The space may feel cool, yet the air still feels stuffy or odors linger inside. The reason is that cooling the air and renewing it are two different functions; running the AC does not necessarily bring in new air from outside.
To understand what happens, we explain how air circulates within the space, the role of the fresh air system, and why ventilation should be planned alongside air conditioning.
Does an AC draw in new air from outside?
Many common AC units recirculate indoor air. They draw it from the room, pass it through the filter and the cooling coil, then return it to the room at a lower temperature.
As for the outdoor unit in a split system, it rejects heat to the outside air and does not carry outside air into the room through the copper pipes.
Therefore, the presence of an outdoor unit on the roof is not enough to prove that the space receives fresh air.
What is the difference between return air and fresh air?
Return air is the air that comes back from the interior space to the AC unit to be reconditioned and recirculated.
Fresh air, on the other hand, is outdoor air brought into the building as part of a designed ventilation system, to help dilute indoor pollutants and renew the air. Ventilation is defined as replacing indoor air with air from outside for this purpose.
Depending on the design, the system may combine return air with outdoor air, or use separate equipment to treat fresh air.
Does an AC filter replace ventilation?
The filter plays a role in trapping particles according to its type and efficiency, but it does not mean the air is renewed.
Recirculating and filtering air is different from bringing in outdoor air and exhausting part of the indoor air. For this reason, relying on the coolness of the space or the cleanliness of the filter is not enough to judge whether ventilation is adequate.
Daikin states that most AC units do not perform the ventilation function on their own, which makes it necessary to verify that a suitable solution exists for renewing the air.
How is fresh air provided to a building?
Several solutions exist, and their selection depends on the nature of the project. They include bringing outdoor air in through an air handling system prepared for that purpose, using a standalone fresh air unit, or using an energy recovery ventilation system where suitable for the application.
Selecting an intake location alone is not enough; its filtration and its thermal and humidity treatment must be studied according to conditions, along with how the air is distributed and coordinated with exhaust air.
Outdoor air may add a load to the AC system, and this load must be accounted for in the design.
Does every AHU provide fresh air?
An air handling unit (AHU) may operate on return air, outdoor air, or a mixture of both, depending on its configuration and the project design.
Therefore, the unit’s name alone does not determine the amount of fresh air. What is required is a review of the system drawings, the outdoor air intake, and the treatment and control equipment.
The same principle applies to concealed units, cassette units and others: the presence of an actual ventilation connection designed for that purpose must be verified, rather than assumed from the unit’s appearance.
Is the fresh air percentage fixed at 10%?
There is no single percentage that suits all buildings.
The figure cited in the explanation of one project may represent a specific design condition. The required quantity, however, is determined by the use of the space, the number of occupants, its area, and the approved ventilation requirements.
Once the required outdoor airflow has been determined, its ratio to the total supply air can be calculated. Therefore, 10% should not be adopted automatically for any majlis, office or hall.
What should you review before approving the AC system?
Ask the designer to clarify:
- The source of the outdoor air and its quantity for each area.
- How it is filtered, treated and distributed.
- The location of the intake relative to sources of odors and pollution.
- The coordination between supply, return and exhaust air.
- How the outdoor air load is included within the AC loads.
- How airflows will be measured and verified during operation.
Frequently asked questions
Is cold air a sign of good ventilation?
No. Temperature alone does not indicate how much outdoor air is entering the space.
Does the presence of a duct mean the system renews the air?
Not necessarily; the duct may be used to distribute only recirculated indoor air.
Can fresh air be added to an existing system?
This can be studied, but the solution requires a review of loads, routes, equipment and controls, and is not limited to opening a connection to the outside.