Is there a sensor device or thermostat setup that can inexpensively switch on a fan based on the difference between two temperature points (one inside, one outside)?
I have a gable fan. I use it at night to help flush cool air from outside into my house. I was thinking a smart way to do this would be if it would automatically go on if it were, say, 5 degrees cooler outside than inside. e.g. if it is 80 inside and 75 outside, stay on 'til either a) outside goes up to 80 or b) inside cools down to 75. And of course, if it is 80 inside but 90 outside, don't turn on at all. (Obviously you would want a few degree buffer so it is not switching on and off rapidly when the two temperatures converge, I believe this is referred to as hysteresis.)
Apparently, this is not a frequently requested task. The closest thing I've seen is expensive stuff originally designed for custom solar water heating setups, like this
. (I have emailed to get a price, but my guess off-hand is S-P-E-N-D-Y!)
This seems like it could be a simple thing to accomplish with a few sensors and some sort of home automation scripting language, but I haven't gotten anywhere near that far tricking out my home.
Also, I am a little intimidated by relays, so the idea of something designed to simply switch on/off an attached household current device appeals, but maybe this fear isn't justified.