How auto mode (heat/cool auto changeover) works
In Auto, the thermostat holds two setpoints at once: a heat setpoint below and a cool setpoint above. It heats if the room falls below the lower one, cools if it rises above the upper one, and does nothing in between. Most thermostats keep the two setpoints a minimum distance apart, and some hide Auto until an installer enables it.
How does Auto mode decide between heating and cooling?
In Heat or Cool mode the thermostat works toward one setpoint and can only run one kind of equipment. In Auto it holds two setpoints, one for heating and a higher one for cooling, and chooses the equipment for you. Honeywell's VisionPRO manual describes it as the thermostat that "automatically selects heating or cooling depending on the indoor temperature."
Sensi gives a worked example. With the cooling setpoint at 70°F and the heating setpoint at 67°F:
- Above 70°F, cooling turns on and brings the room back to 70°F.
- Below 67°F, heating turns on and brings it back to 67°F.
- Between 67°F and 70°F, nothing runs.
That gap in the middle is the point of the mode. The house can drift a few degrees either way without any equipment starting. Google Nest calls the mode "Heat • Cool" and says the system "either heats or cools to try to keep your home within the temperature range you've manually set." Google suggests it for climates that need both on the same day, such as a desert where days call for cooling and nights for heating.
How far apart do the two setpoints have to be?
Every brand enforces some separation, so that heating and cooling never chase each other. How much, and whether you can change it, differs:
| Thermostat | Minimum separation(°F) | Can it be changed? |
|---|---|---|
| Emerson Sensi (all models on its Auto page) | 2 | Not stated; Sensi moves the other setpoint to keep the gap |
| ecobee | Not stated | Yes: "Heat/Cool Min Delta" in the Thresholds menu, on the thermostat only |
| Honeywell Home T5, T5+, T6 Pro Wi-Fi | 1.5 | Not stated |
| Honeywell Home T10, T10+ Pro | None for setpoints; 1.5 changeover differential when ISU 3015 is 0 | ISU 3015, 0 to 3, installer setting |
| Honeywell Home FocusPRO 6000 | 3 | No, fixed |
| Google Nest | Not stated | Not stated |
Sensi explains how it keeps its gap. If you raise the heating setpoint too close to the cooling one, the cooling setpoint moves up with it. Lower the cooling setpoint too far and the heating setpoint moves down. If one of your setpoints seems to change on its own in Auto, that is usually why.
ecobee's thresholds page describes "Heat/Cool Min Delta" as the minimum temperature difference between the Heat and Cool setpoints in Auto mode. ecobee says threshold changes can only be made on the thermostat, not in the app, and recommends contacting its support first if you are not familiar with your equipment.
What is the auto changeover differential?
Some Honeywell Home thermostats add a second rule. Besides keeping the setpoints apart, they require the room to move a set number of degrees past a setpoint before switching to the opposite mode. On the T6 Pro, installer option 303 defines it as the minimum number of degrees from setpoint "needed to switch from the last mode running (heat or cool) to the opposite mode."
The T10 and T10+ Pro guide gives an example with the differential at 3. With heat and cool both set to 70°F and heat used last, the room has to climb at least 3 degrees above the heat setpoint before cooling starts. Cooling then cycles to hold 70°F. For heat to return, the room has to fall at least 3 degrees below the cool setpoint. Honeywell adds that if the air conditioner is also used to dehumidify, the T10 requires an extra drop below the overcooling amount before it switches back to heat.
This is a different setting from the ordinary temperature differential, which decides how far the room drifts before the same mode starts again. For how the setpoint gap can make the house feel off target, see why the room never reaches the set temperature.
Why is Auto missing from the mode list?
Often because nobody turned it on. Several thermostats ship with Auto disabled or available only in some places:
- Honeywell Home T6 Pro. Installer option 300, System Changeover, is set to Manual from the factory. Auto appears only if it is changed to Automatic.
- Honeywell Home T10 and T10+ Pro. Auto is offered when the thermostat is set up for at least one heat stage and one cool stage and ISU 3000 is set to "Auto".
- ecobee. The "Auto Heat/Cool" threshold setting is what allows the System Mode to be set to Auto.
- Emerson Sensi. On the Sensi smart thermostat and Sensi Lite, Auto can only be turned on in the Sensi app. Changing the mode at the thermostat afterwards cancels it.
- Google Nest. Heat • Cool only appears if your system has both heating and cooling.
If the mode is an installer setting, ask whoever set up the thermostat before changing it, since it sits alongside settings that describe your equipment.
Does Auto mode follow its own schedule?
On a Google Nest, yes. Heat, Cool and Heat • Cool each keep their own temperature schedule, and Google warns that switching modes can make the system turn on and off at different times. If you edit the schedule, make sure you are editing the one for the mode you use.
On the Honeywell T6 Pro, each schedule period already stores a heat setpoint and a cool setpoint, so the same schedule serves Heat, Cool and Auto. See schedule periods for how those periods work.
When should you use Auto?
When your weather really does swing between needing heat and needing cooling, such as spring and fall days that start cold and end warm. Leave a comfortable gap between the setpoints. A gap close to the minimum lets the thermostat flip between modes more readily, which is the most common reason people see their system switch between heat and cool.
In steady summer or winter weather, Heat or Cool alone is simpler. And if you want neither, Off mode explains what still runs.
Frequently asked questions
- Is Auto mode the same as fan Auto?
- No. The system mode Auto chooses between heating and cooling. The fan setting Auto only means the blower runs while the system is heating or cooling. They are separate controls, and you can use either one without the other.
- Can I set the heat and cool setpoints to the same temperature?
- It depends on the model. Honeywell says the T10 and T10+ Pro let you set both to the same temperature in Auto, while enforcing a changeover differential behind the scenes. Sensi always keeps them at least two degrees apart and moves one if you push the other too close.
- Why can I not find Auto on my Sensi smart thermostat?
- On the Sensi smart thermostat and Sensi Lite, Auto Mode can only be turned on from the Sensi app. The Mode button on the thermostat itself cycles through Heat, Cool, Off and Aux only.
- Does Auto mode use more energy?
- None of the manufacturers cited here say so. Auto runs heating or cooling only when the room leaves the range you set. What it changes is that both systems are available, so a narrow gap between the setpoints lets the thermostat switch more readily.