Makeup Air Unit

Why buildings need makeup air

Every cubic foot of air an exhaust fan pushes out must be replaced by air coming in from somewhere. When exhaust outpaces the available leakage paths, the building goes negative: doors become hard to open, drafts whistle through every gap, fireplaces and water heaters can backdraft combustion gases into the living space, and exhaust fans themselves lose effectiveness. A makeup air unit solves this deliberately, pulling in outdoor air through a filter and, in most designs, tempering it with a heating coil so winter air does not arrive as an icy blast. Commercial kitchens are the classic application, where powerful hood exhaust makes dedicated makeup air essential, but the issue increasingly reaches homes: modern codes commonly require makeup air once a residential range hood exceeds 400 CFM.

Makeup air in tight new construction and older buildings

Older, leaky housing around Southeastern Pennsylvania historically supplied its own makeup air through gaps in the shell, which is one reason the problem went unnoticed for decades. Air-sealing, new windows, and tight renovations remove those accidental pathways, so a powerful new range hood or extra bath fans can suddenly create pressure problems the house never had before. Warning signs include lingering cooking odors, a hood that seems weak despite its rating, fireplace smoke rolling into the room, and combustion appliance odors. If a remodel adds serious exhaust capacity, plan the makeup air path at the same time, and have combustion safety checked whenever backdrafting is suspected.