The flame rectification sensor is commonly used in high efficiency furnaces that use direct burner ignition.
Direct Burner Ignition System Flame Rectification Circuit — Direct burner ignition systems light the burners using a hot surface ignition (HSI) or direct spark ignition method. In direct burner ignition systems, the flame sensing electrode (FSE), also called a flame rod, is used to detect that the main burner flame is established across the entire length o f the burners. When the flame rod is encircled by the burner flame, the hot gas-ionized particles conduct electricity, thus completing a circuit through the flame rod to the furnace control or ignition module. Current flow through this circuit applies a flameproving signal to the furnace control circuit microprocessor, signaling the microprocessor to keep the gas valve energized.
Flame sensing electrode operation (Figure SP-11-3) can be tested by measuring the flame-proving current signal applied to the microprocessor. This can be done by connecting an accurate ammeter capable of measuring microamps in series with the sensor input to the microprocessor. With this furnace, the ammeter should indicate a current of at least 0.5 microamps DC. Other furnaces may require a higher or lower flame sensing current. If the flame sensing current is low or nonexistent, check the following:
• Flame sensor rod position.
® The furnace is properly grounded per the manufacturer’s instructions.
• The gas valve is grounded through the gas valve ground wire.
• There is good electrical contact at the sensor connection.
• There is no oxide film coating the flame sensing electrode.
If there is a coating, clean it off by lightly rubbing with fine abrasive and wiping clean.
