The air is supposed to enter from the back, filtered by the HEPA filter and then flow out from the front side. A simple tool to test the flow direction is a piece of tissue. The tissue should be adsorbed to the back side but blown away at the front side.
Besides, the inlet airflow should be much larger than the outlet since HEPA filters will compromise the flow rate.
Learn more: Why do I feel little air coming out of my purifier?
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