What instruments are used to control draft in the system?

• A. Pressure transmitter and pressure control controller
• B. Thermocouple and temperature controller
• C. Flow meter and valve
• D. Level sensor and controller

Answer: (A)

Draft control relies on measuring the actual pressure difference (draft) and adjusting the air flow to keep it at the set value. A pressure transmitter continually senses the draft pressure and sends that signal to a pressure control controller. The controller compares the measured draft to the desired setpoint and drives an actuator—usually a damper motor or control valve—to increase or decrease air flow until the draft matches the target. This closed-loop setup is essential for stable combustion, proper furnace operation, and safety.

Other options don’t fit as well because they measure or control conditions that aren’t directly draft. A thermocouple and temperature controller manage temperature, not pressure. A flow meter and valve can influence flow but don’t provide the sensing and feedback needed to maintain a specific draft. A level sensor and controller monitor liquid level, which is unrelated to draft control.