JJ wrote:

Is there a simple way to hack the thermostat on a hot tub to get above
the 104 cutoff? I was thinking of maybe covering the temp sensor with
some kind of insulating material...

That might work, if you could raise the thermal resistance between
the water and the sensor or lower the air-sensor resistance.

Has anyone successfully hacked their hot-tub thermostat, and how did you
do it?

Sure.

Article 663748 of alt.home.repair:
From:
Subject: Hot Tub Thermostat adjust
Date: 4 Nov 2004 13:30:36 -0500
Organization: Villanova University

Marcia wrote:

HOw can I trick the thermostat on my Marquis Spa, so that I can heat
the water to higher than 104 degrees?

You can't. That's the upper temperature limit for UL approval :-)

I want to get the tub up to about 107.

The latest Marquis Coastal Reward spas use a Balboa M7 controller with
2 thermistors near the heater. Balboa says the first (as water flows by)
controls the water temp and the second is a 118 F safety limit. They must
agree within 2 F for the controller to work.

If you wanted to make the controller think the water is 104 F when it's
really 110, you might put a resistor in series with each thermistor near
the control board. They have a negative tempco, with 22117 ohms at 90 F,
17636 at 100, and 14158 at 110, so 104 F would make them about 16245 ohms,
adding a 2.2K ohm resistor in series with each thermistor exposed to 110 F
water will make the controller think the water temp is only 104 F.

(Marcia got someone to do this, and it worked :-)

Nick