What is the name of the limit switch that senses flame outside the heat exchanger?

• A. Draft switch
• B. Flame sensor
• C. High-limit switch
• D. Rollout switch

Answer: (D)

The main idea is a safety limit device that responds to abnormal flame behavior in a furnace. A rollout switch is positioned to detect flame that extends or rolls out of the combustion zone into the cabinet area outside the heat exchanger. When this condition is sensed, the switch interrupts the control circuit (often shutting off the gas valve) to prevent a fire or heat damage, which is why the rollout switch is the one that senses flame outside the heat exchanger.

Draft switches monitor the vent or chimney pressure, not flame location. High-limit switches track the temperature inside the heat exchanger to prevent overheating. A flame sensor checks for the presence of flame in the burner itself, not where the flame is located relative to the heat exchanger.