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This article explains the operation, use, and reset button on Cad Cell Relay Controls: 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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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.
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.
How to Reset the Cad Cell Relay on an Oil Burner for Boilers & Furnaces
How to Test the Oil Burner Cadmium Cell Relay Switch & Switch Sensor
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.
Button & Switch Tests of The Cad Cell Relay
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.
Resistance Testing the Cad Cell Sensor using a VOM or Digital VOM
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:
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:
Other Cad Cell Sensor & Oil Burner Cad Cell Primary Control Troubles
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"
Automated LED Readouts Give Cad Cell Relay Condition
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:
Notes to the Cad Cell Resistance Table
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.
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).
To order a new cad cell for most Honeywell Protectorelay controls order Honeywell part no. 130367 Cad Cell.
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Frequently Asked Questions (FAQs)
Question: Cadmium poisoning report: could my heating boiler cad cell be involved?
I received and respect your response to my recent email. I do not expect a response to this email, but wanted to ask a question because I have exhausted every resource I can think of. In June 2010, I learned that I had long-term carbon monoxide exposure.
After months of treatment, I am very improved. I do biochemical testing and I ordered several tests on myself as no one had done any lab testing or even thought it could be relevant to the downstream effects of CO poisoning. I have very high levels of cadmium in my blood, indicating a "recent" (un definable term) exposure. I wonder if the cadmium could have come from the cad cell relay on the gas hot water heater.
To my knowledge, I was not in contact with any of the common sources of cadmium. I will be testing my home and my family members' blood. Over the past six months, I have put together a team of over 20 people to help treat the effects of the poisoning. I am doing well and believe that some of the data I have collected might be of interest to you. Please let me know if that is the case. I know you are very busy--and I love your site. - A.S. Louisville KY
Reply: Cad Cell Seems Unlikely to Be a Hazard, Here's What & How to Check
The cad cell that is used as a flame sensor in heating equipment is quite well sealed-up. After all cad cells are used in a combustion chamber and exposed to challenging conditions such as high temperature and vibration. We do not know how much cadmium is used in that flame-sensing device - the manufacturer could tell you.
But if nevertheless you suspect that the heating boiler or water heater oil burner cad cell sensor itself is a source of cadmium, it would make sense to have the cad cell removed with care, placed in a sterile clean container, and tested for leakage. I'm somewhat doubtful that you'd find something. My reasoning is that if the cad cell were damaged most likely it also would not be working and you'd have noticed - you'd have no heat because the oil burner would stop running.
Cadmium is used in other products, including some paints. If you've been hanging around an artist's studio ask about her paints. Some other common sources of cadmium include those listed below, all of which seem to us to be potentially greater total cadmium reservoirs than a single sensor in a heating appliance:
As you are consulting a panel of experts, it makes sense to ask them to look at your personal case history and to compare that with the environments in which you've spent time, checking to see if you've been exposed to a particularly likely source of cadmium.
Question: proper ohms readings at a cad cell relay
at times i can`t get an under 2000, or even 3000 ohm reading after ruling out or correcting the following:
- Samuel Zapata
Question: my oil fired furnace fires up but the blower in the air handler doesn't turn on
I have a oil-furnace,my system fire up,blower didn't come on system shut down .I replaced the limit control, system fired up,blower came on ran a few sec. then shut down. I suspect cleaning or replacing the flame detector. I see soot on cabinet,am I close? - Victa
Victa, that's a reasonable first guess.
Question: my oil burner keeps tripping at the cad cell relay
My oil burner keeps tripping at the cad cell relay I replaced the cad cell sensor and it's still doing it the burner will run for 10 to 12 hours fine and then trip off should I replace the whole relay - Bill
My oil furnace comes on sometimes I have to hit the reset button on the relay, then it goes on then off periodically while heating the house. What is likely the problem? - Ed
Bill and Ed: if the oil burner keeps tripping the cad cell relay and the relay sensor has been replaced, more likely the burner is running at too high a smoke level - running dirty- and needs cleaning and adjustment. The root cause could be several things: inadequate combustion air, bad nozzle, improper electrode adjustment etc.
Question: My furnace is not firing even though the thermostat is calling for heat
My furnace is not firing even though the thermostat is calling for heat. It is only when I hit the relay reset will the burner fire. The furnace then will run until its satisfied then turn off like normal. It runs correctly one or two more times when the thermostat is calling but then the same issue will occur. I tried to clean the eye on the sensor but it didn't make any difference. I also jumped out the thermostat (without hitting the reset) and the furnace did not fire. Should I just buy a new cad eye sensor or does this sound like a different issue all together? - Adam Kelly
Question: red reset button on the Protectorelay control keeps popping
The reset button on the oil primary keeps popping daily. I have had the oil furnce tech in to clean the furnace and replace the nozzle, filter, etc. I have replaced the fan motor last year and recently replaced the photo cell. I have heard the fan motor turn on however no light-off since the cleaning. What could be my possible problems and fixes? Tranformer?? Weak primary?? Thanks, Trent - Trent 1/6/12
Question: we keep losing heat and the cad cell and protectorelay trips off
My oil furnace occasionally does not light. It has been cleaned, inspected and gapped and the transformer was changed. It fired w/o problem for approximately one week then began to trip the cad. When reset, the transformer can be heard working when it lights (a faint buzz) yet when it does not light there is not a sound from the transformer. Both the old and new transformers were bench tested and function fine. Since the blower and transformer are wired to the same leads and the blower always works what should I have the technician check next? - Brian C 2/12/12
A repeated tripping cad cell usually means the system is running dirty or losing flame; it's easy to check and rule out a bad cad cell sensor itself as part of the troubleshooting - described in the article above.
Question: oil fired water heater won't start - I pressed the reset, nothing happens
my hot water oil-fired burner does not come on just put 50 gallons of fuel in, the boiler must have been down at least 1 to 2 months,what should i do, i have call for heat but no flashing lights on the transformer for my diagnostics so i cant reset the system nothing happens this is the first time this happend ran out of fuel before but always had no prolem starting back up, what should i do,it cant be the cad cell because i cant start the motor i need some help asap thank you!!!!! - Harvey 3/12/12
If really NOTHING happens, then your system has lost electrical power.
If the burner motor turns on but the burner won't ignite, quite possibly if the burner won't start and run it's because someone needs to bleed air from the fuel line; also when a system has been shut down for a long time it makes sense to change the filter, fuel unit screen, and to inspect, clean, and if appropriate change the oil burner nozzle, then check air settings and flame smoke level.
Question: thermostat calls for heat but nothing happens, how do I test the safety switch?
Eventhough I have the thermostat calling for heat my furnace still hasn't fired up. Is it possible that the (R8184G) not sending the signal or needs replacing? Is there anyway of testing this switch? - Russ 11/11/12
Yes as in the article above
Question: flame sensor keeps tripping, service tech says cleanout is needed
so last winter we had a power outage, all was well until then. After, the flame sensor would trip once and a while, then got progressively worse. I've replaced the sensor, and the controller, but the issue remains. If I short the sensor (after ignition) the system runs. I've cleaned the sensor, thinking it was fouling, but if I move the sensor to a viewing port (holding it in place) it appears to work fine. The port where the light comes in appears to be clear. We did have a service person come out shortly after the initial issue, but he said "I don't know, I can do an annual service, maybe it will help" - nope. Short of replacing the furnace, I'm stuck. - Gunter 12/14/12
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