Overview:

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 during certain times of the day.

Background:

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.

Goals:

This hack has the following goals:

1. Don't allow the heater to come on at night, when the noise bothers my neighbor.
   
2. Don't allow the heater to come on during the day when electricity costs the most.
   
3. Allow the heater to come on so the water is my desired temperature at my desired time.
   
4. Automate this on a timer.
   
5. Do this cheaply.

Process:

1. (Turn off the power to your hot tub.
   
2. Remove the access cover to the controller circuitry.
   
3. Located the wire running from the thermistor to the controller unit.
4. Cut this wire (it should have 2 conductors) mid-way between the thermistor and the controller unit.
   
5. Strip the wires from the thermistor and from the controller unit.
   
6. 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.)
   
7. 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.
   
8. Run the 4-conductor wire through a hole in your access cover and replace the cover.
   
9. 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.
   
10. ) 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.
11.  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. 

Inside of Box, showing Relay and smal circuit board

Outside of box showing switch and trim pot access hole

Parts:
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. (https://www.alliedelec.com/), part number 821-6004, MY2AC24(S) for about $5.

Operation:
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.

Risks:
I assume no liability if you do not know how to safely build circuitry, deal with voltages, soldering irons, etc. Also, if you wind up harming your hot tub controller, don't come whining to me. You will probably void any warranty you may have on your hot tub with this operation. DANGER: If you engage the circuit and the circuit resistance is set to a value LOWER than the temperature you want your hot tub to be, your heater will run forever. Your water will get scalding hot, and/or your heater will burn-out, possibly causing injury, fire or other life-threatening situations!

For example, if you set your value of R2 such that when the circuit is engaged your hot tub thinks it is 102 deg. F, and you set the front panel controls to 104 deg. F, the hot tub will never think it got to 104 and it will heat things until something burns out! Now, in California, they have recently passed laws that prohibit hot tub manufacturers from allowing people to set the temperature above 104 deg. F. If you have some other thermometer handy, and you want to temporarily engage the circuit to fool it into thinking it is only 102, then you can actually get your hot tub up to 106 or wherever you want it to be. There are obvious health consequences for people on medication, or intoxicated, or pregnant, etc... I do not recommend doing this.