how do i fix a temperature reading that's way off? it says 72 but my room feels like 65.

Diagnosis

The Honeywell Home RTH6580WF uses a thermistor embedded on the main circuit board to measure ambient air. When the reading is significantly higher than the actual room temperature—such as the 7-degree discrepancy you are seeing—it is usually due to "internal heating" or "wall draft." Internal heating occurs when the WiFi radio or the backlit LCD generates enough heat to warm the sensor, causing the thermostat to think the room is warmer than it is, which prevents the furnace from kicking in. Alternatively, if your thermostat is mounted on an exterior wall, cold air can seep through the hole where the wires enter the wall cavity. This creates a localized "micro-climate" behind the device. If the sensor is being hit by a draft of 60-degree air from inside the wall, but the room is 65, the device may struggle to stabilize, or conversely, if the device is mounted near a heat source (like a lamp or kitchen appliance), the sensor will be artificially inflated. You can confirm this by placing a calibrated digital thermometer directly on top of the thermostat casing for 30 minutes; if the digital thermometer reads 65 but the screen says 72, the sensor is skewed.

How to Fix It

To correct this, you must apply a Temperature Offset (Calibration). This tells the software to subtract or add a specific number of degrees to the raw sensor reading to match your actual room temperature.

1. Press the MENU button on the main screen to enter the system settings.
2. Use the arrow keys to navigate through the menu options until you see Preferences. Press SELECT.
3. Navigate to the Temperature Offset setting. The default value is 0.
4. Since your thermostat reads 72 but the room is 65, you need to lower the reading by 7 degrees. Use the down arrow to change the offset to -7.
5. Press SELECT to lock in the change.
6. Press HOME or wait for the screen to timeout to return to the main display.
7. Wait at least 2 hours for the system to stabilize before making further adjustments, as the HVAC system may have been running unevenly due to the previous incorrect reading.

If That Didn't Work

• Seal the Wall Hole: If the offset doesn't hold or the temperature fluctuates wildly, remove the thermostat from the wall plate. Use a small piece of plumbers putty, expanding foam, or heavy-duty duct tape to seal the hole where the wires emerge from the wall. This prevents "chimney effect" drafts from the attic or crawlspace from hitting the sensor.
• Check for Heat Sources: Ensure the RTH6580WF is not installed in direct sunlight or within 3 feet of a heat-emitting appliance (like a television or toaster). Even a small LED lamp placed too close to the unit can raise the sensor temperature by 3-5 degrees.
• Verify C-Wire Power: If the device is overheating internally, it could be a power delivery issue. Ensure you have a dedicated C-wire (Common wire) providing 24VAC. If you are using a power-stealing workaround or a faulty transformer, the circuitry can run hot, skewing the thermistor.
• Factory Reset: If the software is glitching, perform a hard reset. Press MENU, go to Advanced Settings, and find the Reset option. This clears any corrupted calibration data and allows you to start the offset process from a clean slate.

Heads Up

• Avoid setting an offset greater than +/- 10 degrees; if you need more than that, the sensor is likely defective or the installation location is fundamentally flawed.
• Always use a calibrated digital probe thermometer for testing, as handheld "laser" thermometers measure surface temperature, not air temperature, and will give you a false reading on the plastic casing.
• Be aware that changing the offset affects both the Heating and Cooling thresholds simultaneously.
• If you notice the screen flickering or the WiFi disconnecting frequently alongside the temperature drift, the internal voltage regulator may be failing, necessitating a full unit replacement.