What is the purpose of the firestat in an air handling unit?

• A. Increase Fan Speed
• B. Shut Down Air Handler When Excess Heat Is Present In The Duct System
• C. Mute Audible Alarm
• D. Open Ventilation Damper

Answer: (B)

A firestat is a heat-sensing switch used in HVAC systems to automatically shut down the air handling unit when excessive heat is detected in the ductwork. This helps prevent the spread of fire and smoke through the building’s ventilation paths and supports fire containment and safety. When the duct temperature rises enough, the firestat signals the AHU to stop the fans and may also trigger dampers to close, isolating the system from the fire area. That’s why the primary purpose is to shut down the air handler when excess heat is present in the duct system. Increasing fan speed would risk spreading fire, muting alarms is not the firestat’s function, and opening a ventilation damper would worsen the situation by letting smoke and heat travel through the system.