Overcool to dehumidify: what it does
It lets the air conditioner keep running past your cooling setpoint when indoor humidity is above your target, because longer cooling pulls more moisture out of the air. The room ends up a few degrees cooler than you set. ecobee calls it AC Overcool Max, Sensi calls it overcooling and Google Nest calls it Cool to Dry.
What does overcooling to dehumidify do?
An air conditioner (AC) removes moisture as it cools, so a longer cooling cycle dries the air more. An overcool setting uses that. When indoor humidity is above the level you chose, the thermostat lets cooling run past your setpoint, up to a set limit, and stops when either the humidity target or the limit is reached.
ecobee describes its version as continuing to run your cooling beyond your setpoint, and is clear that it is not a substitute for a dehumidifier. The trade is simple: drier air, at the cost of a room that ends up cooler than the number you set, and a longer run.
What does each brand call it and how far does it go?
| Thermostat | Setting name | Most it cools below the setpoint(°F) | Where you set it |
|---|---|---|---|
| ecobee | AC Overcool Max, plus Dehumidify Using AC | Not stated | Thermostat only: Installation Settings > Thresholds |
| Emerson Sensi (Basic overcooling) | Dehumidification | 3 | Sensi app: System settings > Dehumidification |
| Emerson Sensi Touch 2 (Dynamic overcooling) | Dehumidification | 3 | Not stated |
| Google Nest (Cool to Dry) | Cool to Dry | 3 when you are home | Thermostat, Nest app or Google Home app |
| Google Nest Learning Thermostat (4th gen) | Overcooling limit | 1 to 5 (default 3) | Google Home app: Humidity Helper > Advanced configuration |
| Honeywell Home T10 and T10+ Pro | Dehumidification - Overcooling Limit (ISU 9070) | 0 to 3 | Installer setup |
Sensi says Basic overcooling stops at the target humidity or after a 3-hour time limit, while Dynamic overcooling, which is only on the Sensi Touch 2, has no time limit. The Touch 2 manual puts it in one line: if your thermostat is dehumidifying by overcooling, "the thermostat will cool 3° below your setpoint."
Honeywell's T10 documents do not all agree. The product data pages give the 0 to 3°F range for ISU 9070, and the user guide says the temperature may drop up to 3°F in one section but gives a "Dehum Over Cooling Limit" range of 1 to 5°F in another. Check the value your installer set rather than relying on either figure.
How do you set it up on an ecobee?
Two settings work together, and both are needed:
- AC Overcool Max. On the thermostat, go to Main Menu > General > Settings > Installation Settings > Thresholds > AC Overcool Max and pick how many degrees the AC may overcool. ecobee's thresholds page describes it as allowing the air conditioner to run past the Cool setpoint to reduce humidity.
- Dehumidify Using AC. Go to Main Menu > General > System, enable Dehumidify Using AC and set your Preferred Humidity Level.
On the Smart Thermostat Lite and Essential, both paths start at Main Menu without the General step. The Thresholds menu is only on the thermostat, not in the app.
ecobee says cooling stops at AC Overcool Max or at your Preferred Humidity Level, whichever comes first. Once humidity reaches the target, the thermostat stops calling for the blower fan for 5 minutes so moisture on the coil does not evaporate back into the house. That pause is deliberate.
The feature depends on the humidity sensor. ecobee warns that inaccurate humidity readings mean the feature may not work correctly, and if Humidity Monitoring is disabled, AC Overcool Max becomes unavailable. See thermostat humidity reading seems wrong if the number looks off.
How is Google Nest Cool to Dry different?
Cool to Dry can start the AC on its own. Google says it turns on the air conditioner if humidity rises above 75% while cooling is off, or above 65% if the thermostat has been on Eco Temperatures for 3 days straight. Sensi's overcooling never does this; it only extends a cycle that is already running.
Google sets limits too. Cool to Dry only cools to 3°F below your set temperature when you are home, and Google's help page says it will not cool the home below 75°F. The Nest Learning Thermostat (4th gen) instead has adjustable options: an overcooling limit of 1 to 5°F (default 3), a humidity threshold of 55% to 80% (default 70%) and a minimum indoor temperature of 68 to 85°F (default 80°F), per Google's configuration guide. Because Cool to Dry may stop before it reaches a target, you choose an approximate level shown as water drops rather than an exact percentage.
What should you expect when it is on?
Longer cooling runs and a room below the setpoint on humid days. On a Honeywell Home T10, the troubleshooting page says that if the display shows "Cooling to" with a setting above the room temperature, the thermostat has likely been set to over-cool when humidity is high.
If the house feels too cold, raise the humidity target or switch the feature off, rather than raising the setpoint over and over. If cooling runs constantly for other reasons, see AC runs constantly. For how the thermostat decides when an ordinary cooling cycle starts, see temperature differential. More ecobee settings are on the ecobee hub.
Frequently asked questions
- Why is my AC running below the temperature I set?
- If an overcool feature is on and indoor humidity is above its target, that is the feature working. The Sensi Touch 2 manual says that when the thermostat is dehumidifying by overcooling, it "will cool 3° below your setpoint." Raise the humidity target or turn the feature off if the room gets too cold.
- Will overcooling start the AC by itself?
- On Sensi, no: Sensi says neither of its dehumidification features will trigger a cooling cycle on its own; they only extend one. Google Nest Cool to Dry is different and can turn on the air conditioner when humidity rises above 75% and cooling is off.
- Where is AC Overcool Max on my ecobee?
- On the thermostat only, under Main Menu > General > Settings > Installation Settings > Thresholds > AC Overcool Max. You also have to enable Dehumidify Using AC under Main Menu > General > System and set a Preferred Humidity Level. Lite and Essential models skip the General step.
- Is overcooling the same as a dehumidifier?
- No. ecobee says AC Overcool Max is not a substitute for a dehumidifier. It removes moisture only as a side effect of running the cooling longer, so it cannot lower humidity without also lowering the temperature.