Thermostat shows the wrong room temperature

Why does the thermostat disagree with your thermometer?

Because they are not measuring quite the same thing. Honeywell's support page on this question says its thermostats take "into account not just the ambient air temperature but also the radiant temperature of objects in the room." A cold exterior wall or a sunny window affects that reading. A thermometer on a shelf across the room does not feel the same surfaces.

The display also rounds. Honeywell says its thermostats round the number "to avoid jumping up and down constantly." So a reading that sits close to the line between two whole degrees can show one number while your thermometer shows the other.

A small gap between two instruments in different spots is ordinary. A gap that is large, or that changes through the day, is worth tracking down.

Is the screen showing the thermostat sensor or another sensor?

On many systems the number on the screen is not the thermostat's own reading.

• Honeywell Home T6 Pro. On the TH6320U and TH6220U models, installer option ISU 520 chooses which sensors control the temperature: 1 is Thermostat Only, 2 is Wired Only and 3 is Average. The default is 3, and Honeywell notes that multiple sensors are averaged.
• Honeywell Home T9. Rooms you manually select create an average temperature for the home. With motion priority on, rooms with detected motion are selected automatically, so the average can shift as people move around.
• Google Nest. On the screen, the small number near the edge is the current temperature and the large center number is the target temperature. If a Nest Temperature Sensor is active, the current temperature shown is that sensor's reading. Google adds that if the sensor is having trouble connecting, the thermostat uses its built-in sensors instead.
• ecobee. With SmartSensors, the displayed temperature is the average of all currently participating sensors.

If you compare a thermometer in the hallway with a number that is really an average of three bedrooms, they will not match, and neither one is wrong. Our comparison of room sensors and the thermostat sensor goes into how each brand decides.

Where does a poor location skew the reading?

Location is the cause manufacturers mention most. Sensi puts it strongly: "In nearly all cases where a digital thermostat is not reading accurately, it is because of a poor thermostat location or an improperly sized heating/cooling HVAC equipment."

Google lists the spots that give a Nest Temperature Sensor a misleading reading. They apply just as well to a thermostat:

• Near a heating vent, a stove or a hot water pipe
• Next to a window in direct sunlight
• In a poorly insulated room or on a poorly insulated wall
• In a drafty area, near a door to the outside
• Mounted too high or too low; Google says chest height is ideal
• In a room that is not heated or cooled

ecobee adds return vents to that list, along with windows, doors and direct sunlight. If the thermostat cannot be moved, ecobee suggests using its SmartSensors as the only temperature reading. Moving a wired thermostat is installation work, not a setting, so it belongs with your installer.

Can you adjust the displayed temperature?

Several brands let you offset the reading when it is off by a steady amount. The ranges differ a lot:

Temperature reading offsets by brand and modelUnits: Range in °F; Default in °F.

Thermostat	Setting name	Range(°F)	Default(°F)	Where you change it
Honeywell Home T4 Pro, T6 Pro	Temperature Display Offset (ISU 1420)	minus 3 to 3	0	Installer setup
Honeywell Home T9	Indoor Temperature Display Offset (ISU 14020), in 1°F steps	minus 3 to 3	0	Menu > Preferences > Indoor display offsets
Emerson Sensi	Temperature Offset	minus 5 to 5	Not stated	Sensi app: settings gear > System settings > Temperature Offset
ecobee	Temperature Correction	minus 10 to 10	Not stated	On the thermostat only: Installation Settings > Thresholds
Google Nest	Not stated	Not stated	Not stated	Not stated

A few details matter. Honeywell's guides say the T4 Pro and T6 Pro can display up to 3°F lower or higher than the actual measured temperature, so the setting is described as a display offset. The T9 has a matching humidity offset of minus 12% to 12% in the same menu. Sensi says its offset applies to the thermostat only, not to its sensors. On ecobee, the full steps are on how to calibrate an ecobee.

An offset is for a steady error. If the gap changes with the sun or the time of day, fix the cause instead, or the reading will be right at one hour and wrong at another.

What can you check on the thermostat?

Are you reading the current temperature, not the setpoint? On a Nest, the large center number is the target.

• Yes

  Is the screen showing a remote sensor or an average of several sensors?

  • Yes

    Compare your thermometer with the room or rooms being measured, not the hallway. On models that let you choose sensors, change which ones take part.

  • No

    Is the thermostat near a vent, window, outside door or direct sunlight?

    • Yes

      The location is the likely cause. An offset will not follow an error that changes during the day. Ask your installer about the location.

    • No

      Check the gap at a few different times. If it stays the same, use the offset setting for your model from the table above.

• No

  Look at the smaller current temperature figure instead. The setpoint is the temperature you asked for, not the one measured.

If the reading is correct but the room still is not at the setpoint, that is a different problem: see why the room never matches the set temperature. How far the room is allowed to drift before the system starts is set by the temperature differential. For the ecobee version of this question, see ecobee temperature different from the room.