PIC16F616 duty cycle variability with thermistor

Good Day,

Just as background information:

I have an off the shelf 50A PWM controller using a PIC16F616 for adjusting frequency and duty cycle. The unit came from a Canadian supplier, but it is made in China. Through a jumper the frequency is fixed at 100Hz. in my application. It drives (2) 17A brushless 12V brushless blowers for a Solar Air Heater of my design, for my home. My issue is efficiency. The Solar unit takes some time to warm up, of which I recover 75%.

More to the point, The duty cycle pot is a 2.2K. I paralleled a 2.2K NTC thermistor, to set a start duty cycle of 20% at 70F; the max temp of the solar heater is 140F, at which temp I would like the duty cycle to be 100%, but I can only get to 65%.

I do not necessarily want the answer, but rather the correct information to work through this issue, to get the thermistor and pic to respond accordingly. Obviously, my electronic skills are quite rusty.