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Old 09-14-2013, 10:42 PM
Join Date: May 2010
Location: Neither here nor there
Posts: 11,810
"Circ" v. "auto" setting on programmable thermostat

What is the difference between circ and auto settings for the fan on my HVAC thermostat when the heat is on, when the a/c is on, and when both of those are off?

I know that "on" turns the fan on so that it runs continuously whether the heat is on, the a/c, or nothing is on.

My older thermostat only had on and auto, and I can't figure out what circ is for. Clearly, it means "circulate" or "circulation," but what does that mean?
Old 09-14-2013, 10:46 PM
Charter Member
Join Date: Jun 1999
Location: Milwaukee, WI
Posts: 26,635
On makes the fan run 100% of the time regardless of what is going on with the furnace or compressor. Circulate makes it turn on and off randomly throughout the day. On mine, it aims for being on something like 30% of the time.
I'll sometimes use it in summer to help move the air around or in fall when I have the windows open (again, to help move the air around) but when I don't want to listen to the vents all day and night.
Circulate the air for 5 minutes and then it shuts off for 20, then it kicks back on for a little while, then it shuts off.

Depending on the physical layout of your house and HVAC setup, it might make a world of difference and lower your electric bill...or it might do nothing at all if it's not that good at moving the air where it needs to be to make you feel cooler (and that's what it's all about, feeling cooler/warmer since the air won't be actually heated or cooled).

Try it once, won't hurt anything.

Last edited by Joey P; 09-14-2013 at 10:48 PM.
Old 09-14-2013, 10:50 PM
Join Date: Dec 2011
Posts: 1,422
Yep, on runs the fan all the time, auto runs the fan when the unit is heating or cooling, and circulate runs the fan at least 30% (or some other predetermined amount) of the time.
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