What takes the safety switch heater out of the circuit when operating an oil burner with a stack-mounted primary control?

• A. Float switch
• B. Pressure switch
• C. Thermostat
• D. Relay

Answer: (A)

In oil burners with a stack-mounted primary control, the safety switch is kept energized by a small heater so the flame safeguard stays ready during the run sequence. A float switch in the oil supply acts as the interlock that controls whether that heater can stay in the circuit. When there is enough oil, the float switch allows current to reach the safety switch heater, so the safety switch remains heated and closed, permitting normal operation. If the oil level drops, the float switch opens and removes power from the heater, causing the safety switch to drop out and the burner to shut down for safety. The other devices don’t perform this specific oil-supply interlock: a pressure switch relates to combustion air pressure, a thermostat to space temperature, and a relay is just a generic switch, not the oil-level interlock that takes the heater out of the circuit.