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Flame sensing devices on oil-fired heating appliances: modern oil-fired heating furnaces, boilers, and water heaters use a Cadmium Cell sensor, usually located inside the oil burner tube, to "see" the presence of flame and thus to assure that the oil burner assembly stops pumping oil into the combustion chamber if flame ignition is unsuccessful. The cad cell itself, that is the little cell that "sees" the oil burner flame, is wired (often by a yellow wire) to the cad cell relay switch (see photo) which is usually a gray box with a red "reset" button located on top of or alongside the the oil burner assembly. The cad cell causes the relay to switch the oil burner off when a flame is not established or if flame is lost at the oil burner (or inside the furnace or boiler's combustion chamber). Our photograph shows a modern Honeywell(R) R8184G 4009 cad cell relay for use on (typical) intermittent ignition oil burner equipment. This device is likely to be found on oil burners less tan 15 years old, and on both hot air furnaces and hot water boiler heating systems provided that they are heated by an oil burner.
Here is a photo of what the cad cell itself looks like. You cannot see this part unless the oil burner is disassembled. The cadmium cell is simply a type of photo cell that is mounted in the oil burner tube, usually near the rear, where it can "see" the flame when the oil burner is operating properly. If the oil burner flame is not present the cell passes that information on to the cad cell relay switch which will, after a delay of 15-20 seconds, shut down the oil burner. You can see that if the glass surface of the cad cell sensor becomes coated with soot, it won't "see" the oil burner flame very well and it will, when dirty enough, shut down the oil burner. |
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The photograph shows an older Honeywell cad cell relay switch - the gray box in the right in this illustration. If the cad cell has told the relay that it can't see any flame, the cad cell relay switch will have turned off electrical power to the oil burner and the red button will have popped up. If the red "reset" button on the cad cell relay is sticking up and the oil burner has shut down, the homeowner is permitted to try ONCE to "reset" the system by pressing the red reset button. If the oil burner does NOT turn on and run normally and continuously (no smoke, no loud noises, etc.) for at least 5 or 10 minutes after resetting the relay or pressing the reset button, DO NOT keep resetting the system since doing so can flood the combustion chamber with un-burned heating oil - a dangerous condition. Where are all the heating system reset buttons? If you are looking for the main reset button on heating equipment you'll want to see: Aquastat Functions and Cad Cell Relay Switch Flame Sensors (hot water boilers and some water heaters), Stack Relay Switch on older oil fired boilers and furnaces, SPILL SWITCHES (gas fired equipment), and also Low Water Cutoff Controls on steam heating systems. At ELECTRIC MOTOR OVERLOAD RESET SWITCH we discuss the thermal overload switch and reset button that is found on many electric motors including those operating air conditioning fans, heating system oil burners, and furnace blowers and motors.
Look closely at the mostly-covered gray cover of this Honeywell cad cell relay control. In the upper center of the photo you can just see a hole in the top of the cover. This is where we should have seen a red cad cell relay button. But the button itself, a plastic part affixed to the cover top, has broken away. We can still reset this cad cell relay however, by removing the cover and looking inside for the button which is mounted on the relay circuit board itself. That button is the actual relay switch and it can still be pressed to reset the switch if needed, even if the external button has been lost. |
Testing cad cell relays: is possible by simply pressing the red button down when the oil burner is operating. If the oil burner is operating normally and you press the cad cell relay reset button down it should turn off the oil burner. When you release the button the oil burner should re-start.
This oil burner safety test checks the interrupt circuit in the cad cell relay. We recommend not trying this test unless the oil burner has been on for a few minutes or more (and has had time to warm up) so that we don't produce unnecessary sooting in the combustion chamber during stop and start of the burner.
Above we showed you the face of a cad cell and explained that it can become soiled and blocked by oil burner soot, particularly if the oil burner is itself running "dirty" or improperly.
Cleaning or replacing an oil burner cad cell is pretty easy, but this is a job for the service technician since it's necessary to turn off power to the oil burner and partly disassemble the oil burner to access the cad cell sensor.
Often by removing a lock screw or two, the voltage transformer (the black box atop[ the oil burner in this photo) is simply hinged back and one can see the yellow wire entering the oil burner tube and leading to the cad cell sensor.
The cad cell relay sensor will be mounted in a little bracket and positioned where it can "see" the flame when the oil burner is operating.
If the oil burner has not been operating properly, say has been producing too much smoke and soot, even after it has been adjusted it may be necessary to inspect and clean soot from the cad cell sensor so that it has a non-sooty clear view of the flame. Or the technician, if s/he has any doubt about the sensor, will simply replace it. It's a simple plug-in part (two pins on the sensor plug into a receptacle that is secured by a bracket inside of the oil burner tube).
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