This prevents negative pressure in the building?

• A. Make up air unit
• B. Exhaust fan
• C. Pressure relief valve
• D. Ventilation damper

Answer: (A)

The key idea is balancing air flow to keep the building from pulling a vacuum. When exhaust devices push air out of the building, outside air must come in to replace it. A make‑up air unit does just that: it actively adds fresh outdoor air to the space to offset the air being exhausted, helping maintain neutral or slightly positive pressure. This prevents issues like doors that won’t close properly, drafts pulling in outside air and contaminants, and backdrafts from combustion appliances.

An exhaust fan adds to the problem by removing air, which can drive negative pressure. A pressure relief valve is used to relieve excess pressure, not to prevent negative pressure. A ventilation damper can regulate airflow but doesn’t guarantee a source of incoming air to balance what’s leaving, so it doesn’t specifically prevent negative pressure on its own.