Why Is My Thermostat Showing the Wrong Temperature? 5 Reasons and Fixes

First, check it against a thermometer

Put a thermometer you trust right beside the thermostat, at the same height, and give both time to settle. Then note two things: how big the gap is, and whether it stays the same through the day or changes with sun, drafts and the system running. That pattern points to the cause.

Do not expect a perfect match. Honeywell's support page on thermostats that read differently from a thermometer explains that its digital thermostats are designed to read temperature the way people sense it. They take in the radiant temperature of objects in the room, such as walls and furniture, as well as the air. Most room thermometers read only the air, so Honeywell says they will not match the thermostat.

1. Rounding and the built-in swing

Small differences are built in. Honeywell says its thermostats round the display to the nearest whole number (half a degree in Celsius). If the thermostat is set to 72, the system turns on and off as the room falls to 71 or rises to 73, but the display stays at 72 so it does not jump up and down.

Many thermostats also wait for the room to drift before starting the system again. Google calls this the maintenance band. In its maintenance band example, a Nest set to cool to 70°F with a 1°F band cools the house to 69°F, turns the AC off, and waits until the room reaches 71°F before cooling again. Google says the delay helps prevent wear and the energy use that comes with frequent starts. If you think your system starts and stops too often, see how long an AC should run to cool one degree.

ecobee gives a number for sensor accuracy too: its Temperature Correction page lists the thermostat sensor's sensitivity variance as plus or minus 1°F (0.5°C).

2. A setting that changes the number on purpose

Some thermostats show an adjusted temperature by design.

• Humidity adjustment. Honeywell's X7 troubleshooting page says its Feels Like Temperature setting can make the indoor reading show a few degrees warmer when relative humidity is high. ecobee's page on inaccurate temperature readings says that when Adjust the Temperature for Humidity is on, the thermostat displays a feels-like temperature and an eco+ logo appears. If humidity indoors runs high in summer, AC fan On vs Auto is worth a look as well.
• Sensor averaging. ecobee says that with Smart Sensors connected, the temperature on the screen is the average of all participating sensors, so it may not match the room you are standing in. You can check which sensors are participating in the Sensors menu. Honeywell says thermostats with wireless sensors can be set to measure at the thermostat, at a sensor, or as an average.

3. The thermostat is in the wrong spot

A thermostat reads the air around it, not the whole house. Honeywell's T10 installation guide says to mount it about 5 ft (1.5 m) above the floor, in an area with good air circulation at average temperature, and not where it can be affected by:

• Drafts, or dead air behind doors and in corners
• Hot or cold air from ducts
• Radiant heat from sun or appliances
• Hidden pipes and chimneys
• An unheated or uncooled area, such as an outside wall, behind it

ecobee adds return vents, windows and doors to the list, along with spots in direct sunlight.

Height matters too. In its temperature sensor setup guide, Google recommends placing a sensor about chest high because air is generally warmer near the ceiling and cooler near the floor. It also notes that rooms differ with their windows, the way they face and their ceiling height, and that even in one room a high shelf and a low table read differently.

If you cannot move it, ecobee suggests adding its Smart Sensors and using them as the only temperature reading. Moving a wired thermostat is a wiring job, and Honeywell's guide says installation should be done by a trained, experienced service technician.

4. Air leaking from the hole behind it

ecobee says the most common installation cause of inaccurate temperature and humidity readings is a hole around the wiring behind the thermostat. Air from inside the drywall reaches the sensors, and that air is usually a different temperature and humidity than the room.

ecobee's fix, step by step:

1. Remove the thermostat, then the terminal plate and trim plate if you have one.
2. Push any extra wire back into the wall.
3. Press plumber's putty directly onto the wall so every hole is sealed and the putty sits flat.
4. Take a photo for reference, then put the thermostat back.

ecobee warns that putting the putty on top of the terminal plate, or leaving extra wire outside the wall, will not fix the readings and can make them worse. Turn off power to the heating and cooling system first; Honeywell's installation guide warns to disconnect power before working on wiring.

5. A steady calibration difference

If the thermostat is off by the same amount all day, you can usually correct it in settings.

• Honeywell: indoor display offsets can make the display read up to 3 degrees higher or lower than the sensor (X7 manual).
• ecobee: Temperature Correction adjusts the sensor reading by up to plus or minus 10°F (5.6°C). On most models it sits under Main Menu > General > Settings > Installation Settings > Thresholds. On the Smart Thermostat Lite and Essential, go to Main Menu > Settings > Installation Settings > Thresholds.

ecobee says to use correction only when the thermostat is a consistent amount higher or lower than other readings. If the amount it is off keeps changing, the cause is more likely one of the location or wall-hole problems above, and an offset will only hide it.

What you can check, and when to call

Safe for a homeowner:

• Compare the thermostat with a thermometer placed beside it.
• Check for humidity or feels-like settings and which wireless sensors are active.
• Keep heat-producing appliances away from it and direct sun off it.
• Set an offset or correction for a steady difference.
• With power off, seal the wall hole behind it as described above.

Call a technician when:

• The thermostat needs to move to a better wall.
• Readings stay wrong after the checks above.
• The system starts and stops in ways that do not match the setting. If it also clicks over and over, read why a thermostat clicks.