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Flame Sensor (“Cad Cell”) Checkout Procedure
On the Honeywell oil primary control, to check the electrical resistance of the flame
sensor (referred to as a cadmium sulfide photocell, or “cad cell”), depress the reset
button on the oil primary safety control while the burner is firing. The oil primary control
will report the measured resistance range of the cad cell by flashing the LED (light
emitting diode) one (1) to four (4) times. Refer to the oil burner manufacturer’s
instructions for further details.
For all primary controls, an alternate procedure to check the flame sensor operation is
as follows:
1) Remove the flame sensor lead wires from the terminals (labeled “CAD
CELL” on the underside of the Honeywell control) of the oil primary safety
control module. Start the burner. Shortly after combustion is established,
place a temporary jumper wire across the cad cell terminals, or leads, of
the control. Connect an ohmmeter across the flame sensor lead wires.
The measured resistance should be less than,
• 1600 Ohms for the Honeywell cad cell,
• 1500 Ohms for the Carlin cad cell, and
• 1200 Ohms for the Riello cad cell.
NOTICE: On the Riello 40-F3 burner only, it may be difficult or
impossible to measure the actual operating resistance of the cad cell with
the burner operating. With the electrical power off, carefully remove the
printed circuit board from the Riello oil primary control and subject the cad
cell to direct sunlight. The measured resistance of the cad cell should
reduce to 50 Ohms, or less. Cover the cad cell and measure the dark cell
resistance, refer to step 3 below.
2) Stop the burner and remove the jumper wire.
3) With the burner off, measure the resistance of the flame sensor with the
ohmmeter. The resistance of the sensor without “seeing” a light source
(so-called “dark” cell resistance) should be greater than,
• 20,000 Ohms for the Honeywell cad cell,
• 50,000 Ohms for the Carlin cad cell, and
• 40,000 Ohms for the Riello cad cell.
4) If the sensor resistances are outside the acceptable ranges given above,
confirm the lens of the cell is clean and the cell is located correctly in the
burner housing. If the cell is clean and correctly located, replace the flame
sensor.
NOTICE: All resistances are approximate values only and will vary
depending upon the intensity of the light source (flame or sunlight) and the
condition or age of the cad cell.