Smart attic fan controller ?

Posted by CJT on August 27, 2006, 2:24 pm

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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Posted by dpb on August 27, 2006, 2:36 pm

CJT wrote:
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...
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.

Posted by Pj on August 27, 2006, 2:49 pm

CJT wrote:
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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.

Posted by DT on August 27, 2006, 2:55 pm

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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

Posted by CJT on August 27, 2006, 4:07 pm

DT wrote:
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Thanks for the info. When I look at that particular page, it seems
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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