Where does the stack switch primary control pick up the signal that a flame has been proven? Explain the operation of a cad cell primary control. What are the two types of primary controls used to supervise the operation of an oil burner?
True or False: A stack switch uses pressure to prove the flame of an oil burner.
The correct answer and explanation is :
Answer:
False – A stack switch does not use pressure to prove the flame of an oil burner. Instead, it detects heat in the flue gases using a bimetallic sensor.
Explanation:
The stack switch primary control picks up the signal that a flame has been proven by detecting the heat of the flue gases in the exhaust stack. It contains a bimetallic strip that reacts to heat by bending or expanding. When the oil burner ignites and produces a flame, hot combustion gases pass through the flue pipe. The heat from these gases warms the bimetallic strip inside the stack switch, causing it to bend and close an electrical circuit. This confirms that combustion is occurring and allows the burner to continue running. If no flame is present, the stack switch does not heat up, and after a set time delay (usually 45-90 seconds), it shuts off the burner to prevent unburned fuel from accumulating, which could cause a dangerous condition.
A cad cell primary control operates differently. It uses a cadmium sulfide (cad cell) flame detector to sense visible light from the flame. The cad cell is a photoelectric sensor that decreases in electrical resistance when exposed to light. The primary control monitors this change in resistance; if a flame is present, the resistance drops, signaling the control to keep the burner running. If the flame goes out or fails to ignite, the cad cell detects the absence of light, causing the control to shut down the burner within a few seconds.
The two primary types of controls used to supervise oil burners are:
- Stack Switch (Thermal) Primary Control – Detects flame presence based on heat.
- Cad Cell (Photoelectric) Primary Control – Detects flame presence based on light.
Both controls are safety devices that prevent unburned fuel from accumulating, reducing the risk of fire or explosion.