How Are Dryer Exhaust and Makeup Air Designed for Laundries?

I start with the manufacturer's exhaust requirement for each dryer — volume and maximum equivalent duct length — and treat those numbers as hard limits. Branch ducts run from each dryer collar to a main that grows as branches join, with velocity kept high enough to carry lint to the termination rather than letting it settle in the duct. Cleanout access goes at every turn and at regular straight intervals, because a duct that can't be cleaned is a fire hazard accumulating over time. Fire dampers, listed materials, and proper terminations complete the design, and I coordinate the routing with structure and other trades early — dryer mains are big, and they don't fit in the leftover space.

Makeup air volume matches the total exhaust — dryers plus general exhaust — and I design it as a dedicated system with its own tempering. The unit heats incoming air in cold weather and provides at least ventilation tempering in warm weather, with controls interlocked to dryer operation so supply tracks demand. Distribution matters: dumping all the makeup air in one spot creates drafts and dead zones, so I lay out diffusers to wash the dryer rows and the folding areas evenly. Building pressure stays slightly positive to neutral with everything running, verified by calculation and confirmed at commissioning.

An exhaust and makeup air system is only as good as its startup: • Measure airflow at every dryer collar and confirm it meets the manufacturer's minimum • Verify building pressure with all dryers running — doors should open normally, no whistling • Test the makeup air interlock: dryers should not run at full production without makeup air operating • Check gas appliance draft under worst-case exhaust conditions to rule out backdrafting • Set a duct cleaning schedule and make sure every cleanout is reachable without dismantling the store • Document design airflows on the drawings so future service techs know what right looks like