I put a thermocouple down in the pipe chase and it is within 10 to 15 degrees of outside temp (Fahrenheit). That would explain the freezing when Temps drop below the 20s and into the teens and single digits. I ordered this fan and it will be here in a few days. I have an idea how I will mount this and reduce output down to a 2 inch pipe to feed into the pipe chase below the raised floor in the bathroom and draw warmer ambient cabin air. Ultimately, I think I will design a small PWM speed control circuit that increases speed the lower the temperature gets with a cutoff at a set (warmer) temperature. That will make this automatic so I don't need to think about it anymore.
I actually have a DC PWM light dimmer switch at home I bought and did not use in this build. I may be able to remove the variable resistor (rotary dial) and replace with a remotely mounted thermistor if I can match it up same resistance range and assuming the PWM circuit can handle up to 2.5A DC at 100 % duty cycle . May save me lot of work.

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