AFG cad cell not seeing very well

archibald tuttle

Got an AFG in a friends shop where i helped him with hydronic piping and its the backup to a wood boiler that's off on the reset. So I've been checking it in short doses on the way home and they're keeping the wood fires burning.

Fire is good. I didn't set it up but did a quick check and got trace smoke. The nozzle is clean as a whistle and checked out of the tube that it throws a beautiful atomized cone. electrodes are 1/16 forward 1/8" gap. everything seems right about the flame but the cad cell was only dropping to around 6000 ohms. didn't look dirty but cleaned it and got no result. replaced it with a new one and the ohms actually went up to 8000. tried a second mount that was a hair tighter fit for the plug in cad but no change. I did see a little dust build up around the 'eye' holes in the nozzle support plate that are just about level with the cad cell near the top of the tube. the electrode clamps are just aft of those 'eyes' and provide maybe 3/8 inch horizontal space where dust can build up. Cleaned those nicely and was sure I had it but no change.

I can't tell with that backview using cellphone inside the tube if its an F3 or F0. The boiler is fired at .75 which is right on the line and should be the lower of those. But I flipped the transformer while running it and see what i would believe is plenty of light in the tube. While I want to clean the head (couldn't do it on first quick stop because, this being an auto shop, they were thinking automotive when they put the burner on and siliconed it to the boiler so I gotta separate the front skin to get a chisel back there and pop it loose and put a fiber gasket back but I just can't see that there is that extensive of a build up on the head that could be making the difference on cad sensitivity to the tune of an extra 6 or 7 grand ohms.

First time for everything and I haven't had this grief before. I can't see that the cad isn't well aimed. I'm stumped but hopefully someone will tell me what I might be missing so i'm fully crowdsourced when i go back tomorrow.

thanks

MaxMercy

A friend of mine had his AFG locking out for the same reason. Combustion numbers were good, and it sounded and looked right. A new CAD didn't help (a little, but still too high). I cleaned the nozzle assy tube and sight holes and it got the resistance down to about 1400 ohms. I remembered having the same problem with another AFG years ago that started locking out after the electrodes were changed, and found the clamp that holds the ceramic was sagged under too much hold down screw torque which moved the clamp in line between the sight holes and the CAD cell. I hammered the gull wing flatter so that when it was installed, it was no longer blocking the sight holes. My friend had the same issue. After straightening the clamp so it didn't block the sight holes, it dropped to the 700 ohm range.

STEVEusaPA

Make/model #of boiler?
Nozzle size/brand/angle/solid, hollow or semi-solid?
Pump Pressure?

An analyzer would help. But there is a tremendous amount of wood dust on the air tube and it wouldn't surprise me to see a lot of dust in the burner housing and on the burner fan wheel. Not enough air moving results in poor combustion too.
Also the oil around the inside of the end cone tells me poor combustion/not enough air moving, or potential back drafting. Maybe it needs a good cleaning.

Don't know what 'a quick check and got a trace of smoke' means.

First steady state, proper draft, then smoke test to dial excess air into true zero smoke, then analyzer.

Next thing I would check is the nozzle line like @MaxMercy stated. Either the holes are blocked by saw dust, or closed from over tightening. Sometimes it helps to replace the cad cell eye AND the wire.

archibald tuttle

@MaxMercy i wondered aboit that clamp. It blocks maybe a quarter of the site holes. They didt really think that through super well. I wouldnt say it was sagged from original shape, and nothing changed. It ran just fine for a year after he put it in. I cant believe, especially where i clea ed out the dusy from within that clamp that its not seeing. I guess if i could get electrodes with thicker ceramic that would push the clamp back for additional visibility.  

@STEVEusaPA. I didnt have a drill so i pulled smoke out of the tee for the barometric vent and got a trace. I got low Co like 15 ppm and 11.5 % CO2. So the combustion. Its a thermodynamics S series steel. Im going to clean the head this weekend and ill pop the top and ck how sooty it is and ill drill a resr hole and dial in the amoke and do complete combustion, but, for the life of me i dont see anything that should be blanking the cad cell.  And it ran as is for a year. Certainly picking up a little dust in that enviroment (auto shop not wood shop but occasionally particulate challenged environment.) But i cleared that from the noozle bracket. It was another used mount and wire assembly that i had to try as an alternative but it seemed to take the plug tighter but made no difference. I guess i should try a new one to make sure if i keep getting higher than expected ohms.

EBEBRATT-Ed

Clean the inside of the blast tube and spray paint the inside silver.

Your OHMs seem way off a good signal is like 700-800 or lower.

Extend the cad cell leads and stick the cell in the combustion chamber peep hole (temporally) to see if you get a better reading. You may have good combustion and the cell could still be seeing a dark spot in the fire.

STEVEusaPA

Oh I can’t agree with painting the inside of the air tube silver. I know that’s an old timers trick, but pretty dangerous for a modern burner. It could allow poor combustion and let a burner keep running that should be shutting down on safety.
Do the proper cleaning and tuning, and let’s see where the ohms end up.
Btw, OP taking a smoke test thru the baro?

archibald tuttle

@STEVEusaPA

sorry, been busy between snow and cold. i actually thought i got a pretty good read through the baro. it was only a foot or so further from the breech than i would have had a test hole and it was veticle so the sniffer wasn't lying on the bottom of the pipe but grabbing from the middle of the stack flow at the entry to the tee. and i was able to use the spring on the smoke tester to gently pull back on the damper to help secure that location and make sure it wasn't admitting much air as i tested. i took it as friendly fly by on the way home and didn't have a drill in my car. i had thrown the testers in but didn't anticipate no test hole.

turns out a tiny down aim made immediate difference on the cad. i wouldn't have thought that a few degrees could make that much difference on a clean new cad cell, but it did.

learn something new every day.

brian

Big Ed_4

Long blast tubes( I think your is) were always a problem .. We use instal Beckett long tubes for GE conversions .. We use to paint the inside silver to get them to work . The Becket rep yelled at us .We ended relocating them .. You can try relocating the eye . They make a clamp which attached to the nozzle assembly tube ... Or loosen the side single screw that attaches the blast tube . Move the cad cell eye and holder to the side of the blast tube , use the last hole in the cad cell holder and retighten the blast tube screw into it. I end up doing this for Reznor waste oil heater , which is over loaded with parts in front of the eye. .... The dirt is side is not helping . Use a oily rag to clean .

EdTheHeaterMan

The fire side of the F3 Head (Yes that is the F3, not the F0) looks dirty. there is an easy to clean that without removing the burner. I invented this 30 years ago when those heads were getting fouled and I did not want to remove the burner to clean it.
RE Michel had this MillRose wire brush at the counter, next to all the fiber brushes for boiler cleaning. I cut off the end and inserted a 3/8" PEX handle. Stuck it in a vice and bent the handle like shown and put another 3/8" PEX handle on the end, then put a wire nut on the end to keep from losing the handle.

Now all I do is put the brush in the air tube and turn the handle counter-clockwise to get the brush to "thread" through the primary air opening and into the secondary air slots. I continue to turn the brush with a little back pressure to clean all the carbon away. Finally, I turn the brush clockwise to remove the brush. It does a great job.

As far as getting a bad Cad Cell reading to improve. That is trial and error. and I have done all the tricks mentioned above. When the bracket is bent from overtightening you just need to reverse the bend in a vice or with a hammer. Drilling the 2 small holes in the static plate a little larger also helps.

archibald tuttle

@EdTheHeaterMan love that. i will make one.

archibald tuttle

@edtheheaterman btw, do you identify that as F3 by the spacing you can see in the picture at the secondary slots?

brian

EdTheHeaterMan

archibald tuttle said:

@edtheheaterman btw, do you identify that as F3 by the spacing you can see in the picture at the secondary slots?

brian

Definitely F3 by the tertiary slots. The primary air is the round hole. Secondary air is the air that goes thru the radial slots. Those slots cause the low pressure to retain the flame at the head. Tertiary air is added (if needed) around the retention head to provide complete combustion for the oil that is burning further away from the burner head several inches inside the combustion chamber.

If you look at come burners like Carlin, Riello, and Wayne, the tertiary air is adjustable by moving the nozzle assembly either in or out of the air tube a few millimeters (or fractions of an inch) to obtain the correct air adjustment.

Respectfully submitted.
Mr.Ed

STEVEusaPA

You can't always tell by looking. F3, F6, F12 look pretty much the same from the inside.

archibald tuttle

got it. shows you what i know. didn't know the F0 didn't have tertiary slots and i was trying to estimate the clearance on the secondaries

Bronxtech

Make sure blower wheel is clean, shops usually get caked up. Make sure correct nozzle/pattern ,type for equipment.
Check Z dimension ! Imp.
Check oil pressure at nozzle line, install nozzle line solenoid valve/ cutoff /delay.
Good luck ! Let us know. John

Bronxtech

Btw, just rechecked ur photo, what happened to the end come / blast tube being back a tad ??? 

archibald tuttle

Bronxtech said:

Btw, just rechecked ur photo, what happened to the end come / blast tube being back a tad ??? 

are you saying that vis-a-vis the carbon on the retention head? of course getting the most accurate Z dimension on this one means taking the gun out of the boiler vs. the clean in place approach with @EdTheHeaterMan 's custom brush.

EdTheHeaterMan

Don know who invented it, but Beckett is now selling it. Another way to reduce the need to remove the burner during normal maintenance.

https://www.youtube.com/watch?v=v3t_EUnIKM0

BTW the proper name for that modified handle brush is "The EDvention"

Mr.Ed

HVACNUT

They've been around forever. Carlin has them also, for the EZ and 99,100 for gap and turbulator spacing. 

STEVEusaPA

HVACNUT said:

They've been around forever. Carlin has them also, for the EZ and 99,100 for gap and turbulator spacing. 

I think I have a blue one laying around that may have been for Wayne or Ducane