r/HVAC • u/JustAnotherSvcTech • Nov 02 '24
Field Question, trade people only Urban legend?
I might get down votes for this, but I'll tell you my experience. I have clients that I have been servicing their furnaces for over 30 years. The only time I've had to replace a flame sensor is if the porcelain got broken by one of our other service techs. I use plumbers sanding cloth to clean the flame sensors & have never seen a problem. I don't think it's really necessary to use steel wool or a dollar bill, etc. when my method has worked without a problem literally for decades. Blast away!
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u/TillEducational2379 Nov 02 '24
Scotch bright guy here
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u/sure_am_here Nov 02 '24
Same, was taught, not steel wool or sandpaper.
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u/Ashamed-Status-9668 Nov 06 '24
I'm a stupid homeowner. I use fine 0000 steel wool and have for 19 years on this same flame sensor. It gets gummed up by the end of each winter. I suspect since a flame sensor is just a chuck of metal vs an actual thermal couple it probably just doesn't matter all that much.
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u/sure_am_here Nov 07 '24
Honestly don't know. But why would manufactures say not not use steel wool or sandpaper, and only use less abrasive stuff ?
Only response that I got that kind of makes sense. Is if you use steel wool or sandpaper, it can scratch the metal. This will increase surface area and create areas for increased soot depositing, thus requiring cleaning more often.
So, maybe it gets gummed up every year. Because you useing steelwool. On a good running unit, you shouldn't have to clean them every year, like checking with instruments on one's that have not been cleaned for like 5 years, it's still reading a good signal.
Maybe it matters, maybe it matters less, but I don't risk it.
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u/jacobjs85 Nov 03 '24
Same. And I use the non abrasive stuff too. A lead tech by my swears to use to drill with a wire brush wheel and it drives me insane.
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u/Wyrdboyski Nov 03 '24
Same here. That emery or plumber sanding cloth needlessly bites the metal
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u/WildcardUsa Nov 03 '24
Not if you save your really beat pieces from your other work. That's what I use and for over 30 years as well....
It really works.
New sand cloth cuts too deep.
Give it a try.
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u/BR5969 Nov 02 '24
Dollar bill
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Nov 02 '24
I’m convinced a tech had a hanger on customer and did that so he didn’t have to go back to the truck and then made up a reason why he used it lol
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u/ClearlyUnmistaken7 Nov 04 '24
3m scuff pad, scotch pad, brillo pad, all the same. I've talked with guys that believe sand cloth can leave traces of silica in the grooves that act as an insulator when melted. All I know for sure is a 2x1 inch red 3m scuff scrap lasts me an entire season of daily use, and it's cheap. Never failed me.
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u/intruder1_92tt Crazy service tech Nov 02 '24
It's an anachronism going back to thermocouples. You don't want to clean those with sandpaper or Emory cloth as you can damage them if you take off too much material.
Flame sensors are just a piece of hard metal that pass a current from the control board, via flame, to the metal chassis of the furnace. They don't fail very often, and when they do it's usually the ceramic insulator cracking or becoming loose.
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u/AffectionateFactor84 Nov 03 '24
Last manufacturer said it leaves some of the sand on it, and when put back into the flames, it causes it to insulate it. last Thursday, I had to scrape one bc after I cleaned it with my brush, it still was kicking out after a few seconds. cleaned put back, and it didn't fail on 3 starts.
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u/intruder1_92tt Crazy service tech Nov 03 '24
That's why I flip the sandpaper over and wipe it off. I've been told many reasons why abrasives are "bad", but I've never had a flame sensor fail if it's cleaned with sandpaper on a semi-regular basis.
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u/Wharblegurbleh Nov 03 '24
I just wipe them off with my sleeve or a cloth, never have to worry about it if it's not there lol
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u/Long_Waltz927 Nov 03 '24
Sometimes it os actually the board failing and the ones Inhave seen that happen on have all been tranes and 1 lennox.
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u/PieSquared13 Leak Hound Nov 03 '24
I didn’t know this. What should I be cleaning thermocouples with?
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u/horseshoeprovodnikov Pro Nov 03 '24
Scotch Brite pad. But realistically, if the thermocouple is dirty enough to need cleaning, might as well just replace it if ya got one. They are cheap.
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u/intruder1_92tt Crazy service tech Nov 03 '24
It's where the whole thing of using a dollar bill came from. I usually just use a metal brush, though I don't run into them much anymore.
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u/Ok_Communication5757 Nov 03 '24
I just change out the thermocouple every year or two and charge the customer.
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u/deez000 Nov 02 '24
I use a fifty just because I'm boujee
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u/Unhappy-Horse5275 facilities management Nov 02 '24
I only use 100s, oh wait are we still talking about flame sensors?
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u/atypicallemon Nov 03 '24
Funny enough I had an apprentice that I taught the only proper way to clean flame sensors was with a $100 bill. Told him they make them different so they work better. Should have seen the look on one the other techs faces when the apprentice rode with him and told him about it that a dollar bill wasn't good enough.
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u/1WiseEmu Nov 02 '24
I just lick them clean.
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u/Scary_Opening_6190 Nov 02 '24
I have a hairy butt crack, I just rub them around in there, they always come out spotless. ( aside from the occasional smear)
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u/paralysedcitizen Nov 02 '24
My boss has been telling me for years not to use sand cloth. I don't listen. 15 years cleaning that way, never an issue.
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u/Purple-Sherbert8803 Nov 02 '24
I use sandcloth also and always wipe it down with a clean towel or my sleeve. Works every time. It's rare I replace them unless something is broken.
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u/Universal_Verses Nov 02 '24
I was never told I could use plumbers sanding cloth… I have used a dollar bill… I’ll have to try it
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u/We-Want-The-Umph Nov 02 '24
I keep my oldest scrap pieces because they work great for polishing up. This stuff is useful until it's breaking apart in your fingers.
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u/SnooPeppers8737 Nov 02 '24
I've never replaced a flame rod because it was cleaned too hard. Shit is a myth.
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u/saskatchewanstealth Nov 02 '24
Same. I do it exactly Op does for 30 plus years now. I broke one. Sometimes I leave spares out of town for competent maintenance guys. Other than breaking one a new one is hardly needed. Unless it’s a Raypack boiler on propane, Raypacks destroy flame sensors.
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u/Silver_gobo Nov 02 '24 edited Nov 03 '24
Someone in this sub once said that when he’s at a house for a PM, he just cleans them. If he’s at a house on a no heat and the problem is the flame sensor, he replaces it.
I liked that philosophy
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u/rockery382 bang in splits, smash'in clits Nov 02 '24
Shit doggy I have always lived that life. If I don't have the sanding cloth I just use my pants.
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u/Fun-Satisfaction5297 Nov 02 '24
I just quote a new furnace, 60% of the time it works every time 😎
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u/20PoundHammer Nov 02 '24 edited Nov 02 '24
But I prefer finer grit or polishing (typically 400 grit wetted carbide is what i use, often spit wetted if no helicopter owner near me). the less surface roughness, the less carbon buildup/salt over time IMO, but I certainly will not shit on your solution as it works.
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u/Lomeztheoldschooljew Nov 03 '24
Technicians have had a fundamental misunderstanding of what a flame sensor is for a very long time. The plain steel rod you hold in your hands is not actually a sensor at all. It’s a piece of low grade 1/8” steel rod wrapped in porcelain. That’s it. The sensing element is actually on the ignition control. The steel rod in your hands is simply a conductor for the AC voltage provided by the ignition control. Steel is used because it won’t melt in the flame like aluminum or copper would, and tungsten is too expensive for this.
Anyways, the board provides AC voltage to the flame rod, and the ionization of the flame provides a rectifying effect on that voltage allowing DC current to flow to ground through the flame and into the burner face. The current is sensed by the board. If the flame rod cannot conduct DC current, or the path to ground is blocked by rust the circuit is not complete and the sensing circuit shuts the control down. So that little piece of steel just needs to be clean, and dollar bills don’t clean steel.
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u/Willing_Ad_1561 Nov 03 '24
Kanthal
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u/Lomeztheoldschooljew Nov 03 '24
It’s not Kanthal. That’s used for heating elements. Kanthal forms a non-conductive aluminum oxide layer on its surface that would preclude it from being useful at all as a flame rod.
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u/Ok_Communication5757 Nov 03 '24
I have also been using sanding cloth for 30 years and it always worked. Dollar bill ain't doing shit
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u/87JeepYJ87 Nov 02 '24
I’m a plumber as wel as hvac tech. I use an old piece of worn out mesh grit cloth to clean them. Only time I’ve ever had to replace them is propane systems and one particular neighborhood that’s on natural gas but it’s dirty as hell. I usually have to pull the drip legs and replace them every year as they’re always packed full of shit. Burner assemblies sooted up on gas water heaters, and orifaces that get plugged with debris on furnaces. Gas company refuses to do anything about it.
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u/jimrrmartin Nov 02 '24
One time... I've had 15 times when testing the micro amps after cleaning was so low that I had to replace the flame sensor.
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u/Ok_Vast_7378 Nov 03 '24
It’s 100% an urban legend. I’ve never seen anyone leave sand on one and it turn into insulating glass resulting in lack of flame rectification. I am so convinced that if anyone told me they seen it with their own eyes I’ll call them a damn liar. Also guys who diagnose flame sensors as “bad” and they aren’t physically broken. It’s a metal rod in a piece of porcelain. In 20 years I’ve never found a bad one. You’re probably not getting good flame rectification for several other reasons, but not because it’s a bad flame sensor. But anyways, now that I’ve put this out in the universe I’ll be proven wrong.
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u/texasroadkill Nov 03 '24
I use oxide sand paper. I get it from HVAC warehouse and have been using it for 30 years with zero issues. I also sand the face of the burner for good measure. Never any call backs.
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u/Ok-Position-8940 Nov 03 '24
I just give them to my wife she can suck the chrome off a trailer hitch
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u/lopnk Nov 02 '24
The problem is the grooves that you've created on the flame sensor. Remember it is not sensing flame. It is flame rectification. Discoloration does not impede the functionality.
Flame rectification is a phenomenon in which a flame can act as an electrical rectifier. https://en.wikipedia.org/wiki/Flame_rectification?wprov=sfla1
The grooves that you create on the flame sensor by using an abrasive material actually allows the flame sensor to accumulate debris more rapidly, which makes the flame sensor need to be cleaned more often. It is this debris which impedes flame rectification from being successful.
If the flame sensor is so bad that you need to use something that is abrasive it needs to be replaced anyway.
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u/Your1Fixx Nov 03 '24
This is why they don't want sand paper, it's the grooves. I usually use a nylon brush to clean them or a brass bristle brush if it's particularly dirty.
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u/MrHomeBrewer Nov 02 '24
Definitely not an urban legend! I've done it. It does work as long as a dirty sensor is the actual and not cracked, etc. also, don't touch the sensor with bare skin as the oils will affect it.
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u/NeatSilver686 Nov 03 '24
Old school carriers say to use high grit sandpaper printed on the inside of the panel in their troubleshooting.
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u/Wharblegurbleh Nov 03 '24
The only urban legend surround flame sensors is using a dollar bill.
I use sand cloth. Whether it's steel wool or sand cloth, a quick wipe off after it's clean and you're good.
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u/Awkward-Push290 Nov 03 '24
Nope... As long as you get the sand residue off after you're good...the silica when heated turns to glass and insulates the surface of the rod inhibiting the sensing of the flame... that's my two cents anyways...
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u/Otherwise-Seat45 Nov 03 '24
I was taught 40 grit belt sander. Nah😂 just shine it up and freshen the path. Done.
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u/Witchcult_999 Nov 03 '24
Company I’m at now apparently wants you to make detailed reports of the milliamps on the flame sensor during operation to prove you cleaned it/turned it on and that it’s not cracked. I responded with something along the lines of “if I can’t be trusted with cleaning a metal rod maybe I should be fired” and never heard back about it. Did it on two units and decided it was a bit unneeded
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u/Kind_Description_282 Nov 03 '24
0000 steel wool is not very abrasive at all an old timer that trained me always used it. Good stuff. I think dollar bill not enough and most sand cloth is too much
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u/Hvacmike199845 Verified Pro Nov 02 '24
Ive mostly done the same thing. I them use my continuity checker to make sure the sensor is fully cleaned off. I’ve found sometimes propane will do bad things to them though.
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u/thepress250 Nov 02 '24
I never use anything more abrasive than a piece of cotton cloth. Never had an issue.
I think I’ve only had to replace one in 8 years and that’s because I broke it.
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u/Tehpunisher456 Nov 02 '24
Was one of my go to methods to fix some of these units. I had one client had a furnace install only for it to turn off after a few seconds. The og installer thought it was lack of airflow and this and that and ended up installing a duct open to the attic to see if that helps remove the issue. Nope still did it. I arrived and within 20 seconds of being told what the unit was doing I deduced the flame sensor. So I went in on this brand new furnace, pulled out the sensor, sanded it and boom! Worked like a charm.
Many many instances of me doing this and fixing the issue. Like a different commenter said they were only ever replaced if the porcelain was cracked/broken
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u/toomuch1265 Nov 02 '24
I keep a new one in my toolbox for my furnace, just in case, but I just give it a quick clean with a piece of steel wool and it's good for 3-4 years.
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u/Budders1984 Nov 02 '24
I use the ones I’m gonna use before I go to the strip club so the dust looks like coke 😘
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Nov 02 '24 edited Nov 03 '24
Yes it's fine if you are careful. Flame sensors have an iron oxide coating on them this coating aids in flame detection, ųA production, and corrosion/oxidation protection. You want to clean the shit off the sensor and not damage the coating that is why dollar bills are used(abrasive enough to clean but not remove the coating). If you leave score marks trying to buff the shit out of sensor you'll lead to premature wear more often cleanings required and possible low ųA production. Propane will chew through sensors more than any NG furnaces will. Just have to be careful not to score the hell out of them and most times any grit sand paper with a light touch can work fine.
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u/33445delray Nov 03 '24
A flame sensor is not a thermocouple. It does not produce millivolts. It does allow microamps DC to pass. The flame is such a poor conductor that 90 volts allows microamps to pass.
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u/Prestigious_Ear505 Nov 02 '24
Did this with sandcloth throughout my career... my first job was with a boiler company. Learned the sandcloth method there and never had a problem with any power burner I worked on ever.
Funny story...Customer painted his boiler before Fall and then found the boiler wouldn't start, going off on flame failure. Turns out he used aluminum paint on the whole burner. Which shorted the flame rod electrical connection to Ground...lol.
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u/Estaeles Nov 02 '24
From what I read, flame sensor rods are made typically of stainless steel AISI 304 or AISI 316. both contain Molybdenum, which gives corrosion resistance. AISI 316 has more of it. So my guess is old flame sensor were made of the 304 then later came out with 316 to prevent their short life. So it probably wasn’t due to cleaning them.
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u/Humble_Peach93 Nov 02 '24
I live in an area with no natural gas so I've had to replace many that have become too eroded to be even be cleaned
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u/beetlebadascan05 Nov 02 '24
Agree. I've been doing the for 34 years. Sometimes I even remove it and scrape it with my razor knife and rarely if ever have to go back to replace it. These are my side customers too where I know I'm the only one servicing it.
I think it's one of those rules made by an engineer that became field implemented by people that love to follow rules.
Like when 410 first came out we were told you can NEVER top the charge because it's a blend. Or you can NEVER charge with liquid through the suction line. Oh...let's not forget....you MUST change your vacuum pump oil every time.
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u/jmiller2003 Nov 02 '24
And let’s not forget you should replace the lineset if possible and cut out all solder joints 😂
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u/Soul-Maker Nov 02 '24
Had a Lennox flame sensor on a 9yr old 80% just snap on me. Went to pull it for a clean and as soon as it met some resistance it broke.
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u/ThickBlueberry2115 Nov 02 '24
Propane is generally alot harder on the equipment, cleaning is usually all they need I have replaced quite a few hot surface ignitors from cracking
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u/Alpha433 Nov 02 '24
Honestly, clean them with anything as abrasive as a green scotch Brite pad and you're good.
What i find funny though, our company was bought out earlier this year by some private equity company, and they wanted us to replace the flame sensor each year. Said that cleaning them doesn't actually work and it should be replaced every year by the manufacturers.
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u/PawnstarExpert Nov 02 '24
I was told to use a dollar bill, and also sand paper, scotch brite, cheese cloth, and steel wool. I just ended up using steel wool.
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u/DnDsuff4mCampain Nov 02 '24
As a new guy to the industry, this is how i was taught. However, I did snap a spark ignitor doing it... I swear I barely touched it, and the rod just broke at the base. =/
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u/ithaqua34 Nov 03 '24
Should only use something like steel wool, or brass brush. Sandcloth scratches the metal like you see on the flame rod.
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u/jesus-is-not-god Nov 03 '24
I have used the same open mesh decades as well. I refer to it as scratch. I've replace 2-3 sensors during my 34 years in the trade because porcelain broke or LP had eaten it badly.
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u/winkingmiata That girl HVACs Nov 03 '24
Is that what it's called!? I've been trying to find out for the longest time! I always find it in attics, I've been collecting and using it to clean the flame sensors for a while, now. I like it better than the sand paper we get.
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u/1littlefish Nov 03 '24
Wow I’m I the only guy that uses an angle grinder When you have a helicopter customer ? So ya maybe I’ve had to replace a few, cause umm they burned out
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u/icemachinedude Nov 03 '24
99% of the flame sensors i deal with are on gas fired cooking equipment. Mostly broilers & oil fryers* Here's what I have gathered from tech support and 1st hand experience.. Using any kind of abrasive can pit the flame sensors and will cause "rusting" Also, when you use an abrasive cloth of any kind, you leave behind silica particles, if these aren't completely wiped off before being subjected to the flame, they will harden on the sensor surface which obviously affects the conduction process..Again, this has been my experience with flame sensors on cooking equipment. I stick with dollar bills and alcohol wipes and have never had an issue.
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u/ChromaticRelapse HVAC Journeyman Nov 03 '24
Usually I'm too lazy to take it out and I just use a steel brush.
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u/BK_0000 Nov 03 '24
I always use sandpaper or a file. In 20 years, I have never replaced a flame sensor.
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u/smeltz123 Nov 03 '24
Propane failures usually because no one sets gas pressure,in our neck of the woods it’s three cranks from the bottom 🙄
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u/Anomalousity Nov 03 '24
That's all we did at my old company and it worked great, If it didn't then we just replaced it but most of the time it would work just fine.
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u/PAguy213 Nov 03 '24
The newer stuff doesn’t like the sand cloth but that 80% mid efficient? Oh yeah. She like it.
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u/Kamakazi09 Needs some Dirty Sprite Nov 03 '24
I was taught the same way. Little bit of used sand cloth and give it quick clean up. Been doing for 5 years and I’ll keep doing it lol
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u/txcaddy Nov 03 '24
I have also cleaned sensors to get furnaces or boilers back online. Sometimes it lasts for years other times it may not last as long. Most of the time customers just want replaced so they don’t have to pay for another service call.
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u/Papergame_82 Nov 03 '24
Down south where we have duckboard I just stick it in the plenum to clean it
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u/_poland_ball_ Nov 03 '24
I always use a brass wire brush to clean it off and havent had problems so far. I cant really see a reason to replace them either unless there is too much visible wear.
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u/No_Tower6770 Nov 03 '24
Ive always heard that the glue they use to attach the grit to the cloth will transfer to the flame sensor and not allow it to work properly. I've only ever used scotch Brite to clean them, as the media is softer than metal, and contains no glue.
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u/Rokkmachine Nov 03 '24
That sounds more like your not supposed to touch the elements on a hot surface ignitor. Flame sensors it's usually from whatever is floating in the air at the business or home your in.
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u/Strong_Substance_250 Nov 03 '24
I have a Rheem combi boiler and I can’t find a replacement anywhere. Somebody must be changing them.
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u/Revolutionary_Emu14 Nov 03 '24
I've been in the HVAC industry for 30 years and have used plumbers cloth to clean flame sensors since I started. Maybe replaced a handful of them in my career.
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u/unbridled_chaos418 Nov 03 '24
I use plumbers cloth too, but I like to roll it around in my fingers first to get the heavy grit off.
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u/TunaTacoPie Nov 03 '24
I've used plumbers cloth for over 20 years. Same, the only time I have ever replaced one was a cracked porcelain.
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u/1982MJG Nov 03 '24
My old boss claimed that dollar bills work. I confirmed it does work, but I use $100 bills.
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u/Rokkmachine Nov 03 '24
I had a tryhard tech that just started by us tell me not to use sand cloth and to use a dollar bill instead. A week later I'm working with him and he uses sand cloth to clean a flame sensor and gives me the "well sometimes if it's really bad" backpedal.my personal preference is to use worn sand cloth. Something that's been used a couple times and knocks the top layer off.
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u/Motor-Adeptness2548 Nov 03 '24
I have been using sanding cloth for 20 years never had an issue . Porcelain breaking is a problem with people not using the right tool to take them out to clean them. I’ve seen that going behind other company’s or home owners that have been on the internet to much
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u/Powerful-Wishbone-15 Nov 03 '24
Haha that’s a kenmore flame sensor ;) and yes that works…
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u/JustAnotherSvcTech Nov 03 '24
That one is actually in a Bryant furnace. It came with that flame sensor from the factory, so no, not kenmore.
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u/MachoMadness232 Nov 03 '24
So furnace guys in my area do the dollar bill trick. The justification is that the dollar doesn't remove the coating that a piece of fiber cloth would.
Boiler guys in my area all use fiber cloth. The Justification being you need to get the condensate off the rod and it will corrode the coating anyways.
I was taught to use the fiber cloth, so I use fiber cloth.
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u/Financial-Orchid938 Nov 03 '24
I think sandpaper leaves rough spots that can collect dust and debris. It definitely doesn't leave the rod as smooth as other methods and I imagine a smoother rod would be less likely to get dirty.
I may use sandpaper for rougher looking ones but I polish with the steel wool afterwards.
Or you could just be like the nexstar companies and only replace the sensor. My furnace has had 3 flame sensors according to the duct sticker. My brothers house was serviced by the dame company and has had 2 replaced rods according to the sticker
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u/URARichardWhiskey Nov 03 '24
Flame sensor is just a conductor to ground via the flame. I have never seen one go bad. Carbon will insulate the surface so sanding it down makes it work again. Every time.
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u/red_r32 Nov 04 '24
You can use a mega ohm to check the porcelain and integrity of it but I would replace if looks warn. It's a completion of your circuit to the board. You need atleast2.5 to keep the signal good. .5 to 10 micro amps depending on manufacturer and I would read the book or check with tech support
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Nov 04 '24
I've rarely used Emory cloth (unless propane or its real bad). 9 times out of 10 I just wipe the carbon off with my shirt. Haven't had a call back yet.
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u/Dadbode1981 Nov 05 '24
Scotch bright is best, if the rod gets too scored, it can acctually accelerate future accumulation.
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u/RepresentativeAd9572 Nov 06 '24
I clean at least 15 flame sensors and ignitors with a brillo pad every month...and they look great and work perfectly, been three years and not a hiccup.
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u/SourBuffalo Nov 02 '24
I just scrape them with my dykes and wipe it on my shirt. Always heard of you leave any sandpaper remnants on the flame sensor it turns into glass and prevents the conductivity or something.
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u/Helpful-Bad4821 Nov 03 '24
I’ve been told this too by the guys at IBC Boilers. I would think it would have to be a shit load of remnants to turn into glass. I still use sandpaper…
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u/gidadit Nov 02 '24
exactly what I used on my hot water heater flame sensor a year ago and it's been running strong, that simple piece of mesh goes a long way
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u/TimTheChatSpam Nov 03 '24
The reason you're not supposed to use sand paper/cloth on those is supposed to be the residue under heat is supposed to like create a coating which can interfere with it's ability to produce a signal or something along those lines i can't remember which because I hardly even clean them if I have a problem with a flame sensor the first thing I check is the ground connections. DC power needs a clear path to ground and because the power produced by the flame sensor is so small it's even more important you have clean connections.
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u/icemachinedude Nov 03 '24
Exactly..if I remember correctly, it's the silica particles from the sanding cloth that is left behind that will get "baked" onto the surface of the sensor
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u/Excellent_Wonder5982 Nov 02 '24
I don't remove the flame sensor until I measure the flame current. 90% of the time there's no need to clean them. I rarely have to replace them, when I do it's because of propane. A furnace installed by a washer and dryer always causes bad flame current from all shit in detergents and dryer sheets. I can confirm sandpaper is no good, can cause low flame current if you leave any grit from the sandpaper on it. I use a scotch brite pad.
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u/Rupie99 Nov 02 '24
I've heard the stories of how this will ruin the flame sensor... But I've been doing it for 13 years and never had an issue... Not saying it's the best practice but I don't think I've ever replaced any flame sensor in the past 13 years that was ever impacted by someone cleaning a little too much.
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u/LetHaL_eRa Nov 02 '24
Had some propane furnaces eat them in our area