How to Ventilate a Room with Air Conditioner?

Some people wish to bring more fresh air into their house to stay refreshed. However, they also need to use the air conditioner. So, how do you ventilate an air-conditioned room?

To ventilate an air-conditioned room, you need to install a fresh air duct that connects your air conditioner to the external wall of your house so that the air conditioner can draw in outside fresh air and supply it to your room.

The fresh air duct that connects to your air conditioner must be installed properly to prevent condensation and foreign objects and organisms from entering your air conditioner which can damage it.

Types of Air Conditioners that Ventilate

Not all air conditioners can work together with a fresh air duct to bring you outside air. Only certain types of air conditioners are designed to have a fresh air duct.

Window Air Conditioner

Higher capacity window air conditioners, typically starting from 10,000 btu/hr, tend to have a fresh air vent where you can manually open it to introduce outside fresh air to your room. Low capacity window air conditioners usually don’t have such a function. You may check read the specification of the air conditioner to confirm. One example is Frigidaire FFRE103WAE.

Ceiling Cassette Unit

A ceiling cassette unit has a fresh air intake opening that is designed to connect to a fresh air duct to draw in outside air. Many ceiling cassette units start at 9,000 btu/hr of cooling capacity which makes them one of the options for residential application. Often, a ceiling cassette unit can be found in the living room of a large house.

Fan Coil Unit

A fan coil unit is also known as a ceiling mounted or ceiling concealed or simply, a ducted unit. Most of the time, a fan coil unit comes with a return air plenum box. A return air duct and a fresh air duct can be connected to the return air plenum box. Thus, the fan coil unit mixes both the air and supply to a room.

Air Handling Unit

Air handling units are mostly found in commercial buildings. They are large air conditioners that mount on the floor of a mechanical room. Needless to say, they are extensively designed to work perfectly with a fresh air duct. Air handling units are customizable to suitable various needs.

Fresh Air Unit

A fresh air unit is an air conditioner that draws only the outside fresh air and supplies it into a building. Normally, an air conditioner circulates air within a room. A fresh air unit is designed to take in outside air, filter it, cool it and supply it either to another air conditioner or directly into a building.

How to Install a Fresh Air Duct to Ventilate an AC Room?

The easiest and cheapest way of introducing fresh air to your house is to use a window air conditioner with a fresh air vent. Using a simple lever, you can toggle on/off the fresh air vent of a window air conditioner. However, the outside air is mostly not filtered in a window air conditioner.

Secondly, if you have a ceiling cassette unit or a fan coil unit, you can connect a fresh air duct to it. The major components in a fresh air system are as follow:

  • Weatherproof external grille
  • Galvanized iron duct
  • Polyethylene insulation (PE)
  • Air volume control damper

Then, you’ll need to find a breakable panel usually located around the body of the ceiling cassette unit. Poke it open and connect the fresh air duct onto it. As for a fan coil unit, you probably need to cut the return air plenum box open in order to connect a fresh air duct.

Following is an example of the fresh air duct connection for a 9,000 btu/hr ceiling cassette air conditioning unit:

A fresh air duct connected to a ceiling cassette unit

A 9,000 btu/hr ceiling cassette unit usually has around 300 cfm of airflow. Since we only take the fresh air at 10% of the supply air, the duct is calculated based on 30 cfm of airflow.

You can use an online ductulator to find out the duct size if you have a different amount of airflow. The external louvre size follows the size of the duct as well. Small size GI ducts usually are about 0.5mm thick only.

To determine how much PE insulation you need, simply sum up the duct surface area. Don’t forget that the duct has four surfaces. If the fresh air duct will not be exposing to cold air, located above the ceiling, for example, you don’t need to insulate it. For a fresh air duct, insulation with a thickness of 3mm is sufficient because the temperature differences are not much.

Also, you can use an insulated round duct instead of the rectangular GI duct as shown above. A round flexible duct will be easier and cheaper but be careful when handling fiberglass insulated flexible duct as it is harmful when inhaled. Alternatively, you can get all of the accessories from Amazon and DIY a fresh air system at home.

[Tutorial] Building a Ceiling Cassette Fresh Air System

Following is an example of building a ceiling cassette fresh air system. All materials used in the example are available on Amazon. The materials are not fixed and so, feel free to change them but make sure to check the compatibility.

Materials to prepare:

Other Accessories:

Assembly Drawing:

Assembly of a ceiling cassette fresh air system

Installation procedures:

  1. After the ceiling cassette unit is installed, open up the fresh air intake panel.
  2. Mount the round duct collar onto the body of the air conditioner.
  3. Secure the flexible round duct to the duct collar with a duct clamp.
  4. Cut the flexible duct and install the air volume control damper.
  5. Connect the flexible duct to both ends of the damper and secure it with duct clamps.
  6. Mount the external louvre on the exterior wall.
  7. Connect the flexible duct to the louvre and secure it with a duct clamp.

During the installation, support the flexible round duct using the duct support webbing. After you completed the installation, touch up your exterior wall using the drywall patch repair kits.


In this article, we discussed 5 common types of air conditioners suitable for a fresh air system with the window air conditioner being the easiest and cheapest solution. Then, we had a list of components needed for a fresh air system.

Also, we included an illustration of an example of a fresh air duct connected to a ceiling cassette air conditioning unit. Finally, we have a tutorial on building a ceiling cassette fresh air system with a list of materials from Amazon, an assembly drawing and a list of procedures.

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

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Yu Chang Zhen

I enjoy working on air conditioners and ventilation fans because Malaysia is a very hot country.

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