Thermostat switches between heating and cooling on its own
Almost always because it is in Auto mode, called Heat • Cool on Google Nest, which is designed to pick heating or cooling. It flips more often when the heat and cool setpoints are close together. If you want only one, set the mode to Heat or Cool. A Nest in Heat can still cool to hold its Safety Temperatures.
What does it mean when the thermostat switches by itself?
In nearly every case, the thermostat is in Auto mode, and Auto is doing its job. In Auto the thermostat holds a heating setpoint and a higher cooling setpoint and runs whichever one the room needs. Google Nest calls this mode "Heat • Cool" and says the thermostat "automatically switches your system between heating and cooling as needed." Honeywell Home and Sensi call it Auto, and ecobee shows it as Auto in the System Mode.
Look at the mode on the screen first. If it shows Auto, Heat • Cool, or on a Sensi Touch 2 a snowflake and flame together, the switching is expected. How the mode works in detail is on the Auto mode page.
Why does it switch more often than I expected?
Auto switches whenever the room crosses one of its two setpoints. A few things make that happen more often.
The setpoints are close together. Sensi's example uses 67°F for heat and 70°F for cooling: below 67°F the heat comes on, above 70°F the cooling does. Narrow that range and a small drift is enough to start the other system. Every brand keeps some minimum gap, and on a Sensi it is two degrees, but a gap at the minimum leaves little room. Honeywell says its T5, T5+ and T6 Pro Wi-Fi enforce a 1.5 degree difference in Auto "to prevent short-cycling."
The changeover differential is at zero. On the Honeywell T6 Pro, installer option 303 sets how many degrees the room must move past a setpoint before the thermostat switches to the opposite mode. It ranges from 0 to 5°F and ships at 0°F. On the T10 and T10+ Pro, when that setting is 0, Honeywell enforces 1.5°F "to ensure the heat doesn't come on after cooling shuts off or vice-versa."
Cooling went past the setpoint. Some thermostats run the air conditioner below the cool setpoint to remove humidity. If the room then drops to the heat setpoint, Auto can start heating. Honeywell's T10 guide addresses this directly: when the air conditioner is used for dehumidification, it requires an extra temperature drop below the overcooling amount before switching back to heat. The overcool to dehumidify page explains that feature.
The schedule changed. On a Nest, Heat, Cool and Heat • Cool each have their own schedule, and Google warns that switching modes can change when the system turns on and off. On thermostats that store a heat and a cool setpoint for each schedule period, a new period can move the two setpoints closer.
Adjusting one setpoint moved the other. Sensi says that raising the heat setpoint can push the cool setpoint up, and lowering the cool setpoint can push the heat setpoint down, to keep the two apart.
Can it switch when it is not in Auto?
A Google Nest can. Google says that in Heat mode the system "won't start cooling unless your Safety Temperatures are reached," and the reverse in Cool mode. Even in Off, a Nest heats or cools to hold its Safety Temperatures. If your Nest briefly ran the opposite system on a very hot or cold day, check the Safety Temperatures in its settings.
On a heat pump, what feels like switching may not be. A heat pump can blow cool air for a while during heating, and that is covered under heat pump blowing cool air while heating. The thermostat is still in Heat when that happens.
What can you change on the thermostat?
Does the screen or app show Auto or Heat • Cool?
- Yes
Do you need both heating and cooling on the same day right now?
- Yes
- No
Switch the mode to Heat or Cool. The thermostat will then only run the one you chose.
- No
Is it a Google Nest, and was it very hot or cold indoors?
- Yes
- No
The thermostat is not choosing to switch. If the wrong system runs in Heat or Cool, check the signs at the bottom of this page.
If you want Auto but less switching, these are the settings involved:
| Thermostat | Setting | Who changes it |
|---|---|---|
| Honeywell Home T6 Pro | Option 303, Auto Changeover Differential; option 300 turns Auto on or off | Installer setup |
| Honeywell Home T10, T10+ Pro | ISU 3015, auto changeover differential; ISU 3000 enables Auto | Installer setup |
| ecobee | "Heat/Cool Min Delta" and "Auto Heat/Cool" in Thresholds | On the thermostat only, not in the app |
| Emerson Sensi | The heat and cool setpoints themselves; Auto on or off | You, at the thermostat or in the Sensi app |
| Google Nest | The Heat • Cool range and its schedule | You, on the thermostat or in the Google Home app |
Installer settings sit next to options that describe your equipment, so ask whoever set up the thermostat before changing them. ecobee also recommends contacting its support before changing thresholds if you are not sure what your equipment can handle.
If you would rather stop both systems for now, Off mode explains what keeps running. And if the switching comes with a system that starts and stops over and over, or a mode that runs but never delivers heat or cooling, the signs below mean it is time for a licensed HVAC technician.
Frequently asked questions
- Is it bad for the system to switch between heat and cool?
- Switching is what Auto mode is for, so an occasional change is normal. The manufacturers build in a minimum gap and, on some Honeywell models, a changeover differential so the two do not chase each other. If it flips every few minutes, widen the gap or use Heat or Cool alone.
- My thermostat is set to Heat, so why did the air conditioner run?
- On a Google Nest, Heat mode will still start cooling if the home reaches your Safety Temperatures. Google says the same of Cool mode and heating. If you have a heat pump, also check whether what you noticed was cool air during heating rather than the thermostat changing modes.
- How do I stop my Sensi from switching modes?
- Choose Heat or Cool instead of Auto. On the Sensi smart thermostat and Sensi Lite, changing the mode at the thermostat turns Auto Mode off, and it stays off until you choose Auto again in the Sensi app.
- Can I make Auto mode wait longer before it switches?
- On some models an installer can. The Honeywell T6 Pro auto changeover differential (option 303) ranges from 0 to 5°F and starts at 0°F, and the T10 and T10+ Pro equivalent (ISU 3015) ranges from 0 to 3°F. On ecobee, the Heat/Cool Min Delta threshold sets the gap between setpoints instead.