09/03/2026
**Potential thermostat issues**
For the last couple of months, we’ve noticed that our HVAC unit has been running a lot harder than normal. We had a new unit installed in July 2025 and a new thermostat installed in January 2026. There wasn’t anything wrong with the old thermostat; we just upgraded it.
We live in Southeast TN, and we’ve had a couple of major heat waves over the past 6 weeks. We started noticing that sometimes our thermostat would read 79° even though we had it set to 70°. The strange thing is, it never actually felt like 79° inside. Throughout the heat wave, the house felt more like 70–72°.
Even stranger, when the thermostat would actually reach 79°, the temperature inside the house would be around 65°. We have a couple of other temperature sensors throughout the house, and they consistently show that it’s much cooler than what the thermostat is reporting.
We also noticed that during the warmest part of the day, the wall directly above the thermostat gets warm or even very hot. I attached a picture showing the area where the wall feels the warmest. There are no outlets or light switches nearby that feel warm—it's only around the thermostat.
The warm area is basically a narrow, straight line running from the thermostat up toward the ceiling.
Any ideas what could be causing this?
**Edit:** The HVAC tech came out and found that Stage 2 wasn’t turning on, so he replaced the wiring for it. The house has felt cooler since he came out about 5–6 weeks ago, but the thermostat still doesn’t seem to reflect the actual temperature inside.
**Edit 2:** I did some more investigating and found something interesting. There’s a smoke alarm directly above the thermostat on the ceiling, and that area of the ceiling/wall is the warmest. I unplugged the smoke alarm and removed the mounting bracket, and it feels like a lot of hot air is coming through the opening—presumably from the attic.
What would you recommend doing to fix this? Should I go into the attic and check for a gap around the smoke alarm fixture? Would it be appropriate to seal any gaps around it with some type of gap-and-crack spray foam?
Adding pictures in the comments.
