I would think that a standard household thermostat would not work for=20
this situation.  What he is (we are) trying to do is to enable an=20
exhaust fan if the outside air is cooler than the inside air.  When the=20
fan is ON it will suck the outside air in through vents and such and=20
force the hotter inside air out.  If we only sense the inside air it=20
could, and in most cases would, suck the warmer outside air in and blow=20
the relatively cooler air out.  By my, possibly mistaken, thinking this=20
requires two thermostats or thermal metal switches and some 'knowledge'=20
of the temperature difference.

-Phil-

--------------------------------
Long ago when men cursed and beat the
ground with sticks,  it was called witchcraft..

Today, it's called golf.

On 7/14/2013 1:54 PM, Mark Hanchey wrote:
> On 7/14/2013 10:52 AM, Phil Keller wrote:
>> A friend of mine was wondering if I could build a simple device for
>> him.  He is looking for something that senses the outside and inside air
>> temperatures.  If the outside temperature is greater than ~70 and if the
>> inside temperature is 5 degrees greater than the outside temperature it
>> would enable an exhaust fan.  (We would probably need some hysteresis on
>> the temperatures to keep the fan from oscillating ON-OFF.)
>>
> This is one of those situations where I wouldn't use semiconductors. A
> household thermostat for heat and air will do what you want and only
> require the addition of a power supply and a relay.  An older mechanical
> type with the thermal metal and mercury switch wouldn't even consume any
> power until needed.
>
> Set the thermostat at 75F and if the inside is greater  then the relay
> triggers. The outside would have to be over 70F for the inside to be
> warmer to enable the exhaust fan.
>
> That system for exhaust fans was/is actually used in thousands of
> business that control the temp in hog/chicken/turkey production houses.
>
> Mark
>

--=20
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