# "Circ" v. "auto" setting on programmable thermostat **URL:** \ **Category:** Factual Questions\ **Created:** [September 15, 2013, 2:42am UTC](https://boards.straightdope.com/t/circ-v-auto-setting-on-programmable-thermostat/668884 "2013-09-15T02:42:58Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![ThelmaLou](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thelmalou/32/390_2.png) [@ThelmaLou](https://boards.straightdope.com/u/ThelmaLou)\ **Post date:** [September 15, 2013, 2:42am UTC](https://boards.straightdope.com/t/circ-v-auto-setting-on-programmable-thermostat/668884/1 "2013-09-15T02:42:58Z")
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? ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [September 15, 2013, 2:46am UTC](https://boards.straightdope.com/t/circ-v-auto-setting-on-programmable-thermostat/668884/2 "2013-09-15T02:46:14Z")
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. ---
**Author:** ![Emtar\_KronJonDerSohn](https://avatars.discourse-cdn.com/v4/letter/e/58956e/32.png) [@Emtar\_KronJonDerSohn](https://boards.straightdope.com/u/Emtar_KronJonDerSohn)\ **Post date:** [September 15, 2013, 2:50am UTC](https://boards.straightdope.com/t/circ-v-auto-setting-on-programmable-thermostat/668884/3 "2013-09-15T02:50:08Z")
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.