I have a Trane XL 850 that I have controlled through Hubitat. This controls a variable speed system. The thermostat sucks. The AC can run for hours and hours and I can watch the "current temp" that it shows go up a degree. It does not accurately read the air temperature. My conjecture is that the system itself generates heat that interferes.
Anyway, I have Zooz temperature sensors everywhere now and have a good idea of what the temperature should be, vs what the thermostat says it currently is. The thermostat scheduler/controller combo is working great at keeping the house at the actual temperature I want it at.
What happens though is this. Let's say I have the TC set to 74. The temperature goes up to 75 and so it wants to tell the actual physical thermostat to make cold air. It might set the thermostat to 66 and then turn it off once then temp reaches 74... which is what I want. Actually, what happens is because of how much the XL850 sucks, the actual house temp might be 75, but the TS says 78 and then TC sets it to something like 68 so it thinks there is a huge demand for more cooling when there really isn't.
The issue is that I have a variable speed system and I would rather the temperature of the thermostat be set just a couple of degrees lower (than what it currently thinks the temperature is) so that the compressor maybe runs at 20% for an hour instead of 100% for 15 minutes. It has basically turned my variable speed system into a regular single stage unit.
EDIT: I actually just realized I will probably have the same issue in Winter but worse. Instead of running the heat pump the high wattage electric heaters will be engaged. So definitely need to know if this can be changed.
I know it can be calibrated. The issue is not really that it's off by X number of degrees. Let's say the air temperature is 76 and the air has not been running for a while. The thermostat might correctly show 76, or within a degree.
But now lets say I set the air down to 72. The AC will kick on and it runs and runs and runs, all while we can feel the temperature dropping and the sensors show the air dropping... the thermostat might say the indoor temperature is still 76 degrees. I have even seen what it thinks is the indoor temperature creep upwards, so it might now say it's 78 degrees when in reality it's dropped down to near 70. We get around it by just manually changing the setpoint temperature a bunch. So I might set it back to 76 at this point.
Where is your thermostat installed? Is it very close to a return or a supply, or does it perhaps have a remote device that reads the return temperature?
Sounds like you are getting false heat around the thermostat. Where is it located? Any lamps or pulldown stairway near it? Also make sure the hole behind the tstat where the wire comes thru the wall is plugged. Toilet paper works well. I was in HVAC 45 years until I retired 4 years ago.
I don't think it's the location. Google "Nexia XL850 inaccurate" and you'll find lots of complaints. I installed a Zooz ZSE44 right next to it and the temps it records rises and falls as expected.
As an example of the behavior I want to change, here is what the thermostat looked like after I adjusted the TC down by 1 degree.
It would have been much better if it had set the thermostat to 75 instead and the compressor would have probably come on at like 25%. I suppose I will need to turn off the thermostat scheduler and build my own schedule in RM using time triggers and setting the TC based on what the thermostat thinks the temperature is. I think it's doable.
Seems like you really just want to set the thermostat's target to [thermostat reported temp] + [actual temp] - [desired temp]. So the thermostat sees the actual direction and magnitude of the desired change, regardless of what the numbers themselves are.
