A pressure differential switch is used to prove air flow in a hydronic system.

• A. Hydronic
• B. Ducted system
• C. Steam system
• D. Electrical

Answer: (A)

Pressure differential switches measure the pressure difference between two points and trigger when that difference reaches a set value. This lets you prove air flow by showing that air is actually moving through the intended path; if air is flowing, a measurable pressure drop across a restriction exists and the switch completes or interrupts the control circuit to indicate airflow is present. In a hydronic heating system, confirming combustion air flow and proper venting is crucial for safe operation, so the pressure differential switch is used to monitor that flow and prevent ignition or continue operation if airflow is blocked. The other options don’t fit this purpose: a ducted system focuses on general duct airflow, steam systems center on steam pressure and water flow, and electrical systems don’t rely on airflow proving in this context.