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Protectorelay, Cad cell relay & sensor questions & answers

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Cad cell relay / protectorelay FAQs for oil burners:

Questions & answers about the reset-button or resetting cad cell relays / protectorelays used on oil burners.

This article series explains the operation, use, and reset button on Cad Cell Relay Controls, including 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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Oil Fired Furnace or Boiler Cad Cell Relay / Protect-O-Relay FAQs

Honeywell R7294U 1004/U Universal electronic oil primary at

Questions & answers about cad cell relay or protect-o-relay primary control reset and reset button problems posted originally at this article RESET SWITCH, CAD CELL RELAY - topic home.

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:
- poor atomization
- dirty burner fan
- electrodes
- continuity
- cad cell eye
- re-positioning cad cell etc...

- Samuel Zapata



Sometimes as continuity check can miss a bad connection, but in any case given what you describe, it would be an inexpensive and trivial step to just plug in a new cad cell sensor and repeat the tests. The reset of the components are very straight forward, as you've described. You can test the cad cell's response to light before placing it back on the burner assembly. That step will tell you if the problem is the cad cell and its wiring or a smoky dirty burner operation.

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



Presuming your thermostat is calling for heat properly, it's certainly a trivial plug-in repair to try a new cad cell eye. But if the cad cell were bad I'd think that the burner would not continue to fire after startup. Try it anyway and let us know.

As you may already realize, when a burner is going off on reset that normally indicates a problem with the flame sensor (your theory) or with the burner flame itself.

IF that doesn't work you'll need to check some other possible causes:

- loose or bad electrical connection at the sensor or controls
- dirty nozzle, electrodes, burner turbulator
- bad transformer

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



If the oil heat service tech watches the burner start cycle s/he may at least be able to observe whether or not there is a difficulty with the flame start-up or not. A range of problems including a weak transformer, even a bad electrical connection, a defective fuel unit, dirty oil filter, etc. can cause intermittent startup difficulties.

There are oil heat equipment tests that can check out the transformer output, the oil line vacuum into the fuel unit, output pressure from the fuel unit, cad cell relay sensor (have you tried replacing the sensor element), etc.

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



I'm not sure your oil delivery of just 50 gallons was sufficient to guarantee an oil flow to the oil burner - it depends on tank size and what was in the tank.

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



Some other reasons that a good cad cell and control keep tripping off the oil burner include

- the cell mount position is not quite correct, or a mounting bracket has been bent, so that the sensor eye doesn't properly see the flame;

- the oil burner is out of adjustment, running dirty, making a smoky flame - your service tech could be right on


(Dec 19, 2012) Fergus said:

my primary control keeps tripping the safety because no flame is detected. I have bench checked the ignition transformer(work fine). Changed the nozzle, checked oil pump is working, when the reset is placed it starts. when the thermostat reaches requested temperature shut down.

Will not start no ignition on next thermostat request. How can I bench test the primary control to see if this is working intermittent.

(Dec 26, 2012) bob said:

good morning...oil furnace stopped working...there was a broken knob from the electrical box on the floor. the knob is similar to the one for the oil line. any way that may be the problem. thanks bob



Watch out: I'm not sure what's broken but damaged oil burner controls can make the system unsafe. It's time to ask for help from a trained service tech.

Question: oil burner firing trouble - cad cell ?

(Feb 28, 2013) steve said:

I have a oil burner, it has a delay in fireing which causes a combution in box. I put new cads eye in. steve 2/28/13

(Mar 9, 2014) Jg said:

I have an old Oneida royal oil burner, it has a delay in fireing which causes a lot of oil to enter in chamber box. I have to loosen the transformer and open it to get the burner to fire. Is this the Cad eye?


I'm not sure JG but I'm guessing the answer is no; the cad cell sees a flame and then keeps the burner going, or doesn't see flame and shuts it down; that's not associated with a delay in firing; the burner should start firing as soon as it receives oil (your system might have an oil delay or quickstop valve in place).

Question: no spark at oil burner

(Aug 2, 2014) Frank said:

two things ... motors running oil flowing seems i have no spark...1. is there a wiring schematic to replace the transformer. 2. could this be a cad cell problem .. af II beckett is there something I'm not checking / please help


Frank with power on and oil burner motor running if there is no ignition spark I too suspect the transformer. Wiring is trivial, most likely just 2 wires to follow.

If there is no spark for sure the cad cell relay will and should shut down the system.

Watch out: excessive un-burned oil in the combustion chamber can cause as puffback explosion when ignited.

Question: cad cell gasket to transformer?

(Sept 16, 2014) Cheryl Hlavaty said:

What is the black dense foam gasket called that is used between the cad cell eye and the transformer?
Where can this be obtained?



You've stumped me on this question. Having removed and installed cad cell sensors on oil burners I recall the sensor, a mounting clip, wiring, but no "dense foam gasket" - and I'm baffled since a foam gasket on an oil burner tube or close to the boiler might just melt or burn up.

Perhaps you can use the email found at our CONTACT link to send us some sharp photos for further comment.

Question: dirty cad cell & other questions

(Sept 27, 2014) sharlene said:

what is the wiring diagram for the cad cell to the furnace. We have wiring coming in from the electric switch we have the transformer and the cad cell. how do you wire the cad cell to the furnace older moncrief our furnace isn't kicking off keeps blowing heat even tho thermostat is turned off.

(Oct 15, 2014) Anonymous said:

Hi, i'm a boiler operator in Nigeria. I turn on the steam boiler in the morning and it fired normaly. When there's no demand for steam, i locked the outlet valve and the boiler pressure rised to the maximum level and it cut off according to setting. After 3hrs when steam is needed, i switched on the system but the burner refused to fire

. I checked every love thing but it won't fire. I powered the burner manually and i observed the diesel pump is not rotating. What could be the possible cause of this, and what's the possible solution.
Tunbosun (Nigeria)

Pls , you can send ur response to me through my mail,
It's very urgent, Thanks.
Tunbosun (Nigeria)

(Oct 28, 2014) kevin said:

oil boiler rans good cad cell will trip sometimes after 22 sec. someyimes after30 sec sometimes runs find could primary control be bad?

(Nov 21, 2014) Paco said:

I have a steam/oil residential system in a large 1929 mansion. My reset button keeps popping up. Maybe one time in five the low water cut-off red light comes on. My guess is the two problems are not directly related as the pump that returns the condensate does not seem to work all the time. Do you think the Cad Cell is just dirty?

Reply: just press the cad cell relay control ONCE -


when we re-start a heating system by pressing the re-set button we do it just ONCE. If you keep pressing the button, depending on what's wrong and on the fuel type, you could cause a dangerous puffback explosion.

The LWCO warning ? I'm unclear which reset button you're pushing. If it's on an oil burner control I would leave the system off and call for an emergency heat service. If a cad cell is dirty it's trivial to wipe it off, but if the dirtifying is caused by sooty oil burner operation that's a more serious trouble that means the burner needs to be cleaned and serviced.


12/6/2014 Linda Calvert said:

I have oil burner (Texaco Fuel Chief) has stopped running. Called person that I used last year, really liked the service job. Called him last week, but now he's retired. Recommended person, so I called for service repair. Tells me: Honeywell Junction Box has power in but no power out? Said need new junction box? Well the part cost $500.00 & his install charge would cost about the same. I no longer use boiler for heating house, just for hot water. The expense to remove oil boiler, hire plumber & unexpected cost, I decided to run boiler (hot water on demand).

But now, the price to repair, that's if he diagnois problem correctly? He was to quick diagnoising & didn't comment, boiler needed cleaning. Your information has been very helpful, think I need second opinion, but again Who? How can I find oil boiler service I can trust to diagnois problem and do repairs correctly. Linda from Kent County, Delaware.


Linda, If there is power in to an electrical junction box and no power out that sounds like a wiring or connector error that's pretty easy to fix. Even a regular electrical junction box complete is just a few dollars. All of the work is the labor. I suspect that someone is not communicating clearly about the problem and its solution. If you are uncertain start by calling the service manager at the heating service company and ask politely for a more understandable explanation of what's going on. Keep us posted. - Daniel


12/6/2014 Paul Cluxton said:

On two occasions a week apart in the night my oil furnace burner has abnormally come on and off rapidly (like a stutter or cough) until I could get it turned off by moving the thermostat down (moving thermostat up did not smooth it out). Fifteen minutes later I turned up the thermostat and it lit smooth and stayed on and cycled normally. What could be the cause and what can I do to not have this reoccur when we are not at home?



Any of several problems (dirty nozzle, clogged filter, low on oil, water in oil, air leaks into the oil line, bad draft) can cause an oil burner to have trouble starting. Although the unit may run "OK" later, I urge you to ask for a service call by an experienced oil heat tech not only to assure reliable heat in cold weather but to reduce the chances of a messy puffback explosion.

Setting the thermostat down will let the heat run less often but I'm not sure that reduces the risk since sitting "OFF" longer might actually increase the malfunction risk - depending on the underlying cause.

Keep us posted - what the tech reports will help others.

Question: can't reset the "stack reset button" after running out of oil.

(Jan 25, 2015) brian said:
Stack reset button does not restart the furnace. It buzzes for about 20 seconds. I had ran out of oil, and like before, I bled the lines and hit the reset. Heater started and ran, both flame and blower, for a few minutes, then stopped. Now when I hit reset, it only buzzes. What's wrong?



If you ran out of oil it's possible that the air bleed-out procedure remains incomplete.

If you really have a stack relay and not a cad cell relay as your safety control, see the repair procedures at


Anonymous said: "Your relay is dead"


Continue reading at RESET SWITCH, CAD CELL RELAY or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see CAD CELL PROTECT O RELAY FAQs-2 - more-recent questions and answers about cad cell control troubleshooting


Or see RESET SWITCH, HEATER REPAIR - how to fix a cad cell relay that has lost its red reset button





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