Differential pressure switches are often used across which component to indicate impeded airflow?

Differential pressure switches are often used across which component to indicate impeded airflow?

• A. Duct
• B. Filter
• C. Damper
• D. Plenum

Answer: (B)

Differential pressure switches detect how much air resistance a component adds to the flow. The place where a buildup of resistance most clearly signals reduced airflow is the filter. As dust collects on a filter, its permeability drops, causing the pressure on the upstream side to rise relative to the downstream side. A switch across the filter taps will float a clean, actionable signal when this pressure drop reaches its setpoint, indicating the filter is clogged and airflow is impeded. Placing the switch across the duct would reflect overall system pressure changes rather than a specific restriction, across a damper would primarily indicate the damper position, and across a plenum isn’t a typical source of a meaningful differential for detecting blockage. So the filter provides the most direct, reliable indication of impeded airflow.

Differential pressure switches detect how much air resistance a component adds to the flow. The place where a buildup of resistance most clearly signals reduced airflow is the filter. As dust collects on a filter, its permeability drops, causing the pressure on the upstream side to rise relative to the downstream side. A switch across the filter taps will float a clean, actionable signal when this pressure drop reaches its setpoint, indicating the filter is clogged and airflow is impeded.

Placing the switch across the duct would reflect overall system pressure changes rather than a specific restriction, across a damper would primarily indicate the damper position, and across a plenum isn’t a typical source of a meaningful differential for detecting blockage. So the filter provides the most direct, reliable indication of impeded airflow.