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Why does an air handling unit need 2 filters?

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Many people who change filters for the first time are often surprised to find out that their air handling unit needs 2 filters - after all, it needs to filter the air from the outside, doesn't it? But the air entering the AHU comes not only from outside, but also from inside - indoors.


The main purpose of the filters in the AHU is to protect the heat exchanger and its other elements from fouling. Therefore, a separate filter is required for both streams. For the exhaust flow, filters of class G3 to G4 (Coarse) are sufficient. Higher class filters, such as F7 (ePM1) or M5 (ePM10), are normally placed on the supply air duct and improve the quality of the supply air.