Cad cell relays on oil burners:
This article explains the operation, use, and reset button on Cad Cell Relay Controls.
We provide : a guide to inspecting and re-setting the cad cell relay safety device on oil burners. We describe how the cad cell relay and the cad cell sensor itself are tested.
We explain how the cad cell works to "see" the oil burner flame, what goes wrong in an oil burner that the cad cell "eye" can detect, and how cad cell sensors are tested using several methods including a VOM. We include cad cell trouble spots - particular oil burner installations or conditions that can make it hard for the cad cell to sense even a good flame.
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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 during oil-burner startup ("trial for ignition or TFI" in some manuals) or if oil burner flame is later 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.
Just below 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 Oil Burner Cad Cell Relay:
The photograph shows an older Honeywell aquastat relay switch that may also make use of a cad-cell sensor. 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 that looks like the gray box above, you'll want to
and CAD CELL RELAY SWITCH (hot water boilers and some water heaters).
Your oil fired boiler or furnace might not have either an aquastat like the one just above or the separate cad cell relay control box shown earlier in this article. Instead some older oil fired boilers, furnaces, and water heaters use a separate STACK RELAY SWITCH and mounted on the flue vent connector piping.
Other heating system reset buttons may be found
at SPILL SWITCH, FLUE GAS DETECTOR (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.
The photograph tells a lot about this oil burner and heating system. The debris on top of the cad cell relay contacts (pointed to by our ruler) indicates that the system is running dirty and perhaps with a puff-back at startup or with backpressure in the combustion chamber.
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. Marked by an orange arrow, this is where we should have seen a red cad cell relay button.
Watch out: sometimes the red plastic button on older heating equipment primary controls can break off - like the one we show at left. .
At RESET SWITCH, HEATER REPAIR we show how to reset the equipment safely if this happens to you.
Watch out: Honeywell's instructions for R4184D and R8184G,M,N,P Protectcorelay® series of oil burner controls advises 
However this instruction pertains to the control itself. The cad cell relay sensor is a plug-in replaceable part that can be cleaned or field-replaced if appropriate.
There are three possible heating system trouble or failure modes addressed by this control:
In the first two cases the device will shut down the oil burner and require manual pressing of the reset button to attempt a re-start.
In the third case the burner should re-start when electrical power is restored.
Function-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.
Using a Honeywell R8184G primary control as an example we illustrate the reset button (red arrow in our photo, below left) and the safety switch lever (black arrow, below left). The red button is the "reset button" on this control.
On some equipment the round reset button is accompanied by a small thin lever or "switch" (the black arrow in our photo above left). On some older controls the safety switch lever is a thin silver metallic lever protruding through the control cover rather than the nice chubby black switch shown above.
If your control incorporates that little lever (which will also protrude out through the control cover) you can test the control by holding the lever down or in the opposite of its normal position (you may have to move it to one side instead of down depending on how the control is mounted).
Holding the switch lever out of its normal position for a short time will cause the safety switch to shut down the equipment and the reset button will pop out - sticking out further from the control cover. To re-start the burner, push the red reset button back in.
Honeywell's description of their cad cell sensor used on their R4184D and R8184G,M,N,P Protectcorelay® series of oil burner controls describes it as follows:
All [Protectorelay series oil burner controls] models use the C554A Cadmium Sulfide (cad cell) Flame Detector to monitor the burner flame and shut down the system on ignition failure or on flame failure during the run cycle. 
Testing the Cad Cell Sensor Itself: Our photo of a cad cell relay being tested by our oil heating company service tech (Bottini Oil, Poughkeepsie, NY), shows how an expert can check on the cad cell function without disassembling the equipment.
When the oil burner is firing, an ohmmeter reading of 1/2 ohm (close to zero) to a maximum of about 500 to 600 ohms (see note below) is normal.
When the oil burner is not firing, the reading across the cad cell terminals should be close to infinity, that is, high resistance.  That is, the cad cell is not seeing any flame.
If the cad cell fails, or more likely if a dirty oil burner flame soots up the face of the cad cell device, the soot blocks view of the flame and cad cell resistance will increase. (That's why at zero flame and when it's dark in the combustion chamber the cad cell will read infinite resistance.)
Significantly, Honeywell's Protectorelay installation and testing instructions do not include a recommendation for testing the resistance of the cad cell sensor. Instead the company recommends the test sequences described above, and note that if the cad cell is not responding to the flame, you should replace it  But before doing so
Thanks to reader Tony for the following comment:
Hi, I love the work you guys do and think it can be a life saver when you may have exhausted all options. My comment is with regards to the section entitled "How to Test the Oil Burner Cadmium Cell Relay Switch", the sub titled, "Testing the Cad Cell Sensor Itself" section.
You mention that an ohm reading on the flame sensor terminals when the boiler is running should be .5 to .6 I took an ohm reading after having cleaned soot from an obstructed cad cell and got an ohm reading of 500 to 600.
I though my flame sensor was faulty. But then i opened the canopy containing the flame sensor and shined a 100 watt light (137 foot candles of illumination) bulb at the flame sensor with my friend taking the readings and compared the results to a specs chart on the flame sensor that charted foot candle power vs ohms and saw that 500 to 600 ohms was correct.
I realize i took the reading while the boiler was off but the wires from the cad cell to the terminal screws where connected and that may have given me a skewed reading but i just wanted to point out the difference between your recommended ohm results and what is observable to an average joe like me. - Ton6 2/11/2012
Watch out: here are some other cad cell sensor troubles that can give odd readings, or that can cause the cad cell control to switch the heating system off on "reset"
On more advanced primary oil burner controls such as the Honeywell R7184 Series Primary Controls, the control unit itself is capable of monitoring the status of the cad cell relay.  A series of LED flashes indicates the cad cell resistance in ohms (for this control) as follows:
|Cad Cell Resistance (R7184 Series Primary Controls)|
|Number of LED Flashes||Cad Cell Resistance (ohms)|
The control will work properly at any of the resistances indicated in the first three lines of the cad cell relay table above. In other words, the cad cell resistance should be below 1600 ohms for this control. The utility of reading the number of flashes and thus inferring the cad cell resistance is that higher resistances can indicate trouble brewing and dirty oil burner operation or a poor sooty flame.
To observe the cad cell condition for this primary control, Honeywell instructs "While the [oil] burner is firing, and after the ignition has been turned off, press and release the reset button (hold 1/2 second or less) to check cad cell resistance. The LED will flash 1 to 4 times depending on the cad cell resistance. For proper operation, it is important that the cad cell resistance is below 1600 ohms.
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.
Try cleaning the sensor first.
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).
To order a new cad cell for most Honeywell Protectorelay controls order Honeywell part no. 130367 Cad Cell.
Currently (2017) Honeywell recommends replacing an older primary controller for an oil buner using the Honeywell R7274U shown here.
The electronic primary controller model Honeywell R7284U will replace most older Honeywell (and some other) primary controllers used on oil burners including the widely-used
This article series answers most questions about central heating system troubleshooting, inspection, diagnosis, and repairs. We describe how to inspect, troubleshoot and repair heating and air conditioning systems to inform home owners, buyers, and home inspectors of common heating system defects.
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