Return Air Vent vs Supply Vent: Which One Pulls Air In

Aaron, our duct cleaning technician, reaches up in a headlamp and white N95 and drops a ceiling return grille out of the ceiling. Behind it, the return duct is lined with gray dust. Then a wall return opening, grille already off, shows the same buildup on the metal edges.

Returns are where air goes in, so they're the first thing to catch the dust, hair and lint floating around the house. Supply registers, the smaller louvered grilles on floors or ceilings, push air out and stay cleaner longer.

The difference between supply and return vents is easy to test: hold a tissue up to one with the fan running. If it sucks the tissue flat, it's a return.