Heating Systems

Where does an air conditioner pull air from?

Heating Trends, last updated on December 7, 2022

Many people wonder whether their air conditioner brings in fresh outside air, or just recirculates the room air. It's worth to know in case it's the outdoor pollen season or the air is polluted.

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Where does an air conditioner pull air from?

Whether an air conditioner pulls air from outside, recirculates indoor air or mixes both depends on the type.

  • Split type air conditioner recirculates the indoor air through its evaporator coils.
  • Central air conditioner units or air handling units mix indoor air with the fresh outside air based on the predefined parameters of the type of building.
  • Some older window air conditioners have a flap in the air duct which, when opened, pulls the fresh outside air a mixes it with the air from the coils.

Split type air conditioners

The split model air conditioner consists of two parts, which is why it's called a split type. One in which the gas expands and one in which the gas is compressed again to a higher pressure. That work is performed by the compressor, and the process takes place in the indoor and outdoor units.

If we cool a certain space, the outdoor unit heats. When the indoor unit is in the process of heating a certain space, the outdoor unit cools. That is why the outdoor unit must be placed outside the air-conditioned space.

Home inverter air conditioners don't pull fresh air from outside. They are designed to use a fan to pull the indoor air into the AC unit, heat it or cool it (based on the setting) and disperse the air back into the space.

The air that is being recycled first passes through evaporator coils, than through fan (rotary turbine), and lastly through air filters before being blown out through the unit vent.

Because the same air is recirculated, it can get stale with time unless there's a window or a door opened to allow some fresh air to circulate. But even with all the windows and doors closed, since no room is 100% airtight, there will always be some small amount of fresh air pulled in through the tiny leakages. Keep in mind to ventilate your room to replace the stale air and remove carbon dioxide every once in a while for a few minutes to let the fresh air come in, but not to let your walls loose their heat (or to get warm if you're cooling the room).

Split heat pumps are so efficient because they heat the indoor air by only a couple of degrees instead of heating the cold outside air. But if we always keep a window open, the compressor inside the AC unit gets overworked and has to always run to achieve the set temperature which results in increased power consumption and shorter lifespan.

Mobile air conditioners work the same way as the fixed, split type systems, except it's expected for some fresh air to circulate depending on how the exhaust hose has been mounted to a window.

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Central air conditioner units and air handling units

Big HVAC systems are designed with a fresh air intake feature which allows them to use inlets to pull fresh air from outside and and an outlet to vent out the stale air from the inside.

Ceiling mounted air conditioner unit

This is important for office spaces in which windows can't be opened, but also for supermarkets, halls or storage rooms containing produce smells.

It's also a part of the building design and required by law to mix fresh air in a specific ratio to to reduce the levels of CO2. To put it simply, the room needs fresh and clean air if people are going to be in it.

Window air conditioners

Although some older window AC units from the 90's had a feature to let some fresh air in by pulling a lever on the front panel, modern-made windows air conditioners aren't equipped with that feature which means they are designed to operate the same way as split system air conditioners.

This is because it's important to seal the interior space and reduce energy loss to achieve better efficiency and minimize the consumption of electricity.