I have to ask -- are you aware of the existence of bimetallic strips, and how they can perform this function in a completely mechanical way? (No electricity.) It doesn't get you active air movement, of course, but it has worked for years in greenhouses.
I know about them; that would be the simplest and safest possible solution indeed, however they are on-off switches and also have a fixed (sometimes excessive) hysteresis which couldn't be ideal for some uses.
