First-Year HVAC Service Certification Practice Test 2026 – The Comprehensive All-in-One Study Guide for Success!

During the heating cycle, the flame sensor operates using AC voltage. This voltage is typically derived from the transformer and is part of the ignition system. The purpose of the flame sensor is to detect the presence of a flame during the heating process, ensuring safe operation of the furnace or boiler.

AC voltage is suitable for this application because it allows for efficient detection of the flame's presence by generating a signal that can be easily monitored and interpreted by the control system. In the context of HVAC systems, flame sensors work by measuring the ionization of the combustion gases that occur when a flame is present, which is facilitated by the alternating current.

Using other types of voltages, such as DC or battery voltage, would not provide the same functionality or reliability for flame detection in this context. High voltage is also not typically applied directly to flame sensors as it could potentially damage the sensor.