I have a gable mounted, thermostatically controlled attic exhaust fan which I use to hold down attic temps. I know some people think those are not energy efficient, but I've made my decision and it is not the subject here.
It occurs to me that there are times when the fan is running needlessly, because even though the attic is hot enough to trigger the thermostat, the attic temp and outside temp are almost the same, so the fan is simply replacing hot air with air that is almost as hot.
I don't want to just increase the thermostat setting to be above the maximum outdoor temperature (about 105 here in Austin, Texas this year), because then there will be many times when the fan could run to advantage but would not.
Does anybody produces a reasonably priced fan controller with sensors for outdoor temps that is smart enough to run the fan only when it makes sense to do so (e.g. when the outdoor temp is more than a specified number of degrees cooler than the attic temp)? Or has anybody built something like that for which they'd be willing to share their design?