Diagnosis
The discrepancy between the displayed temperature and the actual room temperature on the Emerson Sensi Touch ST75 is typically caused by "ghost heating." This occurs when the thermostat's internal thermistor is exposed to a localized heat source that does not represent the ambient temperature of the room. Because the ST75 is a high-performance digital interface, it generates a small amount of internal heat from the backlit screen and WiFi radio; if the unit is mounted in a spot with stagnant airflow, this internal heat traps against the sensor, causing an artificial spike in the reading. Another common culprit is "wall drafting." If the hole behind the thermostat where the wires emerge is not sealed, warm air from inside the wall cavity (which is often warmer than the room air) can blow directly onto the sensor. You can confirm this by placing a calibrated digital thermometer directly next to the ST75. If the thermometer reads 68°F while the screen says 75°F, the sensor is being influenced by an external or internal heat source rather than the air in the room.
How to Fix It
To correct the temperature reading, you must first eliminate environmental interference and then apply a software calibration offset.
- Inspect the Installation Site: Ensure the thermostat is not located within 3 feet of a lamp, television, or kitchen appliance. Check that it is not in the direct path of sunlight during the day. If it is mounted on an exterior wall, the thermal mass of the wall may be skewing the sensor.
- Seal the Wall Hole: Remove the ST75 faceplate from the wall bracket. Use a small piece of plumber's putty, foam insulation, or electrical tape to plug the hole where the wires exit the wall. This prevents warm attic or crawlspace air from leaking into the device housing.
- Access Temperature Calibration: Wake the screen and tap the Menu icon. Navigate to Settings and select Advanced Settings.
- Apply Temperature Offset: Look for the Temperature Calibration or Offset setting. Since your unit is reading 7 degrees too high (75°F vs 68°F), adjust the offset to -7°F.
- Save and Reboot: Confirm the setting and return to the home screen. Wait 15 minutes for the sensor to stabilize and verify the reading against your handheld thermometer.
If That Didn't Work
- Check for "C-Wire" Overheating: If you are using a Power Extender Kit (PEK) or a non-standard wiring configuration, the thermostat may be drawing excessive current, causing the internal components to overheat. Verify that the C-wire is securely seated in the terminal and that there are no frayed wires touching the backplate.
- Perform a Factory Reset: A glitch in the firmware can occasionally cause sensor drift. Navigate to Menu > Settings > Reset and perform a full factory reset. This clears the cached calibration data and forces the thermistor to re-baseline.
- Verify WiFi Signal Strength: A very weak WiFi signal causes the ST75's radio to work harder, increasing power consumption and generating more internal heat. If your signal is below two bars, install a WiFi extender to reduce the load on the thermostat's internal processor.
- Test with a Remote Sensor: If your HVAC system supports it, integrate a remote temperature sensor. This allows the ST75 to ignore its own internal reading and instead trigger the HVAC system based on the temperature of a sensor placed in the center of the living area.
Heads Up
- Avoid cleaning the thermostat screen with ammonia-based cleaners; these can seep into the edges of the bezel and corrode the sensor circuitry.
- Never install the ST75 directly above a heat vent or radiator, as the rising hot air will cause the system to shut off prematurely, leaving the rest of the house cold.
- If you notice the temperature jumping erratically by 5-10 degrees in seconds, this usually indicates a failing thermistor, which requires a full unit replacement.
- Keep the area around the thermostat clear of curtains or furniture, as these block the airflow necessary for the sensor to accurately read the room's ambient temperature.