Flame rectification is a method of flame sensing in modern gas furnaces where the flame sensor (flame rod or ignitor) is located in the pilot or burner flame and a current applied to the sensor flows through the flame to the pilot assembly or the burner head and then to ground.
Because the area of the flame sensor is smaller than the ground electrode (the pilot assembly in pilot ignition systems, or the main burner in hot surface or direct spark systems), an alternating current applied to the flame sensor will only flow in one direction. The AC current is rectified to a pulsating direct current, or rectified current. When the flame is present, this current sends a signal to the control module. The control module then keeps the gas valve opened, the gas valve will remain open as long as there is a call for heat and the DC signal from the flame rod is present. When the signal ceases, or drops below a specified level, or is interrupted, the control module closes the main gas valve immediately to prevent an unsafe condition. The system either recycles itself or locks out and must be reset by cycling the thermostat or in some instances, be checked by a qualified service technician.
Since the flame sensor is just a metallic rod, it will never need replaced unless it is broken. The most common cause of a gas furnace or boiler not working because of a flame sense problem is a dirty, corroded, or rusted flame sense rod. Simply remove the flame sensor and clean it thoroughly with sand cloth. Do not use a green scrubbing pad, it cleans the flame sensor but leaves a residue that also inhibits the small micro amp flame signal.
A gas furnace with a dirty flame sensor will ignite the burners for a few seconds then shut down the gas valve. In most cases, it will try this 3 times before it locks out.
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by Ron Walker
Because the area of the flame sensor is smaller than the ground electrode (the pilot assembly in pilot ignition systems, or the main burner in hot surface or direct spark systems), an alternating current applied to the flame sensor will only flow in one direction. The AC current is rectified to a pulsating direct current, or rectified current. When the flame is present, this current sends a signal to the control module. The control module then keeps the gas valve opened, the gas valve will remain open as long as there is a call for heat and the DC signal from the flame rod is present. When the signal ceases, or drops below a specified level, or is interrupted, the control module closes the main gas valve immediately to prevent an unsafe condition. The system either recycles itself or locks out and must be reset by cycling the thermostat or in some instances, be checked by a qualified service technician.
Since the flame sensor is just a metallic rod, it will never need replaced unless it is broken. The most common cause of a gas furnace or boiler not working because of a flame sense problem is a dirty, corroded, or rusted flame sense rod. Simply remove the flame sensor and clean it thoroughly with sand cloth. Do not use a green scrubbing pad, it cleans the flame sensor but leaves a residue that also inhibits the small micro amp flame signal.
A gas furnace with a dirty flame sensor will ignite the burners for a few seconds then shut down the gas valve. In most cases, it will try this 3 times before it locks out.
If you want to learn more about HVAC, sign up for our online HVAC certification course.
by Ron Walker