Booth pressure balancing fundamentals

The basic equation is exhaust outflow > intake inflow. The exhaust fan moves a designed volume of air out of the booth; the AMU (Air Make-Up unit) supplies a designed volume into the booth; the differential is the design negative pressure margin.

Filter loading shifts the balance. Loaded intake-ceiling and AMU pre-filter media restrict inflow, the booth runs MORE negative because exhaust is still pulling at full volume. Loaded exhaust-pit media restricts outflow, the booth runs LESS negative because outflow has dropped while inflow is unchanged. Severe exhaust loading flips the booth to positive pressure (overspray escape).

The AMU's role. The make-up air handler conditions outdoor air (heating in cold weather, occasionally cooling in hot/humid weather) and supplies it to the booth at the design volume. AMU pre-filter loading restricts AMU output, contributing to inflow restriction. AMU heater output drift (over time, due to wear) can shift the supply temperature and humidity profile that the booth's intake-ceiling sees.

Damper position. Most installations have automatic damper compensation that adjusts to maintain pressure within range. The damper's range of motion is finite, when filter loading goes far enough to exceed the damper's compensation range, pressure drifts outside design.

Door seals. The integrity of the door seals is what makes the negative pressure work, if the seals leak, air enters from the shop side at the booth's lowest-resistance path. Worn seals magnify any pressure imbalance.

NESHAP Subpart HHHHHH (6H) and Subpart IIII both require negative-pressure operation for overspray containment. Positive pressure documented during inspection is a violation. The maintenance log demonstrating consistent filter cadence (which prevents pressure drift) is the compliance documentation that ties everything together.