Hot Tub Hack
#temperature sensor
#thermistor
#24VAC
#irrigation timer
#hot tub
#hack
#adjustable
#controller
#automation
Description: This modification replaces the thermistor (temperature sensor) of a hot tub with an adjustable resistor that deceives the hot tub controller into believing the water is at a specific temperature. Additionally, the circuit is activated and deactivated by a 24VAC line controlled by an irrigation timer, allowing the hot tub to remain inactive during certain times of the day. The hot tub features a basic controller that filters water every 12 hours for a duration of 2 hours and monitors water temperature, activating the heater if it falls below the user-defined setting. The filtering time can be programmed by power cycling the unit at the desired start time, but the heater cannot be prevented from activating at other times. To implement this modification, the resistance of the original thermistor must be measured using an ohmmeter. The circuit is constructed using a 4-conductor wire to connect the thermistor to the controller. A fixed resistor (R1) and a rheostat (R2) must be selected to match the resistance of the thermistor. The rheostat should be a 10-turn potentiometer for precise adjustments. For automation, a wire from the irrigation timer can be connected to a DPDT 24VAC relay, allowing for automatic control of the circuit. If an irrigation timer is unavailable, the circuit can still be used manually. Components such as resistors, switches, and wiring can be sourced from electronics retailers. The relay can be obtained from suppliers like Allied Electronics. After assembling the circuit, initial testing should involve switching to a manual override mode, where the hot tub's temperature reading can be observed. By adjusting the variable resistor, the hot tub's temperature reading can be manipulated to ensure the heater does not activate unnecessarily. The irrigation timer can be programmed to provide accurate temperature readings at specific times, allowing for efficient heating of the hot tub.
The circuit design involves a straightforward approach to replace the thermistor with a combination of resistors (R1 and R2). The fixed resistor (R1) is chosen based on the resistance of the original thermistor, while the rheostat (R2) allows for fine-tuning. The circuit is connected in such a way that when the DPDT switch is engaged, the thermistor is effectively replaced by the resistor network, tricking the hot tub controller into believing the water temperature is higher than it actually is.
The irrigation timer plays a crucial role in automating the operation of the circuit. When the timer activates, it allows the hot tub controller to receive the true temperature from the thermistor, enabling the heater to operate as needed. This setup not only ensures that the hot tub remains at the desired temperature during specific periods but also conserves energy by preventing unnecessary heating when it is not in use.
In terms of safety and reliability, it is essential to ensure that all connections are secure and that the components used are rated for the voltages and currents involved in the hot tub's operation. Proper insulation and protection from water exposure should also be considered to prevent any electrical hazards. The overall design provides a practical solution for managing the heating of a hot tub while incorporating automation features for user convenience.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 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, but you will have to manually engage the circuit during the day and/or at night. I got the 2 resistors, the switch, the wire, and the little project box all at Radio Shack. I had to search the Internet to find the 24VAC relay, which I got from Allied Electronics, Inc. ( ), part number 821-6004, MY2AC24(S) for about $5. Turn on the power to the hot tub after you have completed testing your circuit. For the initial test, you should switch the DPDT switch to the position that engages the thermistor (referred to as Manual Override.
Your hot tub should tell you what temperature the water is, and it may turn on the heater, depending upon what you have set the water temperature to be. I like to set my hot tub at 104 deg. F. With the circuit engaged (switching OUT the thermistor and replacing it with the combination of R1 and R2), you can adjust the variable resistor (R2) and you should see your hot tub respond by showing you a range of temperatures close to the original value.
Increase the resistance (R2) until your hot tub reads 1 degree higher than the value you like the water to be. In my case, I increase R2 until the temperature on the control panel read 105 deg. F. With this circuit engaged, your hot tub will never turn on the heater. It will always think the water is 105 and since it is only trying to reach 104, it will never turn on.
By setting the Irrigation timer to come on at certain times of the day, you can disengage the circuit, providing the hot tub controller with the true reading from the thermistor. In my example, the Irrigation timer turns on at 7:00 AM, telling the hot tub the true temperature of the water (which has been cooling all night).
The hot tub will respond by heating the water until it has reached its goal (104). At 8:30AM the Irrigation timer turns off, engaging the circuit, and telling the hot tub there is no need to heat the water any more. I set the Irrigation controller to do the same thing at 7:00 PM. This allows me to have hot water for my morning tub (usually around 8:30) and again in the early evening if I so desire.
I assume no liability if you do not know how to safely bu
The circuit design involves a straightforward approach to replace the thermistor with a combination of resistors (R1 and R2). The fixed resistor (R1) is chosen based on the resistance of the original thermistor, while the rheostat (R2) allows for fine-tuning. The circuit is connected in such a way that when the DPDT switch is engaged, the thermistor is effectively replaced by the resistor network, tricking the hot tub controller into believing the water temperature is higher than it actually is.
The irrigation timer plays a crucial role in automating the operation of the circuit. When the timer activates, it allows the hot tub controller to receive the true temperature from the thermistor, enabling the heater to operate as needed. This setup not only ensures that the hot tub remains at the desired temperature during specific periods but also conserves energy by preventing unnecessary heating when it is not in use.
In terms of safety and reliability, it is essential to ensure that all connections are secure and that the components used are rated for the voltages and currents involved in the hot tub's operation. Proper insulation and protection from water exposure should also be considered to prevent any electrical hazards. The overall design provides a practical solution for managing the heating of a hot tub while incorporating automation features for user convenience.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 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, but you will have to manually engage the circuit during the day and/or at night. I got the 2 resistors, the switch, the wire, and the little project box all at Radio Shack. I had to search the Internet to find the 24VAC relay, which I got from Allied Electronics, Inc. ( ), part number 821-6004, MY2AC24(S) for about $5. Turn on the power to the hot tub after you have completed testing your circuit. For the initial test, you should switch the DPDT switch to the position that engages the thermistor (referred to as Manual Override.
Your hot tub should tell you what temperature the water is, and it may turn on the heater, depending upon what you have set the water temperature to be. I like to set my hot tub at 104 deg. F. With the circuit engaged (switching OUT the thermistor and replacing it with the combination of R1 and R2), you can adjust the variable resistor (R2) and you should see your hot tub respond by showing you a range of temperatures close to the original value.
Increase the resistance (R2) until your hot tub reads 1 degree higher than the value you like the water to be. In my case, I increase R2 until the temperature on the control panel read 105 deg. F. With this circuit engaged, your hot tub will never turn on the heater. It will always think the water is 105 and since it is only trying to reach 104, it will never turn on.
By setting the Irrigation timer to come on at certain times of the day, you can disengage the circuit, providing the hot tub controller with the true reading from the thermistor. In my example, the Irrigation timer turns on at 7:00 AM, telling the hot tub the true temperature of the water (which has been cooling all night).
The hot tub will respond by heating the water until it has reached its goal (104). At 8:30AM the Irrigation timer turns off, engaging the circuit, and telling the hot tub there is no need to heat the water any more. I set the Irrigation controller to do the same thing at 7:00 PM. This allows me to have hot water for my morning tub (usually around 8:30) and again in the early evening if I so desire.
I assume no liability if you do not know how to safely bu
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