What is the cad cell purpose in a cad cell primary control?

• A. To prove ignition
• B. To sense flame
• C. To shut off if pilot goes out
• D. To regulate gas pressure

Answer: (A)

The cad cell’s job is to sense the presence of flame. In a cad cell primary control, a cadmium sulfide (cad cell) sensor detects the light from the flame and changes its electrical resistance, which the control reads to know that combustion is actually taking place. If flame is present, the control keeps the gas valve energized and the burner running; if the flame dies or never lights during the ignition trial, the control shuts the gas off to prevent unburned fuel from accumulating. So the core purpose is flame sensing, not directly proving ignition or regulating gas pressure. The shutdown if the flame goes out is a safety response to the loss of flame sensing, not the primary purpose of the sensor itself.