What Are "Return Air" and "Fresh Air"? Why Can't A Regular Air Conditioner Be Used As A Fresh Air System?

When selecting or installing air conditioners, many people will hear two words, "return air" and "fresh air". But a more common question is, "As it blows air all the time, can an air conditioner supply the fresh air to the inside like a fresh-air machine?"

The answer is absolutely not; most of the traditional air conditioners do not equal a fresh air machine. To understand the reason, we must know the basic distinctions between "return air" and "fresh air".

I. Return air—A systematic indoor air-circulation

Return air means there already exists air in the room; it is taken in and processed by the air conditioner and then returned to the room.

The whole process is indoor air → being taken into the air inlet → passing through the filter → passing through the evaporator (cooling/heating) → blowing out from the air outlet.

It features a source of air from indoors, a function of temperature adjustment, and a characteristic of hermetic or semi-hermetic circulation.

The function of return air lies in the improvement of cooling and heating efficiency, the reduction of energy consumption (no need to cool the new air), and the even coverage of the room temperature. It stresses whether cooling and heating are comfortable rather than whether the air is fresh or not.

II. Fresh air--- A true replenishment of outdoor air

Fresh air means supplying air from the outdoors to indoors, in which the air has been filtered and processed.

The whole process is outdoor air → air inlet → multiple filters (dust-proof, PM2.5, pollen, etc.) → heat recycle/cold recycle → fresh air supply to the indoors → dirty air exhaust to the outdoors in the meantime.

It features a source of air from outdoors, a purpose of replenishing oxygen and diluting the dirty air, and a nature of air exchange.

The fresh air can solve many health problems of the room, such as a high concentration of carbon dioxide, strange smells, formaldehyde, VOC, dirty air caused by long-term closed windows, and the lack of oxygen in confined spaces.

III. Why can't traditional air conditioners be used as fresh air machines?

1. Traditional air conditioners have no air inlet design from the outdoors

Wall mounted air conditioners and floor standing air conditioners—their indoor units do not contact the air outside. The air inlet and air outlet are all inside of the room. The outdoor units are only responsible for heat dissipation.

The new air didn't come in, and the air just circulates inside.

2. Air-conditioned wind is not same with fresh air

Some people believe that it seems the air is fresh when the air conditioner is turned on and the air flows. But in fact, only the air circulates; the content of oxygen does not increase, and carbon dioxide and strange smells accumulate in the room continuously.

That's why people will get tired and have headaches in the air-conditioned room for a long time, and offices need to open the window regularly or install a fresh air system.

3. Traditional air conditioners don't have filtration and protections that a fresh-air system requires

A true fresh-air system needs high-efficiency filtration (dust-proof, PM2.5, pollen, etc.), anti-condensation, anti-backflow design, and pressure control between indoor and outdoor.

Traditional air conditioners have a filter, but it is only used for dust-proofing, which cannot process the outdoor polluted air directly. If we insist on supplying the air from the outside, it may lead to condensation, a surge in energy consumption, and even malfunctions.

IV. Is there a win-win plan that covers air conditioning and fresh air needs?

The answer is YES, but they are not traditional air conditioners. Here are applicable solutions:

· Air conditioner + individual fresh-air system (Common and stable plan);

· Central air conditioning/ducted air conditioning with fresh air function;

· Full heat exchange fresh air + air conditioning system.

Those systems had taken into consideration fresh air volume, energy consumption balance, and heat/cool compensation during the designing process.

V. In which conditions should we choose a fresh-air system

If you meet the following situations, a fresh-air system is not optional but a must.

· Long-term hermetic spaces (closed windows)

· Offices, hotels, apartments

· Live with the elderly, kids, and pregnant women.

· High pursuit of the air quality (allergies, dust, formaldehyde)

The core task of the air conditioners is to adjust the room temperature, while the fresh-air system is to refresh the air. They are not in a relation of replacement but of complementarity. If you only install the air conditioner, your room will be cool and heated, but if you have a fresh-air system at the same time, your room will be alive.