Typical Cad cell readings

zepfan

What are typical cad cell readings on a Beckett AFG burner with a Honeywell R7184B primary control? The reason I ask this is that I had a no heat on a Kenmore furnace that was installed in 2005. I reset the burner, it fired off fine with no delay or sputter, and then locked out as soon as the lockout time (15 seconds) expired. I checked the cad cell and it was above 2700 ohms, too high. I did the normal maintenance changed the oil filter, nozzle and cleaned the furnace (it was not dirty at all) The new homeowner has only owned the home two years but it appears that the previous owner had the furnace serviced on a regular bases. While adjusting the burner to get the below readings:
Co2- 11.8% it was 9
overfire- .02"
in the stack- .05"
smoke- 0

After adjustment of the burner the cad cell reading was 900, low enough for the burner to stay online. As many oil burners that I have worked on I never check the cad reading unless I think it is an issue, this is something that is going to change moving forward. I have a hard time to believing that through adjustment the readings was so much lower, and it does not make sense how the burner was able to stay online up until this point when the burner was not that far out of adjustment, or dirty. I did not change the cad cell when I found that it was able to read within range. Any help would be much appreciated I am not completely sold on this issue not coming up again soon. Thanks to all

EBEBRATT-Ed

Havent done any small oil burners in a while. My memory says 900 is ok I think the danger zone is 1500 or so but others who know more will comment @STEVEusaPA ??

SuperTech

I typically see 100-600 ohms on your average AFG burner. I consider readings higher than 1000 ohms a problem that needs to be addressed.

STEVEusaPA

Yes Ohm readings are a good diagnostic indicator of good/bad flame. All new primary controls can display ohms while running.
Most will lock out at 1600 ohms, however there are burners that won't lock out until around the 2700 ohm readings. This is due to their design and the small hole in the head (turbulator) allowing less light to reach the cad cell. NX is a good example where if you don't have everything properly aligned, you could get nuisance lockouts.

The usual check out for a control is to put a 1500 ohm resistor across F-F. If the burner starts, the control is bad. Remove one wire, start burner. When it fires, reconnect the resistor across F-F. If at anytime it locks out, replace control. Important to make sure you do not have flame failure when checking control as it will not shut off and could flood the chamber with oil.

A properly tuned burner will display the lowest ohm reading.

As always, run to steady state.
Properly set draft.
Adjust to true zero smoke.
Read analyzer settings.
Air has a big influence on ohm readings. Which is why true zero smoke is important. So setting to true zero smoke, reading the CO2 number, and dropping it one percent is vital for proper operation. After you have that setting you can read the running ohms. To me, if your above 1200, you are either underfired, improper nozzle, impinging, or the heat exchanger is dirty.

Luckily I am the only person who works on my customers equipment, so I know they have the correct nozzle. With larger companies (and new customers) they can be way off, and a likely scenario of small nozzle/wrong head or just wrong nozzle, will equal poor combustion with Beckett burners.

So to answer the OP question, best combustion equals lowest ohms. Any number can be good, but higher numbers, approaching 1600 leave no room for error and could cause nuisance lockouts.

Riello doesn't have a way to check ohms, nor do they (I think) publish a reading that will cause their burner to lock out.
But you can put a regular cad cell/holder/wire into the air tube and take a cad cell reading and even adjust the air for true zero smoke.
This method was shown by the always awesome George Lanthier in articles and in the best book for oil burners Advanced Residential Oilburners:
https://www.firedragonent.com/ (book store opens back up in the spring.
Every oil burner tech should at least have:
-Advanced Residential Oilburners
-The Riello Oil Handbook
-Fuel Systems Handbook
There are also 6 wiring books and others.

Here's an article that you may find informative.
https://fueloilnews.com/2005/02/01/a-setup-wakeup/

Jellis

I Test the CAD cell during any tune up and no heat call. if it is over 1000 with good flame then i change either the EYE or the entire assembly. typically with a new eye it reads 400-600 ohms or so.
Riello cad cells can be tested by removing the cad eye from the primary control and taking an ohms reading. if you read less than 40,000 ohms without light and more than 150,000 ohms (if i remember right) with light than they say the cad eye is good in a riello.

Leonard

That was a Very interesting link about using cad cell as a smoke sensor.

STEVEusaPA

Leonard said:

That was a Very interesting link about using cad cell as a smoke sensor.

There's a better, full description in his book. I thought there was a complete description in one of his articles.

SuperTech

Thats an excellent post Steve, 100% agree with everything stated. Thank you so much for the recommended reading, I need to buy the Riello handbook. I've had a couple of them kick my butt this season with random lock outs.

STEVEusaPA

If you're up his way, taking his Advanced Riello class for oil is a good idea too. Get your NORA credits too, if you need them.

GBart

Beckett AF should have 300ohms or so.

The problem most Becketts suffer is this thing called a Mek-A-Nik, these are the people who CRANK everything until their eyes pop out of their heads.

What happens to the Beckett is a person can CRANK on the screw-bolt that holds the electrodes until the plate goes down and blocks the two holes that are supposed to allow light to the cad cell, the cure is to take it apart, put the plate in a vise and re flatten it, reinstall and realign the electrodes making sure not to block the hole again.

see pic, holes are to be clear

SuperTech

@GBart , thank you for that advice. Thats something that I have to keep an eye out for.

GBart

It happens so often and is easily overlooked.

Tighten VS Torque

STEVEusaPA

I usually give it a few whacks with the head of an adjustable to flatten it (removed of course). I also keep small pipe cleaning brush to poke thru the holes if they have lint blocking them. Same brush to clean a burner blower wheel.