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My Ecobee was working great all winter but now that the cooling season has started the temperature on the Ecobee is quite a bit off (it seems to be a new problem), 5-6F above the room temperature, measured with an adjacent wireless sensor and a type T thermocouple. Note, I'm looking on the sensors screen to make sure I'm looking at the temperature from the thermostat sensor and not the average of the active sensors. When I first installed the Ecobee I had to make sure to fill the hole in the wall behind the thermostats where the wires come in with fiberglass insulation to prevent problems with drafts from the walls. I tried to put more fiberglass insulation in the hole but the problem remains. I contacted Ecobee supported and they suggested I put in a manual offset of -5F to "calibrate" the sensor. I'm not happy with this as the problem appeared all of a sudden. The Ecobee seems quite warm to the touch so I'm not sure if its self-warming of the sensor that is the issue. I was wondering if anyone else has experience with incorrect readings from the main thermostat temperature sensor?
I know the nest uses a Sensirion SHT21 combined temperature/humidity sensor and the PCB is notched to prevent heat transfer from the device electronics to the sensor. I haven't found en Ecobee teardown but I suspect they do something similar.