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I have an attic fan with a thermostat, but with the high heat these past weeks, the outdoor temperature is often above the thermostat setting. Thus, I'm just replacing all the air in the attic with more hot air from outside the attic, at the same temperature. Its a huge waste of electricity.

I'd really like to find a thermostat that will compare the outside temp to the inside one, and shut off the fan only if the temperature inside is w/in 5 degrees or so of the outside temperature.

Does such a product exist?

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Whooo - sounds like a little prototyping project is required here :) a small pic (arduino?) with 2 temperature IC meters and a relay switch. To be honest I have never seen a product like that.. and i have been meaning to make something like for a greenhouse control system.. err no time... –  ppumkin Jul 13 '11 at 13:34
    
I'm capable of building that myself. But I really rather not. :-) –  David Pfeffer Jul 13 '11 at 15:50
    
Yea lots of works and uncertified.. The only other thing i can think of is have a thermostat outside with a setting that will turn on off the thermostat inside.. you know what i mean? –  ppumkin Jul 13 '11 at 15:54
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2 Answers 2

The only easy way (and non dynamic) is to have an external temp switch that turns on the internal main switch. Or the other way around depends how you want it to work.

Outside 35Degrees Inside 25 Degrees External >30(OFF) --> Internal >20(ON) --> FAN OFF

Outside 20 Degrees Inside 25 Degrees External <30(ON) --> Internal >20(ON) --> FAN ON

My logic might be slighltly flawed but its the only way I can think of. Otherwise you have to buy some more inteligent system (usually expensive) or make your own microcontroller based relay

A Thermostat With Adjustable Hysteresis

http://electroschematics.com/6606/temperature-controlled-relay-circuit/

http://www.enercorp.com/contr/ettr.htm

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Yea, the lack of dynamic here really doesn't cut it, unfortunately. I mean, when its 90 out, I still want those fans going. Otherwise, the attic is gonna be in the 130's! When its super hot out is when I need the fan the most. –  David Pfeffer Jul 13 '11 at 16:20
    
so when the attic is hotter than outside the fans must go.. if the attic is near equal then don't bother.. when its colder definitely not? is that correct? –  ppumkin Jul 13 '11 at 16:26
    
Yes, exactly. Standard rule of thumb says its worth the power it takes to run the attic fan if the inside attic temp is 10 degrees greater than the outside temp. –  David Pfeffer Jul 13 '11 at 16:43
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I've never seen one with a dynamic setting and two thermometers, but it makes a lot of sense that they should make one. No need to run the fan when the outside is not more than 10 degrees (F) cooler.

One option that would avoid putting a DIY gizmo on your home electrical system in an abusive environment* is to run some romex down from the attic to a convenient location and install a timer switch. Then you can configure it to be always on in the spring/fall, always off in the dead of winter, and run it on a timer to get the last of the cool air in the morning and exhaust what you can in the late evening:

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*My personal experience with electronics in the attic itself is that they have a bad habit of melting or frying the transistor. And my personal experience with attic fans is that they have a bad habit of breaking and create another hole in the roof that exposes the attic to the elements or a potential leak. So when I had my roof redone a few years ago, I just had them remove the old unit (it was already broken from a failed bearing) and patched the roof with a new sheet of plywood.

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not really on topic .. but i suppose in its an option.. and no-nobody want a hole in the roof- especially another one ;) –  ppumkin Jul 13 '11 at 18:08
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