What does a thermostat with a remote sensor do?

What a remote sensor is

A remote sensor is a separate thermometer that reports to your thermostat. The wireless room sensors from Google, Copeland and Honeywell Home run on batteries. Google says that because its sensor runs on a battery, it needs no special wiring or outlet.

ENERGY STAR, the federal energy-efficiency program, describes two kinds. Temperature sensors monitor rooms that run warmer or cooler because of things like poor insulation or sun exposure. Occupancy sensors detect which rooms people use most and give those rooms priority, without sacrificing comfort in the rest of the house.

Google says most homes aren't the same temperature in every room, because of windows, room size and sunlight. Its sensor measures one room's temperature and sends it to the thermostat. Copeland says its Sensi room sensors monitor both temperature and humidity in different rooms.

Google says its Nest thermostats are currently only compatible with Nest sensors, and that its sensor cannot be used without a Nest thermostat.

How the thermostat uses the readings

One approach is to let a single sensor take charge. For the Nest Thermostat E and Nest Learning Thermostat (3rd gen), Google says that whenever its sensor is active, the thermostat uses that reading to control when the system turns on and off. During those times it ignores its own built-in sensor.

For those models, Google's settings page says one thermostat can connect up to 6 sensors but can only use one sensor at a time to control the system. For the Nest Learning Thermostat (4th gen), Google says you can attach up to 6 sensors per thermostat and select more than one sensor, and your thermostat heats or cools your home based on the average indoor temperature of the sensors you select. You can set a schedule, such as using different sensors at different times of the day. You can also switch the active sensor by hand in the app until the next scheduled change.

Another approach is averaging. Copeland says that by default its thermostat heats and cools to an average reading across all the selected sensors and the thermostat. To favor one room, you select only the sensors you want to prioritize. Honeywell Home's installer documentation also shows wired sensors set up as a temperature averaging network.

Occupancy features are another approach. The ecobee product page says its sensor detects which rooms are in use and adjusts the temperature for comfort in those spaces. Our reading: before you buy, check which of these methods your thermostat uses.

Where to put a sensor

Google suggests a room where you spend a lot of time, such as a bedroom or living room, mounted about chest high. It explains that air is generally warmer near the ceiling and cooler near the floor. A sensor set too high or too low may make the system overheat or overcool the room.

Copeland says 5 feet off the ground, and Honeywell Home says about 5 feet above the floor. Honeywell Home adds that the sensor should go on an inside wall, not behind furniture or doors.

Google and Copeland both say to avoid these places, and Honeywell Home names direct sunlight and spots above air vents: - direct sunlight; - near or above air vents, or near heat sources such as stoves and hot water pipes; - on or near a window or an uninsulated wall; - closets, or behind books or wall art, where air does not circulate.

Google also warns against rooms much hotter or colder than the rest of the house, such as an attic. It says the difference can make the thermostat run the rest of the home at an uncomfortable temperature. Google says that, depending on where you place the sensor, your system might have to work longer and use more energy. Google says not to paint or cover its sensor.

Wireless range

Wireless sensors have to stay within range of the thermostat. The published distances differ by maker. Google gives about 50 feet for its sensor, Copeland says within 45 feet, and the ecobee product page says up to 60 feet.

Google says walls, floors, building materials, large appliances, furniture and other wireless signals can all shorten the range. Copeland names thick walls, cordless phones and baby monitors. Google's app can test the connection after you move a sensor, to check that it is still in range.

Sensors and zoned systems

A remote sensor changes where the temperature is read. Google says that with a sensor your system works the same way, except that it can take the reading from the sensor instead of the thermostat. ENERGY STAR describes temperature sensors for single-zone homes, meaning one HVAC (heating, ventilation and air conditioning) system controls the whole house. It says sensors let the thermostat adjust the overall temperature to manage hot and cold spots.

Related guides: zoning systems and uneven cooling in two-story homes.

Google says that if you have a zoned system, each sensor should go in the same zone as the thermostat it is connected to. Otherwise the zones will probably run too hot or cold and may use more energy than you expect.

Outdoor, duct and wired sensors

Not every remote sensor goes in a room. Honeywell Home's documentation describes a wireless outdoor sensor that feeds some of its thermostats. Honeywell lists heat pump lockouts among the outdoor sensor's uses.

The same documentation describes sensors mounted on the return duct and on the supply duct, each in a location where the air is mixed well. Honeywell Home's documentation for these sensors is its T10 and T10+ Pro product data.

Honeywell Home's instructions for its wired indoor sensor carry an electrical hazard caution: wiring can cause electrical shock or equipment damage. The sensor also has to be selected in the thermostat's installer setup menu. That is work for a licensed contractor.