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A Guide to Cad Cell Relay Switches on Oil Burners
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Here we discuss Cad Cell Relay Controls: a guide to inspecting and re-setting the cad cell relay safety device on oil burners. Readers will also find that this website answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect residential heating systems to inform home owners, buyers, and home inspectors of common heating system defects.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

A Guide to Oil Fired Furnace or Heating Boiler Cad Cell Relays found on Oil Burners

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.

How to Reset the Cad Cell Relay on an Oil Burner for Boilers & Furnaces

How to Test the Oil Burner Cadmium Cell Relay Switch

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.

How to Service or Replace the Cad Cell Relay on an Oil 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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