Does my Nest thermostat average its temperature sensors?

Only the Nest Learning Thermostat (4th gen). Google says that if you select more than one sensor on that model, the thermostat uses the average temperature between them. The Nest Learning Thermostat (3rd gen) and Nest Thermostat E use one sensor at a time to control the system.

Can I stop my ecobee from using its own sensor?

Yes. ecobee says you can include or exclude any sensor from a Comfort Setting, including the thermostat's built-in sensor. ecobee suggests relying on SmartSensors alone when the thermostat sits in a poor spot that you cannot change.

What happens if a room sensor stops working?

On Nest, Google says the thermostat uses its built-in sensors instead when a Nest Temperature Sensor has trouble connecting. On a Honeywell Home T9 set to active rooms, the user guide says the thermostat uses its own sensor when no motion is detected anywhere.

Do room sensors make my system run more?

They can change when it runs, because the thermostat heats or cools until the measured rooms reach the setpoint. If those rooms are harder to heat or cool than the hallway, the system may run longer. Google describes this as the thermostat heating or cooling until the sensor's room reaches your desired temperature.

Room sensors vs the thermostat sensor

It depends on the brand. ecobee averages every sensor you select for the current Comfort Setting. A Nest Learning Thermostat (3rd gen) or Nest Thermostat E uses one sensor at a time, while the 4th gen can average several. Honeywell Home T9 averages the rooms you select or the rooms with motion.

Which temperature does the thermostat control to?

Not always its own. Once you add room sensors, the thermostat controls to whatever reading it has been told to use, and that is also the number most models put on the screen. There are three common approaches:

An average of selected sensors. ecobee, the Nest Learning Thermostat (4th gen), the Honeywell Home T9 and the Sensi Touch 2 all average more than one reading.
One sensor at a time. The Nest Learning Thermostat (3rd gen) and Nest Thermostat E use a single active sensor and switch between sensors on a schedule.
Motion-weighted. ecobee's Follow Me and the T9's active rooms option give priority to rooms where people are.

The built-in thermostat sensor is just one more input in these systems. Some brands let you leave it out entirely.

How does each brand decide?

Which reading controls the system when room sensors are paired
Thermostat	Reading used	How you choose sensors	Can you exclude the thermostat sensor?
ecobee with SmartSensors	Average of participating sensors	Sensor Participation, set per Comfort Setting	Yes
Google Nest Learning Thermostat (4th gen)	Average of the selected sensors	Google Home app, sensor per schedule event	Not stated
Google Nest Learning Thermostat (3rd gen), Nest Thermostat E	One sensor at a time	Sensor schedule in four time blocks, or a manual switch in the app	Yes, by choosing a remote sensor for a block
Honeywell Home T9	Average of selected rooms, or of rooms with motion	Priority: selected rooms or active rooms	Not stated
Honeywell Home T6 Pro (TH6320U, TH6220U)	Thermostat only, wired sensor only, or average	Installer setting ISU 520 (default: average)	Yes, with Wired Only
Emerson Sensi Touch 2	Average of selected sensors and the thermostat	Not stated in the manual	Not stated

From each manufacturer's manuals and help pages, listed under Sources. Not stated means the pages we checked do not say.

The Nest Temperature Sensor works only with the Nest Thermostat E and the Nest Learning Thermostat (3rd gen and 4th gen), according to Google. Up to 6 sensors can be associated with one thermostat.

When does the thermostat sensor take over?

Every brand falls back to the built-in sensor in some situation, and that is often when people notice the number jump.

Google Nest. When no Temperature Sensor is selected, Google says the thermostat defaults to its internal sensor. If a selected sensor has trouble connecting, the thermostat uses its built-in sensors instead. On the 3rd gen and Thermostat E, a manual switch to another sensor lasts until the next scheduled sensor block, then the schedule takes over again.
Honeywell Home T9. With priority set to active rooms, the user guide says that if no motion is detected, the sensor in the thermostat is used and the remote sensors are excluded. A room counts as occupied for 10 minutes after someone walks through, longer if motion continues.
ecobee. There is no fallback in the same sense. The average contains exactly the sensors you selected for the running Comfort Setting, and during a Manual Hold ecobee uses the Home Comfort Setting's list. ecobee phases a sensor in or out over 30 minutes.

When is a room sensor the better reading?

When the thermostat sits somewhere unrepresentative. Google's list of bad spots for a Nest Temperature Sensor includes sunny windows, drafty doors and places near a heat source, and the same spots trouble a wall thermostat. ecobee goes further and suggests using SmartSensors as the only reading when the thermostat cannot be moved away from a return vent, window, door or direct sunlight.

A room sensor also lets the schedule follow where you actually are. Google's example is a living room sensor during the day and a bedroom sensor at night. ecobee does the same through separate participation lists for Home and Sleep.

When is the thermostat sensor the better reading?

When a remote sensor is the one in a poor spot, or when one extreme room would drag an average. A sensor on a sunny bookshelf pulls an ecobee average up all afternoon. A sensor in a room with a closed door may never reach the setpoint, and Google notes that the thermostat heats or cools until the sensor's room reaches your desired temperature, so other rooms can overshoot.

If the reading on the screen does not match the room you are in, start with which sensors are counted. The ecobee version of that problem is on ecobee temperature different from the room, and the general causes are on thermostat shows the wrong room temperature. ecobee sensor lists live inside ecobee Comfort Settings, and Nest sensor blocks follow the time periods described on thermostat schedule periods.

More brand-specific pages are on the ecobee hub and the Google Nest hub.

Frequently asked questions

Does my Nest thermostat average its temperature sensors?: Only the Nest Learning Thermostat (4th gen). Google says that if you select more than one sensor on that model, the thermostat uses the average temperature between them. The Nest Learning Thermostat (3rd gen) and Nest Thermostat E use one sensor at a time to control the system.
Can I stop my ecobee from using its own sensor?: Yes. ecobee says you can include or exclude any sensor from a Comfort Setting, including the thermostat's built-in sensor. ecobee suggests relying on SmartSensors alone when the thermostat sits in a poor spot that you cannot change.
What happens if a room sensor stops working?: On Nest, Google says the thermostat uses its built-in sensors instead when a Nest Temperature Sensor has trouble connecting. On a Honeywell Home T9 set to active rooms, the user guide says the thermostat uses its own sensor when no motion is detected anywhere.
Do room sensors make my system run more?: They can change when it runs, because the thermostat heats or cools until the measured rooms reach the setpoint. If those rooms are harder to heat or cool than the hallway, the system may run longer. Google describes this as the thermostat heating or cooling until the sensor's room reaches your desired temperature.