A Guide to Cad Cell Relay Switches on Oil Burners
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Cad Cell Relay Controls: a guide to inspecting and re-setting the cad cell relay safety device on oil burners
Questions and Answers about Cad Cell Relays and Cadmium
Questions & Answers about sources of cadmium in homes
Reset buttons on heating systems: where are they, how to reset
Oil burner cad cell trouble: what goes wrong and what to do about it
Questions & answers about cad cell relay safety controls on oil burners

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

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.

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Photograph of an oil fired heater stack relay

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).[2]

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

Photograph of an oil burner cad cell relay switch

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 see: AQUASTAT CONTROL Functions 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 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.

The photograph at left 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 Broken - Quick Repair we show how to reset the equipment safely if this happens to you.

How to Test the Oil Burner Cadmium Cell Relay Switch & Switch Sensor

Button & Switch Tests of The Cad Cell Relay

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 the Safety Switch "Test" Lever to Test the Primary Heating System Control

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

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 .5 to .6 ohms 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. [1] 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.)

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"

An oil burner that is adjusted with excessive combustion air can sometimes produce a flame that is abnormally bright, giving abnormal cad cell readings, probably too low.
An oil burner that uses an oil nozzle that produces a very narrow flame, combined with the high mount of the cad cell in the burner tube, can make it difficult for the cad cell to see the flame, even if the burner is operating properly. Some oil heat service techs used to polish up internal components in the burner tube or even spray aluminum paint on surfaces to improve reflectance to help out the cad cell. In our OPINION those repairs are likely to be short-lived.
An oil burner whose squirrel cage fan is dirt-clogged will eventually not get enough combustion air. The result, at least for a while, is that the cad cell sees what was a narrow flame pattern before, even better as the weak flame curls up into view of the cad eye. But of course weak oil burner flame is sooty flame which is likely to eventually soot up the cad cell eye so that it becomes blind. It will then shut the system down. Finally.
Modern oil burners in heaters with a small, short combustion chamber use oil burner nozzles that produce a short flame. When the flame is burning well (without sooty curling up flame ends) it may be harder for the cad cell to see the fire.

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. [2] A series of LED flashes indicates the cad cell resistance in ohms (for this control) as follows:

Cad Cell Resistance (R7184 Series Primary Controls)[2]
Number of LED Flashes	Cad Cell Resistance (ohms)
1	0-400
2	400-800
3	800-1600
4	> 1600

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).

Questions & Answers about Cad Cells and Cadmium in Building Products & Coatings

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:

Auto exhaust can move cadmium into soils around highways (and into the air)

Some batteries or battery manufacturing facilities
Metal containers (some such as some ice cube trays or even bowls)
Foods: certain grains, potatoes, vegetables, depending on the soil in which they are grown - so watch for spinach grown along a busy highway; some shellfish are high in cadmium.
Metal water piping if the water supply is chemically aggressive and leaches out cadmium
Natural occurrences at very low levels in the soil, in air, and even in water.
Nearby industrial waste burning plants, metal plating or metal working businesses
Tobacco smoke, smoking or being around smokers, possibly also from cigarette papers
Paints (some, not all) particularly in some paint pigments ("cadmium yellow")
The U.S. EPA has some helpful and rather authoritative articles on cadmium sources, cadmium poisoning, and particularly worrisome cadmium sources. For example see the U.S EPA website on Lead and Cadmium in toy jewelry - http://www.epa.gov/lead/pubs/toyjewelry.htm

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

Reply:

Samuel,

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

Reply:

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

Reply:

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

Reply:

Adam,

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

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[1] Thanks to reader S.M. for careful editing and noting that the ohmmeter reading on a cad cell test with the burner is NOT firing should be close to infinity.
[2] Honeywell R7184 Series Primary Controls, Technicians Quick Reference Guide, Honeywell International, Inc., 1985 Douglas Drive North, Golden Valley MN 55422 customer.honeywell.com and in Canada: Honeywell Limited-Honeywell Limited, 35 Dynamic Drive, Toronto Ontario M1V 429
[3] Thanks to Bottini Fuel service. Bottini Fuel is a residential and commercial heating oil distributor and oil heat service company in Wappingers Falls, NY and with offices in other New York locations. Bottini Fuel, 2785 W Main St, Wappingers Falls NY, 12590-1576 (845) 297-5580 more contact information for Bottini Fuel

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Fuel Oil & Oil Heating Magazine, 3621 Hill Rd., Parsippany, NJ 07054, 973-331-9545
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Inspection Procedures for Oil-Fired Heating Systems Detailed step by step approaches for inspecting complex systems]
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Oil Tanks - The Oil Storage Tank Information Website: Buried or Above Ground Oil Tank Inspection, Testing, Cleanup, Abandonment of Oil Tanks
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Domestic and Commercial Oil Burners, Charles H. Burkhardt, McGraw Hill Book Company, New York 3rd Ed 1969.
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The Steam Book, 1984, Training and Education Department, Fluid Handling Division, ITT [probably out of print, possibly available from several home inspection supply companies] Fuel Oil and Oil Heat Magazine, October 1990, offers an update,
Principles of Steam Heating, $13.25 includes postage. Fuel oil & Oil Heat Magazine, 389 Passaic Ave., Fairfield, NJ 07004.
The Lost Art of Steam Heating, Dan Holohan, 516-579-3046 FAX
Principles of Steam Heating, Dan Holohan, technical editor of Fuel Oil and Oil Heat magazine, 389 Passaic Ave., Fairfield, NJ 07004 ($12.+1.25 postage/handling).
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"Warm Air Heating Systems". Instructional Technologies Institute, Inc., 145 "D" Grassy Plain St., Bethel, CT 06801 800/227-1663 [home inspection training material] 1987
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Boilers, Boiler Conversions, James E. Brumbaugh, ISBN 0-672-23389-4 (v. 1) Volume II, Oil, Gas, and Coal Burners, Controls, Ducts, Piping, Valves, James E. Brumbaugh, ISBN 0-672-23390-7 (v. 2) Volume III, Radiant Heating, Water Heaters, Ventilation, Air Conditioning, Heat Pumps, Air Cleaners, James E. Brumbaugh, ISBN 0-672-23383-5 (v. 3) or ISBN 0-672-23380-0 (set) Special Sales Director, Macmillan Publishing Co., 866 Third Ave., New York, NY 10022. Macmillan Publishing Co., NY
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Installation Guide #200, The Hydronics Institute, 35 Russo Place, Berkeley Heights, NJ 07922
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A Guide to Oil Fired Furnace or Heating Boiler Cad Cell Relays found on Oil Burners

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

Button & Switch Tests of The Cad Cell Relay

Using the Safety Switch "Test" Lever to Test the Primary Heating System Control

Resistance Testing the Cad Cell Sensor using a VOM or Digital VOM

Other Cad Cell Sensor & Oil Burner Cad Cell Primary Control Troubles

Automated LED Readouts Give Cad Cell Relay Condition

Notes to the Cad Cell Resistance Table

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

Questions & Answers about Cad Cells and Cadmium in Building Products & Coatings

Question: Cadmium poisoning report: could my heating boiler cad cell be involved?

Reply: Cad Cell Seems Unlikely to Be a Hazard, Here's What & How to Check

Question: proper ohms readings at a cad cell relay

Reply:

Question: my oil fired furnace fires up but the blower in the air handler doesn't turn on

Reply:

Question: my oil burner keeps tripping at the cad cell relay

Reply:

Question: My furnace is not firing even though the thermostat is calling for heat

Reply:

Questions & Answers regarding this article

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