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Sudden change in max stove temperature

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by ncturner, Nov 19, 2012.

  1. ncturner

    ncturner New Member

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    I'm new here and read through some old threads to see if I could find the answer for this. No dice. I'm hoping one of you can help.

    I moved my Buck Model 91 (catalytic) to my upstairs fireplace, and it reached insane temperatures for the first few weeks. The thermometer (this is for the catalytic combuster) would regularly read 1000+. I read some threads about over firing, and I'm not sure if the catalytic temperature is the same as stove temperature. If it is, I guess I have been over firing, and this is bad. If not, I liked it. Here's the problem:
    Over the last few days, it has been operating in more normal (and less toasty... my wife is complaining) ranges; I can't get the catalytic thermometer above 700 or so. Same wood. Same weather conditions. I'm super-familiar with the stove. The only variable is that I reseated my door gasket (I replaced it this year) to eliminate a gap I had noticed. When I try closing the door with an imperfect seal to approximate the (about 1/16") gap that I eliminated by reseating the rope door gasket, I'm not able to regain those high temps. Any thoughts? Thanks!

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  2. BrowningBAR

    BrowningBAR Minister of Fire

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    If this is the cat temps, then 1,000+ degrees is fine. Stove temps, not so much.

    How old is the cat?
  3. ncturner

    ncturner New Member

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    Getting up there. 4-5 years old. Could it have suddenly died on me? There was this two-week burst of fantastic operation after I cleaned the cat and moved the stove upstairs, and then it went back to the less-awesome performance I have been used to after I fixed the rope gasket. (This could be a red herring.)
  4. begreen

    begreen Mooderator Staff Member

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    Have you examined the catalyst carefully for cracked or plugged cells?
  5. ncturner

    ncturner New Member

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    I cleaned it over the summer. Like I said, the stove burned beautifully for two weeks. It couldn't be plugged already, could it? It's a bear to remove.
  6. Joful

    Joful Minister of Fire

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    You will NOT hurt a catalyst by running it at 1000F, as this is right in the middle of the range for normal catalyst operating temperatures. Most manufacturers quote minimum "light off" temperature around 500F, and list a max operating temperature of 1800F. Even a few stints above 1800F are not going to hurt much, although you should endeavor to avoid it. When I see my cat holding 700 - 1300F, life is good, I'm happy.

    While you implied the wood before/after is from the same pile/vintage, are you sure you're burning the same species? Also, your implication that the 1000F was only observed after moving upstairs... did you not have a thermometer on it previously? Surely you must have seen it go to 1000F countless times before, if you had a thermometer.

    A loose door gasket will indeed cause the cat to go hot. I see my cat temp probe climb, even in bypass mode, when I have a door open or cracked. One possible scenario is that the bad door gasket may have been that way a while, and causing the catalyst to run very hot. In fact, much hotter than the 1000F you observed. This can cause the catalyst to fail prematurely, with no visible signs that there's a problem with the combuster. If this is the case, you may not be getting the cat to properly light off even at 700F, as a non-cat reburn requires more like 1100F to occur.

    One thing I would try to do is get the cat lit off, at 700F as it may be, and verify there is indeed nothing but steam (no black smoke) exiting my chimney. Then throttle the stove down to ~300F on the stove top thermometer. Then verify the cat is holding > 500F and there is still no smoke (just steam) coming out of the chimney. If you can't do this, I would infer the cat is probably dead.

    I believe Buck offered a later modification to your catalyst housing, to make it much simpler to remove / reinstall. You may want to call them and ask about this, as you're going to be removing it for inspection at some point soon. You mentioned the cat is 4 - 5 years old, which if you're burning 24/7, may just be typical end of life.
  7. begreen

    begreen Mooderator Staff Member

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    Could be just age unless something was burned that might have contaminated the cat. Can the cover be removed for inspection to see if it got plugged with flyash? If it all looks good, it may be failing. You could try to pickle it per Condar's instructions on page 6. http://www.condar.com/combustorcleaningmanual.pdf

    Or maybe the fire just needs a little more air now that the gasket is fixed?
  8. ncturner

    ncturner New Member

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    The cat is 4-5 years old. This summer, I cleaned it for the first time in years, which could account for the sudden improvement in performance. It got really hot a few times last week, possibly as a result of the faulty gasket. Could some temperature spikes have killed an older cat? Would that explain why I can't get good heat anymore?
  9. ncturner

    ncturner New Member

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    Thanks for all of the awesome input. You folks are awesome! I'll test my cat.
  10. Woody Stover

    Woody Stover Minister of Fire

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    The 91 manual states that 1400-1600 on the combustor is ideal. In my limited amount of burns with the stove so far, that is where it has generally been running with a load that's fully involved. It went into the 1700s once when I loaded some smaller lightweight splits onto too large of a coal bed and all of it got burning at once. When the combustor lights off, the probe temp generally goes over 1000 in short order. If there's no flame in the box but a good amount of off-gassing, I see orange reflections when looking in at different angles around the bypass rod. The combustor can be working even when it's not glowing, but the temp won't go as high. If you see smoke out of the stack, obviously the combustor isn't doing its job. I would think that if you can't break 700, you're probably not burning the smoke. I simmered the combustor, per the link that begreen posted, before I ever burned the stove, and it's been working well. I would try that before giving up on the combustor. I'm just wondering if this has something to do with the move upstairs, although you indicate that you were getting heat for the first couple of weeks. How tall is the chimney now? 8" liner all the way up, or what is the present setup?
    BTW, I have a surface meter on the front face above the door, about where the top of the firebox probably intersects the front face. It goes up around 500 when the load is burning well. At that temp, stove is really blowing some heat.
  11. ncturner

    ncturner New Member

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    Thanks for the Buck-specific details. I think the good performance I was getting can be attributed to the cleaning I did on the five year old (I did the math, finally) combustor. That 1400-1600 range is about where I was when I was getting the excellent heat. The visual you describe of the off-gassing in the firebox fits what I was seeing at that time. I think the gap in my door gasket burned out the cat entirely. I don't have a surface thermometer, but plan on putting a kiln pyrometer on there when I get home. 500 is a good, hot surface temp, eh? Good to know. No way I've been at that over the last few days. I'll definitely try reconditioning the combuster before shelling out $300 for a new one. Thanks again for the input. Let me know if you think of anything else.
  12. iceman

    iceman Minister of Fire

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    if you moved it upstairs chances are you may have lost some draft. but that isnt the whole problem, check the cat again..

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