The short answer
Cool lowers the temperature and takes out some moisture, heat warms the room, dry mainly removes moisture with gentle cooling, fan only moves air, and auto chooses between cooling and heating for you.
Most air conditioners offer the same five modes, even though the buttons and names vary between models. Choosing the right one for the weather keeps a room comfortable without the system doing work you don't need. Symbols differ too, but cool is usually a snowflake, heat a sun and dry a water droplet; your manual shows the exact buttons.
| Mode | What it does | Outdoor unit | Best for |
|---|---|---|---|
| Cool | Lowers the temperature, removes some moisture | Running | Warm days |
| Heat | Warms the room | Running | Cold days |
| Dry | Removes moisture, cools gently | Running gently | Muggy, mild days |
| Fan | Moves air only | Off | A breeze without cooling |
| Auto | Chooses cool or heat | As needed | Changeable weather |
In cool, heat and auto modes, the temperature you set is a target. Most systems slow down as the room gets close to it, rather than switching fully on and off.
Cool mode
In cool mode the indoor unit draws in warm air from the room, passes it over a cold coil and blows it back out cooler. The refrigerant carries that heat to the outdoor unit, which releases it outside. That's why the outdoor unit blows out warm air in summer.
Cooling dries the air too. Moisture condenses on the cold coil and runs away through a drain pipe, so the room feels less clammy, not just cooler. If your drain pipe ends outside, a steady drip on a humid day is normal.
Heat mode
Heat mode runs the same refrigerant circuit in reverse. The outdoor unit collects heat from the outside air, even on a cold day, and the indoor unit releases it into the room.
Two pauses are normal. When you first switch on, the indoor fan may wait a few minutes until the coil has warmed up, so it doesn't blow cold air at you.
In cold, damp weather the outdoor unit also stops heating now and then to melt frost off its coil. The indoor fan stops and a light may flash for a few minutes. That's defrost, not a fault.
Dry mode
Dry mode suits days that are muggy rather than hot. The unit runs its fan slowly and cools gently, so more of its effort goes into taking moisture out and less into lowering the temperature. On many models you can't change the fan speed in dry mode, and on some you can't set the temperature either.
Most dry modes still cool the room a little, because they work in the same way as cool mode. That makes them a poor choice for a cold, damp room in winter, since they take heat out along with the moisture. A few models have a dehumidifying setting designed to hold the temperature steady, and the manual will say if yours does. There's more on this in how air conditioning removes humidity.
Fan mode
Fan mode runs only the indoor fan. The compressor and the outdoor unit stay off, so the air isn't cooled, heated or dried, just moved around. Moving air still feels cooler on your skin, and with only a small fan motor running, the unit draws far less power than it does when cooling.
It's useful on a warm evening when a room feels a little stuffy but not hot. It doesn't bring in fresh air, though. A wall-mounted unit recirculates the air already in the room, so for fresh air, open a window or keep your trickle vents open.
Auto mode
In auto mode the unit decides for itself whether to cool or heat, based on how the room temperature compares with your setting. It can make sense in spring and autumn, when a room can be chilly first thing and warm by the afternoon. If it switches between heating and cooling more often than you'd like, choose heat or cool yourself.
Auto mode isn't the same as auto fan speed. Auto fan speed only decides how fast the fan runs, within whichever mode you've chosen.
Other buttons and shared outdoor units
Many remotes add functions such as powerful (sometimes called turbo), eco, quiet and sleep. These change how a mode runs rather than being modes of their own.
Powerful usually runs at full output for a limited time. Eco typically limits how much power the system draws, so the room may take longer to reach your setting. Quiet and sleep are covered in sleep and quiet modes.
If several indoor units share one outdoor unit, as on a multi-split system, they usually have to be heating or cooling at the same time. If one is set to heat while another is set to cool, one of them waits, and on many models a light or message shows the clash. Fan mode can usually run alongside either.
Related guides
- How air conditioning heats a roomTo heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.
- How air conditioning removes humidityAir conditioning removes humidity as it cools. Room air passes over a coil colder than its dew point, so water vapour condenses on the fins, drips into a tray and drains away through a hose.
- What temperature to setAround 23–25°C for cooling and 20–22°C for heating are sensible starting points, adjusted a degree at a time to suit you. A setting far lower or higher than you want won't get the room there much faster.
- Sleep and quiet modesSleep mode usually shifts your setting by a degree or two over the first hours of the night, warmer when cooling and cooler when heating, and turns the fan down. Quiet mode slows the fan to cut noise, at the cost of some output.