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Very high stack temps last night....why?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by muncybob, Feb 16, 2013.

  1. muncybob

    muncybob Minister of Fire

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    The Mrs. was getting ready for her 3rd shift job last night and mentioned she smelled smoke...I thought maybe she had just blew out a candle but she said no. I went down to the basement and right away I smelled smoke and an odd odor. Turned lights on and could see smoke in the air near the boiler and mainly around the 8' insulated horizontal run of pipe to the chimney. I noticed my stack thermometer was pegged too(only goes to 500). Looking closer I could see the high temp insulation was smoldering and the pipes were cherry red! The high temp silicone sealing the pipes had disintigrated in several spots and one area actually had flame. Panic set in, quickly removed the insulation and smothered any flame. Ran outside to check the chimney...could not see anything and the chimney was cool to the touch....whew! I took the time to clean the pipe run into the chimney thinking maybe the 90 degree elbow at the bottom of the chimney was plugged with ash but I didn't get much out of it. Went and cleaned the boiler and since everything had cooled down I loaded up some wood and she ran fine all night. It took awhile for my nerves and the dogs to calm down though.

    A week or so ago I purposely let the firebox burn the coals way down and it seemed it was getting a bit too thick with them and I noticed that the stack temp had hit 500 while doing this. I knew last night that the box was needing wood soon but was stretching the reload until bedtime. What is causing such high stack temps when I'm down to coals and the unit is running? Normally it runs no higher than 425 but more like 375-400. I hesitate to put the insulation back on the pipes now...had originally done this to prevent an accidental burn and to keep the chimney hotter to lessen any condensation.

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  2. KenLockett

    KenLockett Member

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    Bob, whoa that was close! I often let my coals burn down when the bed gets to high like you mention. Never had a problem like that. You mentioned that you cleaned the 8' section. Was there just ash in there or did you have any creosote in there? I know when I disassembled my 6' section to chimney at the end of last season there was a LOT of crusty residue that had accumulated in the base of the pipe. It is later in the season. Did you have this same residue which to some degree probably has crusty creosote residue in it. If so I could see that combusting if it saw spark (from fine coals??). Not sure if stack temps of 500-600 could ignite it but sounds like the most likely culprit to me. Definitely a wake up call on my part. I wouldn't insulate. Mine is simply black stove pipe and I for one don't have to worry about condensing in that section. Of course I have a tall chimney with good draft. Better to see the exposed pipe I would say. Good luck and be careful. Might want to consider a stack high temp monitor as well. A lot of members on the forum have these as do I. I use mine to detect low boiler supply tx to tell me when to reload wood. Also can be used for high stack temp detection. Jim on the forum has one to shut his draft fan off on high stack temp. I use mine to drive a VERY loud buzzer. Let me know if you want the details on the setup for the monitor. You can install for less than $60.
  3. infinitymike

    infinitymike Minister of Fire

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    Woe that is scary. I thought you had it wrapped with a fire resistant insulation.
    I've never seen temps higher than 400 either.
    I assume its single wall?
    Was the cyclone plugged? I've seen chunks of charcoal on my roof and grass when the cyclone gets plugged.
    So maybe the cyclone was plugged and sending red hot embers into the horizontal run that began accumulating and heated up.
    When they burned through the fly ash just got sucked up the stack and thats why you didn't see any real pile of ash.
  4. Bret Chase

    Bret Chase Minister of Fire

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    That was close!

    what kind of "high temp" insulation were you using? At work, we use rockwool for any rated assemblies.... it cannot burn and melts at 2150F...
  5. Gasifier

    Gasifier Minister of Fire

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    Bob,

    How about real fine Ash. When you cleaned out the pipe did you have any real fine Ash in the pipe? It sounds to me like you had a fire in your pipe. Can be a scary thing. I like the fact that I changed my entire pipe over to insulated double wall from Ash cyclone all the way past the roof. The pipe is rated for a chimney fire. And I experienced a strange smell at one time and that is what it was, I had a fire in my chimney. I believe the smell was from the high heat tape I have on the joints. The tape is actually not high enough heat resistant, but it works good. Eventually I will buy some even higher heat tape. But I just shut the boiler down, waited for things to cool down and then took the pipe apart and cleaned everything. And there was fine ash in the pipe. A good bit of it. To fine for anything else in my book. The fire in the chimney can be a dangerous thing. Be very careful.

    I have learned, in my case anyway, that I need to check the Ash cyclone every other day. You have probably seen me say that before. But anytime the cyclone gets plugged you start depositing Ash and possibly moisture/creosote in your pipe. Another reason I do not like AHS's suggested design of horizontal pipe. If you are going to have horizontal pipe, for everyone's sake, make sure you clean them often. I have my short section of pipe from cyclone now at about a 45 degree angle right when it comes out of cyclone and then a short section of about two feet of horizontal when it goes through the wall. I take this whole 45 piece of pipe outside about once a month in the dead of winter when going through a lot of wood and clean it good. It is almost time for me to do that again.
  6. maple1

    maple1 Minister of Fire

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    Sure sounds like you had some creosote buildup that got ignited somehow.

    Glad things are OK - that can be scary.

    Wondering though why the creosote?
  7. flyingcow

    flyingcow Minister of Fire

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    I wouldn't insulate the pipe. Maybe use double wall pipe?

    Plus i would be curious how much ash you had in the pipe, causing air flow problems.
  8. Fred61

    Fred61 Minister of Fire

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    I think I had posted my experience with that particular situation back when several of the guys on the forum were installing their Wood Guns. I've been trying to give you Wood Gun owners a "heads up" on what to expect as time goes on. It wasn't creosote in the stack but rather a collection of small pieces of charcoal that finally ignited.
    I had this scare on several occasions. I also used single wall pipe with high temp silicone caulk on the joints. The silicone burns and turns into a crumbly substance. It also smokes like hell and smells bad.

    I was worried that it would burn through the pipe wall as it is so thin. The pipe would glow red.

    I think Gassifier has made progress in reducing the likelyhood of this happening by eliminating his horizontal run. Thr Wood Gun moves so much air that it carries lots of ash and burning charcoal into the flue and many times up and out of the top of the stack. The asphalt shingles surrounding my stack were all burned on the surface.
  9. Woodsrover

    Woodsrover Member

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    I concur that it was probably a build up of still burnable charcoal in your horizontal run. That, and you had an abundance of O2 in your exhaust gas.

    As the fire burns down and you're heating up a large amount of coal instead of making charcoal from your wood I suspect that two things are happening. One, your dragging a bunch of small charcoal through the chambers and into the cyclone. Second, the amount of fire your making with a small load of mostly burned wood isn't using up all the O2 that you're bringing into the firebox and it's going back up the chimney. O2 and glowing charcoal = fire where you don't want it. Not sure the remedy for this but perhaps closing the intake flapper all the way when you're trying to burn the fire down to nothing? Just a thought.

    Any way to eliminate your horizontal run? I would like to avoid that if possible. I have about 18" or horizontal pipe just where it goes through the wall to the vertical tri-wall pipe.
  10. muncybob

    muncybob Minister of Fire

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    There was fine ash in the pipe but no more than I have seen before. I run with the air intake flap at about 90% closed almost all of the time. I guess I could make the vertical pipe out of the cyclone shorter and make the horizontal pitch a bit more severe. I had thought with the air intake flap almost closed and the vertical pipe out of the cyclone at about 2' that I would not have embers into the horizontal run...guess I was wrong.

    I don't think I will put the insulation back on. It was only smoldering so it did handle the temps but it was hard to determine where the smoke was coming from at first until I pulled some of it off.

    Ken, send me the info on the high temp sensor you mentioned...thanx.
  11. muncybob

    muncybob Minister of Fire

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    Forgot to mention, I saw no evidence of creosote in the pipe run, cyclone or the bottom of the chimney.....unless it looks like fine ash after it has burned?
  12. Gasifier

    Gasifier Minister of Fire

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    Yup. That's what it looks like.
  13. CTFIRE

    CTFIRE Member

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    No fun. Happened to me a few weeks back. Mine was caused by some blockage but the pieces of charcoal definitely lit up. Might get rid of the horizontal run and have more of 45 degree slope in the spring. Although 30 minutes to clean it a month isn't bad.
  14. Gasifier

    Gasifier Minister of Fire

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    DSCF1677.JPG

    DSCF2393.JPG
  15. KenLockett

    KenLockett Member

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    Bob, the first picture below is of the small 1/16 DIN temperature controller. You can pick these up either on Ebay or on Amazon for about $30. I also have one from Automation Direct (currently powered up in picture, Amazon one not powered up on top) that has RS-485 communications for interface to a data logger or HMI and is also a little more sophisticated. That one cost about $100. For the high temp alarm monitoring you can use the inexpensive one. You need to buy one that is powered from 120VAC and that has an alarm relay contact (dry). Don't know if you are electrically inclined, but basically, I use an orange extension cord stripped back as you see in the picture to power the unit. In addition, I use the same source of power to 'wet' the dry alarm contact from the temperature controller (programmed for high Tx with small dead band of course) and in series with that wire into an alarm buzzer also pictured in the picture. I bought the small box and buzzer from Automation Direct (www.automationdirect.com) for about $12. To measure the stack temperature, you need to buy a stainless steel probe type thermocouple (I use a Type K) with a minimum 6' lead length. The thermocouple leads get wired into the temperature controller (these units for the most part have universal inputs (various type TCs and RTDs - type programmed via unit keys). The TC probe would be placed in the same hole as the mechanical type probe in the second picture in the horizontal run. I have seen the probes on Ebay and Amazon anywhere from $5 to $15. After reading your post this morning I am getting ready to buy a probe and wire into the power down temperature controller just as I am explaining to you. I have been waiting to get to this project for a while in addition to the one I have wired into the well for boiler supply Tx. Your incident this morning has motivated me to do it right away. I will send you the link to the one I find in the next day or so. Hopefully this brief explanation is sufficient. If not or if you are not electrically inclined let me know and I will sketch a schematic for the connections. Essentially, anytime the stack Tx (measure via the TC) rises above the programmed high tx limit the alarm contact closes and the buzzer is energized. The contact will open when the stack temps drops below the (limit - dead band). The dead band is also programmed via the keys on TC controller. I set my DB to 1 DegF. You can also wire a SPST switch in series with the alarm contact and buzzer to 'silence' or 'disable' the alarm circuit. There is a picture of a switch in my photo as well. I actually use the switch to disable the oil burner. My overall system is a little more sophisticated in that I wire the alarm contact and Tx back into a PLC and monitor boiler Tx and idle/fire cycles. If you are interested I can give you more info on this sometime. In your situation, a very simple high stack Tx monitor as I describe above would get you started. Hope this helps. If you find a TX controller and want me to take a look at the link or spec before you purchase just let me know. I got started right here on the forum when I saw someone post pics of these inexpensive Tx controllers.

    Tx Controller #1.JPG

    Tx Controller #2.jpg
  16. muncybob

    muncybob Minister of Fire

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    Ken, I'll be discussing this with my friend that is an electrician so any schematics that will be helpful to him will be appreciated. I want to install something that not only has an alarm but will also shut down the boiler.
    My options to modify the horizontal run are very limited and would result in providing only a slight increased incline to the chimney, probably not worth the effort. If the general consensus is a hot coal got into the pipe and lit up some creosote... I'm concerned. In the previous 3 years I've had a minimal amount of build up when the chimney has been cleaned. This year is much drier wood than before yet now I have creosote unless it was coals sitting there and they were ignited? I'll be inspecting the pipe more often now as previously I only pulled the cap mid season to clean out the bottom of the chimney where ash would build up. That end cap is a bugger to remove though, are there any easier ways to do this?
  17. maple1

    maple1 Minister of Fire

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    I'm now thinking Fred got it more right, and it's not creosote but rather pieces of charcoal (coals not burned out) that accumulated in the horizontal section, then started burning when more active ones got blown in on them with the right (wrong) air & draft conditions.

    Not sure though what you can do to prevent it - can you share a pic of your piping?
  18. KenLockett

    KenLockett Member

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    Bob, attached is a picture of the schematic. Let me know if you have any questions

    HIGH TX MONITOR - WIRING SCHEMATIC.JPG
  19. Fred61

    Fred61 Minister of Fire

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    Although I'm almost positive it is coals that ignited and not creosote, I'm surprised that the coals went through after changing the center bricks. I mostly had the problem when the bricks had eroded and allowed more coals to drop into the tube.

    After observing the differences in the speed that the logs break up into coals between super dry wood and wood that is slightly higher in moisture, I would bet that drier wood contributed to the situation.

    My set-up went vertical for about 6 feet out of the cyclone and a dog leg horizontal for about 2 feet then vertical up 4 feet straight into the double wall chimney.

    When these fires ignited in the horizontal area, they would burn the sealant and paint off the pipe all the way to the chimney connection.
  20. KenLockett

    KenLockett Member

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    Bob, with regard to shutting down your drafting fan. You could buy a Tx controller model that has two alarm contacts with one driving the alarm and the other wired in series with the existing draft fan circuit to shut if off. I for one would caution against this until your factory warranty on the boiler controls has expired. Jim has modified his draft fan circuit to accommodate this functionality. Perhaps he can chime in. There are a few other ways to implement the shutoff circuit as well. The alarm contact mentioned above could drive an interposing relay with multiple contacts. One contact to drive the alarm circuit (NO contact) and the other wired into the draft fan control circuit (NC contact). Wiring in this manner isolates the controls from the TX controller. The assumption here is that the draft fan circuit is very simple and not solid-state based. Again be careful if you are not an electrician nor electrically inclined.
  21. infinitymike

    infinitymike Minister of Fire

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    HEY, Don't forget about me! I have NO horizontal pipe. It is 11' from top of cyclone to top of cap.
  22. KenLockett

    KenLockett Member

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    Sounds like if you can't modify the horizontal section to get more incline as others have done then you're just going to have to clean it out more often. I have a white board right next to the boiler and list every time I clean and/or remove ash. Helps remind me when the essential tasks are due.
  23. Yankee

    Yankee Member

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    Creosote would have black granular residue, charcoal chunks would have grey powdery residue.
  24. Gasifier

    Gasifier Minister of Fire

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    It is time for me to check my pipe from the cyclone to the thimble. I am going to take it right apart tonight and clean it good. Then clean the boiler. -2 here early this morning.
  25. muncybob

    muncybob Minister of Fire

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    The residue was all grey...nothing dark and it was a very fine power. Gas, I was hoping the cyclone was blocked as I would prefer that than trying to control chunks of hot coals entering the the horizontal run but when I pulled the ash back toward the cyclone it fell thru the vertical pipe and wound up in the pan. Actually, I don't think my cyclone has ever been blocked. It's cold here today too but not too bad, just came in from the barn and it's creeping toward 30 now.
    Ken, I agree. I clean the boiler on a regular basis, just one more activity to add to the routine. I need to find an easier way to remove the cap on the T if I'm going to be doing this routinely. Thanks again for the info.
    Fred, I am surprised that this happened now with the nozzle being barely a month old, I would have thought this would more likely have happened when I was stretching the use of the old deteriorated nozzle! The higher stack temps that I have observed have happened only when down to nothing but a thick layer of coals. Yesterday I cleaned the box of a lot of ash and some of the coal bed down to probably only a few inches thick. I noticed this morning in mid run that the stack temp was only 325-350.
    My solution will be to install high temp sensor that will shut the boiler down. Giving my electrician buddy a call later.

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