Differentail Temp Controls

I have a variant of the "whole house fan " idea that I'm pretty happy with, and it occurred to me that a differential temp control would be a handy
thing to install in the control circuit. ( The "controls" consist of a wind-up timer, at this stage). Anyhow, can anyone suggest a reasonably priced setup for this application? I'm thinking along the lines of permitting it to run only when the temp outside is at least10 (or so) degrees cooler than inside.
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