Differential temp controller

I am thinking about building a differential temp controller. Basically two thermistors and associated circuitry feeding a comparator, the comparator would then control an output.
I could base the design off of a digital thermometer.
One of the thermister needs to be placed remotely from the other one.
Any Ideas?

what exactly do you want to do at the output

Turn on a blower motor(fan) to move te warm air from the solar colector to the cooler building!

I don't think you want to involve a digital thermometer, just two voltage dividers and a comparator plus a switch for the motor is all you need. Use shielded wire for the remote thermistor so it doesn't pick up noise.