Smart attic fan controller ?

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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CJT wrote:

Don't know of an inexpensive differential controller offhand, but the simplest way to approximate one would be a second thermostat (for outdoor air temp, of course) in line w/ the present controller. Set it to be on when the air temp is at or below your desired external temperature for bringing in exterior air and the interior one at whatever level above that you want it to come on at. The fan will then only run when both conditions are met -- when the exterior temp gets to whatever level you want it will open and cut the fan off.
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CJT wrote:

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
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Sure, what you want is a differential controller. They are commonly used in greenhouses, so search for "greenhouse temperature controller". Here are some, about halfway down, look for the Dayton controllers:
http://www.littlegreenhouse.com/accessory/controls.shtml
-- Dennis
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DT wrote:

the differential they're talking about is what I would call hysteresis (i.e. the difference between the "on" and "off" temperatures), rather than control based on the difference between two sensors (e.g. attic and outdoor). But I do think greenhouse controls are likely to be a fruitful (no pun intended :-) )area for my search.
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Search for a "differential thermostat". I think most are made for solar systems and greenhouses but you'll likely find one that would do the job. This is a low volume item compared with most consumer products so I would expect it to be a relatively expensive device compared to a regular simple thermostat.
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Malcolm Hoar wrote:
<snip>

cost $100 and up, judging by what I found on the Web. I do not believe that level of expenditure is justified by the likely savings in energy cost (which, IMHO, is highly unlikely to exceed $15/year).
I may pursue the homebrew option. If I can build something serviceable for under $25, it might be worthwhile.
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Use two thermostats?
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CJT wrote:

quickly!
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When is it you think this is true? I have an attic fan in the roof and I can't think of any time when it is hot enough to turn the fan on that it it isn't quite a bit hotter inside the roof than outside.

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mm wrote:

approximate the outdoor temp while both are still quite hot.

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I"ve found that the fan turns off between 7PM and 10PM, rarely going as late as 10, depending on how sunny and hot the day had been.
The fact that it turns off implies to me that for some time before then, it was cooling off with cooler outside air.
And my experience when I walk outside after sundown is that it cools off more quickly than even does the second floor of my house. (I rarely use AC)
I have doubts that it is ever as hot outside as it is inside the attic after sundown.
If I ever again see an indoor/outdoor thermometer at a yard sale, maybe I'll get it, so I can know "for sure".
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mm wrote:

Once they're within about 10 degrees F, it's not worth running the fan any more, because the fan uses more energy than it removes as heat.

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