Hot Tub Hack

Posted on Feb 5, 2014

This hack replaces the thermistor (temperature sensor) of a hot tub with an adjustable value that will fool the hot tub controller into thinking that the water is a certain temperature. In addition, this circuit is turned on and off by a 24VAC line controlled by an irrigation timer. In this way, I can make sure that the hot tub will not turn on du

Hot Tub Hack
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ring certain times of the day. My hot tub has a very basic controller circuit. It is programmed to filter the water every 12 hours for 2 hours duration. In addition, it will monitor the temperature of the water and turn on the heater if it drops below the value set by the front panel controls. I can set the time of day that the 2-hour filtering occurs by power cycling the unit at the time of day I want it to start. But I can`t prevent the heater from coming on at other times of the day. Using an Ohm meter measure the resistance of the thermistor. (My hot tub was hot when I started -104 deg. F the thermistor was putting out about 18K ohms yours may be different. ) Using a 4-conductor wire, splice the wires from the thermistor to 2 wires and from the controller unit to the remaining 2 wires. Remember which colored wires go where. Build the circuit shown in Diagram 1. Choose the values for the fixed resistor (R1) and the rheostat (R2) such that you can match the resistance measured in Step 6. R2 should be a 10-turn potentiometer (wired as a rheostat) so that you can accurately match the correct resistance. ) If you want to automate this circuitry, run a wire from your irrigation timer (pretending it is one of the `stations` used to turn on irrigation) and connect it to the DPDT 24AC Relay as shown in Diagram 1. If you don`t have an irrigation timer, then you can eliminate the Relay and use the circuit described in Diagram 1,...

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