CJT wrote:
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?

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You can do this with a microcontroller. Have it turn on when the
difference between the outside air and inside air has reached your
pre-programmed setting. Have it turn off when the difference is a few
degrees apart. Maybe there is something that exists on the market, but
I have no idea. I would build my own.

Pj