Why Do Air Conditioners Need Outside Air?

Many air conditioners need outside air. They need to be vented outside in order to work properly. However, this requirement can be inconvenient as it restricts the placement of air conditioners. So, why do air conditioners need outside air?

Air conditioners need outside air to dissipate heat. Air conditioners cool a room by absorbing heat within the room and reject it to the outside air. Hence, air conditioners need outside air to carry away unwanted heat. Meanwhile, some air conditioners use outside air for ventilation purposes.

Every air conditioner needs outside air for different purposes and thus, the way they use outside air is also different. Understanding the mechanism behind it can help clear up your mind.

Air Conditioner Types and Outside Air Usage

As mentioned earlier, air conditioners use outside air to dissipate heat or reject heat. By doing so repeatedly, the temperature of a room can be gradually decreased.

1. Portable Air Conditioners

Portable air conditioners must be vented in order to cool or work properly. However, the hose used to vent portable air conditioners is actually discharging hot air outside.

Technically, portable air conditioners don’t need outside air. However, they need to discharge hot air outside. Otherwise, the hot air will be recirculated back into the room and no cooling will be done.

However, dual-hose portable air conditioners do introduce outside air as part of their normal operation. They have an extra hose to bring in outside air, unlike single-hose portable air conditioners that only send air outside.

Dual-Hose Portable Air Conditioner

Since single-hose portable air conditioners only discharge air, outside air naturally fills the room with the same volume. The outside air comes into the house through door and window gaps.

As a result, the cooling capacity of the portable air conditioner is actually lower because part of it needs to handle the hot and humid outside air that is infiltrating the house.

So, with an extra hose, dual-hose portable air conditioners show the actual cooling capacity. They already accounted for the outside air. In addition, dual-hose portable air conditioners also provide ventilation.

2. Window Air Conditioners

Window air conditioners must be placed such that the front section is inside the room and the back section is outside the room. They need outside air to dissipate heat.

Some people misunderstood window air conditioners and think that window air conditioners take in outside air, cool it and supply it to the room for cooling. But, window air conditioners don’t work that way.

Instead, the front section of window air conditioners absorbs heat within the room and the heat is transferred to the back section where the heat is then dissipated to the outside air. In other words, outside air does not enter through the window air conditioning unit.

The way window air conditioners are designed to allow them to utilize outside air for heat rejection. As part of the normal operation of air conditioners, heat within the room is absorbed and it must be dissipated outside in order for cooling to take place.

If the back section of a window air conditioner is not placed outside, the absorbed heat will be recirculated back into the room and no cooling will be done.

Certain window air conditioners, especially large cooling capacity ones have an extra vent to allow a small amount of outside air to enter the room to provide ventilation.

3. Central Air Conditioners

A central air conditioner has an air handler and a condensing unit that is placed inside and outside the house respectively. The air handler does not need outside air in order to cool the house.

However, many air handlers do bring in outside air for ventilation purposes. They have a fresh air duct that is normally connected to the return duct and users can control the amount of outside air coming into the house by adjusting the damper.

Meanwhile, the condensing unit is one that needs outside air for heat dissipation. The air handler absorbs heat while the condensing unit rejects heat through outside air.

Air Conditioners vs Fridges on Outside Air

Refrigerators or fridges are similar to air conditioners. But, fridges don’t need outside air. They are not connected to the outside by any means. So, what’s the difference between fridges and air conditioners?

Air conditioners are meant to cool the room or house while fridges are only meant to cool or freeze foods within the fridge.

So, it is fine for fridges to dissipate heat into the house instead of outside because either ways, they are still able to cool and freeze the foods within. Whereas air conditioners cannot dissipate heat back into the house because if they do so, no cooling will be done.

Therefore, air conditioners must dissipate heat outside for cooling. To achieve that, they need outside air to help with the heat rejection process.

Air Conditioners That Don’t Need Outside Air

By now, I believe you sort of understood the reason why air conditioners need outside air. So, the main reason is to reject heat and the second is to provide ventilation.

But, are there air conditioners that don’t need outside air at all?

Actually, there are types of air conditioners that don’t rely on outside air. Anyhow, air conditioners need to dissipate heat in order to cool but they don’t necessarily need outside air to do the job.

One of the types of air conditioners that don’t need outside air is water-cooled air conditioners. So far, what I’ve covered are air-cooled air conditioners. As the name suggests, air-cooled air conditioners are cooled by air, the outside air.

Water-cooled air conditioners use water to dissipate heat. The heat absorbed within the room is rejected to water instead of outside air. Hence, water-cooled air conditioners don’t need outside air.

Unfortunately, using water to cool air conditioners is not cheap. In comparison, air-cooled air conditioners are much simpler and thus, water-cooled air conditioners are rarely available for residential applications. They are mostly for large commercial applications.

Evaporative coolers are another type of “air conditioner” that doesn’t need outside air. Instead, water is constantly needed as evaporative coolers consume water as part of their normal operation.

On a side note, evaporative coolers shouldn’t be categorized as air conditioners because they don’t work the same way.

Sometimes, evaporative coolers are known as air coolers or “portable air conditioners”. Anyhow, evaporative coolers cool by the principle of water evaporation, not the transfer of heat from indoor to outdoor.

Nonetheless, outside air is not needed for evaporative coolers.


Air conditioners need outside air because they use outside air to dissipate the heat that they absorb within the room or house. When air conditioners are able to dissipate heat outside, they are able to cool the room or house.

Also, some air conditioners need outside air to provide ventilation for the room or house. They bring in a small amount of outside air into the room or house as part of their normal operation.

Meanwhile, other types of air conditioners such as water-cooled air conditioners and evaporative coolers don’t need outside air because they don’t use outside air to dissipate heat. Instead, some of them use water for heat rejection.

This article was originally published on aircondlounge.com. Actions will be taken for unauthorised republication of this article.

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Yu worked as a professional in the HVAC industry for more than 7 years. He is an engineer who has a passion for blogging. Ever since he created this website, Yu has helped hundreds of homeowners and engineers on HVAC related matters.

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