The short answer
Air 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.
On this page
On a muggy day, part of what makes a room uncomfortable is the moisture in the air, not just the temperature. An air conditioner deals with both at once. The water it collects has to leave the house, which is why every indoor unit has a drain.
Why cold surfaces collect water
Air always carries some water as invisible vapour, and warm air can hold more of it than cool air. Relative humidity describes how much the air is holding compared with the most it could hold at that temperature.
When air cools, its relative humidity rises. Cool it far enough and it reaches its dew point, where it can hold no more, so any surface colder than that collects water from the air touching it. It is the same effect you see on a glass of iced water or a steamed-up bathroom mirror.
Air at 25°C and 60% relative humidity, for example, has a dew point of about 17°C. When cooling, the indoor coil usually runs well below that, so water forms on its fins.
What happens inside the indoor unit
- The fan draws room air through the filter and across the cold coil.
- The air cools, and some of its moisture condenses on the fins.
- The water runs into a drip tray and out through the drain hose, by gravity or, where that isn't possible, with a small condensate pump.
- The air leaves the unit cooler and carrying less water, then mixes with the rest of the room, bringing the humidity down over time.
Turning vapour into water releases heat onto the coil, and the system has to carry that heat away as well. Engineers divide cooling into sensible cooling, which lowers the temperature, and latent cooling, which removes moisture. In humid weather a good share of the system's effort goes on the latent part, so a room can take longer to cool on a sticky day than on a dry day at the same temperature.
The amount of water can be surprising: on a humid day a single indoor unit can collect litres of it. Water dripping from the indoor unit itself, rather than from the end of the drain outside, often points to a blocked or poorly sloped drain, as explained in water dripping from the indoor unit.
How each mode affects humidity
| Mode | Removes moisture | Effect on temperature |
|---|---|---|
| Cool | Yes | Lowers it to your setting |
| Dry | Yes | Lowers it more gently |
| Fan | No | None, though moving air feels cooler |
| Heat | No | Raises it, so relative humidity falls |
Heating doesn't take any water out of the air. Relative humidity falls simply because warmer air could hold more. What each mode is for is set out in the modes explained.
How dry mode works
Dry mode runs the compressor gently and the indoor fan slowly. With the air moving more slowly across a cold coil, more moisture is removed for each degree of cooling. It still cools, though, so in a room that is already cool it can leave things chilly. On many models you can't choose the fan speed in dry mode.
A few models can warm the air slightly after drying it, so the humidity falls without much drop in temperature. Most can't. That is why dry mode is no substitute for a dehumidifier in cold weather: it cools the room, and in heating mode the system removes no moisture at all.
When a room still feels clammy
- The unit is much too big for the room. It reaches the set temperature quickly and eases right off, removing less moisture along the way, so the room can feel cool but clammy.
- Only a little cooling is needed. At low output the coil runs less cold and removes less moisture. Dry mode can help on warm, humid days that aren't especially hot.
- Moisture keeps arriving. Cooking, showers and drying laundry indoors all add water to the air, and open windows let humid air straight back in.
Drier air feels cooler because sweat evaporates from your skin more easily, so the same temperature can feel more comfortable when the humidity is lower. Many people find somewhere around 40 to 60% relative humidity comfortable. The balance between too damp and too dry is covered in humidity and comfort, and the question of whether air conditioning dries the air too much in does air conditioning dry the air?.
Related guides
- Humidity and comfortHumidity changes how warm a room feels at the same temperature. Most people are comfortable with relative humidity of roughly 40–60%; above that, warm rooms feel close and sticky, and air conditioning helps by taking moisture out as it cools.
- The modes explained: cool, heat, dry, fan and autoCool 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.
- Where the water goes: condensate drainageWater that condenses in the indoor unit as it cools runs through a drain hose to a suitable outlet, with a small pump where it can't fall naturally. In heating mode the outdoor unit produces water too.
- Does air conditioning dry the air?Yes, in cooling and dry modes it takes moisture out, which is part of why a cooled room feels comfortable, but it rarely leaves a UK home uncomfortably dry. Heating mode removes no moisture, although warming air lowers its relative humidity.