Checking the Cad Cell On an Oil Furnace

[@franks9549]

This was helpful - almost ordered a primary control when didn't need it. Key point is after starting burner, immediately jumpering the CAD male leads at the control end to prove burner will continue to run. Best regards and great advice!!!

[@Notascab1903]

New to oil since moving to the east coast, these videos have been gold to me

[@DysfunctionJunctionOhio]

Omg you saved me I have had two different companies look at my furnace and one guy made it back fire !! Now I know the issue and I'm done paying guy to play a guessing game!

[@erinzway]

Very good explanation

[@quimshtgclg]

You might want to check resistance with alligator clips and not your fingers , your body has some resistance and can sque the reading . Just a suggestion.

[@johonneng4105]

My oil furnace one day stopped working, doesn't start. Oil pressure ok. But I changed the oil filter and nozzle. Again oil pressure ok. Again oil furnace doesn't start. Check the spark it's ok. Again oil furnace won't start. Check the peephole to see if I’m getting spark, no spark. Replace the cad cell and transformer. But I get a spark when I took the nozzle/electrode out of the burner head. So I left the peephole open, guess what? The furnace started. This is completely the opposite of what the cad cell does. I double-check the cad cell, high resistance with no light, low resistance with light. Again cad cell is brand new. The oil furnace only starts with the peephole open, then I close the peephole and it runs great, very smooth. But won’t start up the next time the thermostats want heat on. What’s going on???

[@bac4431]

Double check that your transformer is making good contact with your electrodes. Also double check that the new transformer has good spark. Although very rare a (tar baby) transformer can be bad out of the box and really through you off

[@Philosophymineoryours]

Did you ever figure pit what the problem was? I'm having the same issue as I type. Would love to get this thing back running, cold night are here. Any suggestions or how you got yours back running would be greatly appreciated.

[@slipperyjohnson7016]

I'm trying to fix my boiler and think the cad cell may be the issue. If I hit reset the boiler fires and runs fine. After it shuts down it won't come back on unless I hit reset again. I tried checking cad cell like this but I get nothing. With the transformer closed and the cad cell wires disconnected I get no reading at all. With the transformer open I'm getting like 6.9-7 k I believe. Anyone know what this means?

[@randymarcell]

I'm a Journeyman HVAC tech working with more gas/electric furnaces over 20 years. The unit does not sense the flame and locks out. I ohmed and cleaned cell. No change. Replaced cell and wire. Same thing. It keeps running only when I wait then jump F-F. It does not attempt to run as expected if I jump first. It seems it is the flame itself. I have a new primary R7284U to replace R8184G. What are your thoughts?

[@frederickkosier2508]

The cad cell has to see dark before it sees light. If you jump it out it will not light because it sees light first. What are your ohm readings ? It should be in the 1000’s when it sees dark and under 100 when it sees light

[@randymarcell]

@@frederickkosier2508 Yes. That's why i jumped it 1st to be sure the module prevented the sequence from starting. It was proof that the primary was not the problem

[@frederickkosier2508]

@@randymarcellthe primary may still be the problem if it runs after jumping out ff. I assume the bracket is aligned properly so the cad cell has a clean line of sight?

[@TheNYgolfer]

Sir the 10k reading was your body resistance. You can't have your fingers on the probes.

[@frederickkosier2508]

Your body does ad resistance but the difference between a cad cell seeing light or dark is in the thousands or tens of thousands. The resistance of the body has minimal affect

[@TheNYgolfer]

@@frederickkosier2508 You stated the high 10k resistance was due to the CAD " seeing darkness". That's incorrect. It's just the opposite. Light increases resistance. The brighter the light the higher the resistance. The dimmer the light the lower the resistance until total darkness causes the CAD to "open" and resistance goes to infinite. Since your burner was not fired up, the resistance reading through the CAD should have been VERY LOW. Probably under 50 ohms if some faint room light got to it. Otherwise in darkness it should show "open". You got a reading of 10k. So the only place such an egregiously false reading could have come from was from a defective CAD or by you introducing your body's resistance into the measurement by holding onto the wire leads of the CAD. 10k body resistance is not "negligible" when you are dealing with a CAD that responds in resistance variations ranging from "thousands or tens of thousands". Check your body resistance by grabbing one of the meter leads in each hand. You will read around 10k. Unless your hands are very dry. Then properly check the resistance reading of the CAD without your hands not touching anything but the insulated part of the meter leads. You will read very low to infinite resistance depending on whether the CAD is in complete darkness or not. If you checked an open 10k resistor the way you checked the CAD, the meter would read 10k and you would say the resistor is good and chase your tail looking elsewhere for the problem when the resistor was the problem all along.

[@chadharrison5681]

@@TheNYgolfer The cad cell is made from a cadmium sulfide coated ceramic disk with a conductive grid over its surface. Electrodes are attached to this surface, and the cell is sealed in glass to protect it. The cad cell in darkness has a very high resistance to the flow of electrical current. Yet, in the presence of visible light, it has a very low resistance.

[@garagekeys]

I thought no 2 fuel oil is hard to light with just a flame?

[@frederickkosier2508]

It’s a 10,000 - 20,000 volt spark that ignited it after it has been atomized by the nozzle.

[@garagekeys]

@@frederickkosier2508 thanks, I was meaning when you said throw a lit pair towel into the combustion chamber to burn the extra fuel. Does it light that easy?

[@frederickkosier2508]

@@garagekeys yes it does

[@garagekeys]

@@frederickkosier2508 thank you. I enjoy your videos

[@frederickkosier2508]

@@garagekeys thank you so much

[@riley4198]

That motor is definitely shot lmao