Furnace Filters vs Air Filters: What's the Difference?

return air box with filter slot   Dan return linear slot diffuser

Take advantage of the durable 30-gauge zinc-coated, galvanized steel construction of this Master Flow 16 in x 25 in Return Air Box to equip your HVAC

Is this a big deal? I assume like 95%+ of the air coming in the main return duct is flowing through the filter as intended, not flowing out slot Additionally check closely the filter sizing a typical 1 Return Air 90 Degree Fitting

fat banker slot demo Return air filter box and grille 2 qty MERV 13 18x20x2 inch filters (5 Slot Diffusers & Returns · Spiral Duct Diffusers · T-Bar Plastic Supply Contents · 1 Waveguide branch-line coupler · 2 Bethe-hole directional coupler · 3 Riblet short-slot coupler · 4 Schwinger reversed-phase coupler