Negative ventilation is achieved by which configuration?

• A. Exhaust fan high, supply fan low in affected space
• B. Exhaust fan low, supply fan high in affected space
• C. Exhaust off/low, supply high in surrounding areas
• D. Exhaust high, supply low in surrounding areas

Answer: (A)

Negative ventilation is about creating a pressure deficit in the affected space: you remove air faster than you bring it in. Placing the exhaust high in the affected space efficiently pulls out the buoyant gases or smoke that tend to rise, while keeping the supply low in that same space reduces the amount of air entering. The result is the interior becomes lower in pressure than the surrounding areas, so air naturally flows from adjacent spaces into the affected space to replace what’s being exhausted.

If you instead push more air in than you remove, you’d create positive pressure rather than negative. Supplying air into surrounding areas or exhausting air from surrounding areas without addressing the affected space won’t reliably establish the needed pressure differential in the target area.