Cool Mode on AC: What It Does and How to Set It

Cool mode is your air conditioner's standard cooling setting. The compressor and fan run to pull heat out of the room until it reaches the temperature you set, then the unit cycles to hold it there, removing some moisture along the way. Use it whenever the room is warmer than you want, and switch to Dry on humid days that are not hot.

How cool mode works

In Cool mode the air conditioner runs its full refrigeration cycle. Daikin Malaysia's guide to cool, dry and fan modes says Cool mode uses both the compressor and the fan: the unit takes in warm room air, cools it, and blows it back out. Senville's mode guide describes Cool as removing heat, and some humidity, to cool the room.

What the thermostat does

The set temperature is the target. GE's portable air conditioner manual says the compressor cycles on and off to keep the room at the level you set. A lower number makes the room cooler, and a higher number makes it warmer.

How the compressor holds that target depends on the unit. ENERGY STAR's room air conditioner page explains that a conventional compressor works like an on/off switch, running at full capacity or not at all. A variable-speed compressor changes its speed instead, which ENERGY STAR says lets the unit cool quietly and save a significant amount of energy.

One thing that surprises people: there is often a short wait. GE says that when you turn the unit on in Cool, it takes about 3 minutes for the compressor to start, and that after switching off from Cool you should wait 3 minutes before restarting.

The humidity side effect

Cool mode dries the air while it runs, but only as a by-product. ENERGY STAR warns that an air conditioner that is too big for the room cools it before it has a chance to remove the humidity, which leaves the room cool but damp and clammy. Matching the unit to the room matters, and our room size calculator shows the capacity you need.

When to use cool mode, and when not to

Senville sums it up in two words: hot days. Daikin Malaysia says to use Cool when both the temperature and the humidity are high. If the room is too warm, Cool is the setting that brings the temperature down.

It is not always the best choice:

• Mild, muggy days: if the temperature is fine but the air is sticky, dry mode targets the moisture with light cooling.
• Comfortable temperature and humidity: fan mode moves air with the compressor off. Daikin says it changes neither the temperature nor the humidity.
• Cooling fast: many units add a boost. Mitsubishi Electric's feature glossary describes a Powerful Mode that temporarily drops or raises the set temperature to reach your target quickly, and MrCool and Gree call their version Turbo.

Cool mode vs auto mode

Cool mode only cools. Auto lets the unit decide. Senville says Auto switches between heating and cooling to hold your set temperature, and MrCool's remote manual says Auto picks Cool, Fan or Heat on its own and does not let you set the fan speed. If you want cooling and nothing else, choose Cool. Our guide to auto mode on an AC explains when Auto makes sense. On multi-zone mini splits, Senville notes that every indoor unit must run the same mode, all heating or all cooling.

How to set cool mode

On most units you press Mode until Cool shows, then set the temperature and fan speed. Senville uses a snowflake symbol for Cool, and the snowflake is common on other remotes too. Senville suggests starting 2 to 3°F (1 to 2°C) below the current room temperature.

GE's manual also gives three simple recipes. For normal cooling, pick Cool with any fan speed and a middle temperature. For maximum cooling, pick Cool with the high fan and a lower temperature. For quieter nighttime cooling, pick Cool with the low fan and a middle temperature. Our guide to AC modes and remote symbols covers the other buttons.

If cool mode is not cooling

Start with the simple checks. ENERGY STAR's maintenance checklist says a dirty filter can raise energy costs and damage the equipment, and dirty coils reduce the system's ability to cool and make it run longer. It recommends inspecting, cleaning or changing filters once a month. The same checklist says a plugged condensate drain can cause water damage and affect indoor humidity, so check it if the room cools but stays damp. ENERGY STAR also says to keep lamps and TVs away from a room unit's thermostat, because their heat makes it run longer.

If the unit stops responding or acts strangely, a reset sometimes clears it. Our guide covers four ways to reset an air conditioner. If it runs but cannot get the room cool, a licensed technician is the right next call.

Does cool mode save energy?

Cool mode is not an energy-saving mode in itself, but how you set it changes what it costs. ENERGY STAR's tip is to set the thermostat as high as is comfortable, typically 78°F. Daikin Malaysia makes the same point with numbers: cooling a room from 30°C to 25°C uses less energy than cooling it from 35°C to 25°C.

Many units also have an eco setting that works inside Cool mode. MrCool's remote manual says its ECO function is only available in Cool and sets the temperature to 75°F if you had it lower. To see what your own unit costs to run, try our cost to run calculator.