# Michigan Thermostat logic...when and why

Discussion in 'General Discussions' started by BobCurtis, Feb 11, 2021.

1. ### BobCurtisNew Member

Winter cold has finally come to Michigan with overnight lows hitting just about zero F. I replaced the 20 year old original WF thermostat with a wifi stat from Honeywell, in anticipation of getting away this winter and wanting to monitor things while gone. The new stat behaves pretty much like the old one as far as shifting from stage 1 to stage 2, although the furnace run times seem to be a bit more consistent with the new one. Can anyone explain the logic these stats use to change a call from stage 1 to stage 2? Is the stat looking for a certain temp rise over a given time before it decides to kick in 2nd gear? As cold as it has been, this furnace almost never uses 2nd stage, but it has run in 1st stage for nearly an hour before reaching the set temperature. Why didn't it go into 2nd after 20 minutes? It puts out more heat at the ducts in 2nd stage, I've wondered why it would run so long in 1st instead of less than half the time in 2nd. Honeywell is no help, hoping someone here has some practical experience. Thanks...

The thermostat monitors the inside temperature and if it's not increasing quickly enough in stage 1 it will engage stage 2. They won't tell you the exact algorithm. Some higher-end Honeywell STATs use a "Proportional + Integral" algorithm that looks at two things: 1) the difference between the indoor temp and the set point and 2) the cumulative time the temp has been off the set point. Do an Internet search for "PI controller"

The Honeywells have a comfort setting that will engage stage 2 as needed to keep the inside temp within 1-deg of set point. Alternatively, some models also have a stage-differential setting that lets you specify that stage 2 will come on only when the temp drops 2, 3, or 4 degrees below the set point.

Honeywell also has an adaptive recovery feature that will activate the system far in advance of a scheduled set point change, so the temperature will be at the set point when the scheduled time arrives. This mode will also heat up slowly to avoid use of AUX heat.

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3. ### BobCurtisNew Member

Thanks for that, makes sense. I'm not finding any root level info for my stat, but your explanation gets me what I wanted. I have learned one thing that isn't in their knowledge base, the color stat I use will invert the colors of the background and text when it moves into 2nd stage to give a visual cue. This stat also send me a report via email once a month on heating/cooling runtime hours and a comparison of last year which I find helpful.