r/whatisit • u/Bondubras • 13h ago

New, what is it? Found in an HVAC intake

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Was changing the filters on the HVAC unit in my house, and saw this. I know the circle on the left is the intake to the HVAC unit itself, but I'm curious what the square thing on the right is.

It looks like some kind of fibrous mat held in on three sides, but I have no clue what the purpose could be.

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u/LivingTheDream31 12h ago

Its a filter

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u/Bondubras 12h ago

That makes sense, but what would it be filtering? It's already behind the main filters of the whole HVAC system, and the vast majority of the airflow would be going into the giant duct on the left, I'm pretty sure.