Busy
    Commented Sep 5, 2023 at 11:47
  • \$\begingroup\$ @MathKeepsMeBusy Thanks for clarification \$\endgroup\$
    – Eng Sam
    Commented Sep 5, 2023 at 14:48
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It is usually a thermostatic switch.

If the ordinary thermostat fails closed then the other is the safety control.

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