1. Welcome Hearth.com Guests and Visitors - Please enjoy our forums!
    Hearth.com GOLD Sponsors who help bring the site content to you:
    Hearthstone Soapstone and Cast-Iron stoves( Wood, Gas or Pellet Stoves and Inserts)

Flue temp vs. Stove top temp, and cat light-off

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Joful, Mar 4, 2013.

  1. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    So, I've always had trouble with the notion that stove-top temp is a good indicator of when to close the bypass damper for successful cat light-off. There are too many factors (slow time constant of most stove-tops, moisture baking off less-than-ideally-seasoned wood, starting flue and stove temps, etc.) to have any great correlation between cat or flue gas temps, and stove top temp.

    Thinking that flue temp must be a better metric than stove top temp, for this purpose, I've been shooting my single-wall stovepipe with the IR gun before engaging the cat. I find that I always get good light-off when the stove-pipe is close to 600F, and rarely if ever get light-off if the flue wall is under 500F. I'm not sure how well the stovepipe temp correlates to the flue gas temp, as that also has it's own time constants (heat sunk to stove and chimney liner at either end).

    Both stoves are down-draft catalytic, which is known to burn very well after light-off, but can be a little work to achieve light-off. I think it's time to install some flue probes.

    Helpful Sponsor Ads!





  2. charly

    charly Guest

    My Fireview has a steel cat,,, STT is usually 200-250 and internal flue gas temps are from 500-600.. close the by pass, set the draft to one and done! I think a steady flue gas temp works better then a quick shot up above those temps , before closing the by pass.
  3. ddddddden

    ddddddden Minister of Fire

    Joined:
    Oct 20, 2009
    Messages:
    1,329
    Loc:
    Central Va
    A probe on/near the cat itself is best, but I dunno how doable this would be on your stove. Some members have used thermocouples.

    Same stove as Charly here. I have a corrugated flex pipe liner going right into the stove, so I place the thermometer on the smooth surface of the appliance adapter @ the flue collar. Probably a bit hotter here than the typical location 18" up the pipe, but the liner fits inside the adapter, so my readings are probably similar to double-wall pipe. . .heh, my readings are probably pretty unique, but FWIW, I consistently get light off @325°+ here, which I reach more quickly than the recommended 250° stove top, if the stone has cooled off. Actually, Mumsy has misplaced my thermo, so lately I've been going by how the fire looks. When I start getting some secondary flames with the draft ~1 (1 out of 4, which I hear is the recommended max draft when running the cat) I engage the cat. However, I'm not trying to burn "BK style" with just a cat glow. I always go for a nice secondary flames, which last for several hours, so I'm burning the smoke regardless of what the cat does (I do make sure the cat lights off though.) I'm getting good 12+ hour burns without trying to milk it with a cat-only burn. :)
  4. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    I do have a thermocouple probe on the cat on both stoves, but being a downdraft stove, they read only 70F - 100F prior to closing the bypass damper.
  5. ddddddden

    ddddddden Minister of Fire

    Joined:
    Oct 20, 2009
    Messages:
    1,329
    Loc:
    Central Va
    Ah. . .hmm. . .well, if you're getting good light-off when engaging @70-100°, I think the value itself is irrelevant. I prefer flue to stovetop readings, but I think cat readings would be even better. Now, if you are talking about getting a better handle on what's going on with flue temps to facilitate downdraft operation. . .sounds reasonable, but I have zero experience with downdraft stoves. Sorry, I missed that point in your post.;em
  6. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    Well, the point was finding the right moment to close the bypass damper and engage the catalyst. I know if my stovetop is below 500F, it ain't gonna happen, and if my stovetop is over 700F it is definitely going to happen. The trouble is, anywhere in the 500F - 700F range is always a "maybe". So, I was thinking maybe a flue probe would give me a better indication of exactly when it was time to engage the catalyst.

    The caveat here is that my wood is only 1-year seasoned, and a large portion of it is various species of oak. Some was standing dead, some was already down a year or two, but all of it has been CSS'd only 1 year. That's as long as I've been in this house, and I've already gone a long way to improve that, CSS'ing 12 cords per year and using less than 6 per year. Sometimes, a little excess moisture in the wood keeps my cat from climbing above 600F, as the steam passes thru it.
  7. ddddddden

    ddddddden Minister of Fire

    Joined:
    Oct 20, 2009
    Messages:
    1,329
    Loc:
    Central Va
    I must be missing something, Jøful. If you have a cat probe, why are you using the stovetop temp to determine when to engage?

    Re wood, yeah, it'll only get better going forward. It's March already, and although you're about to get some snow, there isn't *that* much time left in stove season in the Mid-Atlantic. I would just coast on through the rest of this season, and see how it goes with drier wood next season. :)

    You may have already heard/done this, but if your wood quality is marginal, splitting it smaller will help. It will dry more quickly, and it will burn better due to increased surface area. I've tried some large splits of 1-yr red oak, and it sux. If you have to use it, resplit it and mix with other stuff in the load. Maybe get some pine to mix in. Word is that pine dries in ~6 mo.==c Some folks think pine is not worth the mess that it makes on the saw. . .how 'bout Poplar?
  8. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    Yep... I've been mixing my oak with well-seasoned poplar, and have plenty of pine and ash stacked and ready to use for the same purpose next year.

    The reason I use stove top temps (or flue temps, per the OP) to determine when to engage the cat is that my cat probe will never in a million years measure much over 100F in bypass mode. That's just a characteristic of downdraft cat stoves, I guess.
  9. mellow

    mellow Resident Stove Connoisseur

    Joined:
    Jan 19, 2008
    Messages:
    3,512
    Loc:
    Salisbury, MD
    Still waiting to see pics of your install Joful, but if I were you I would burn hot then engage with that semi-seasoned wood, you need to get rid of that moisture or it will kill your cats.
  10. Machria

    Machria Minister of Fire

    Joined:
    Nov 6, 2012
    Messages:
    1,117
    Loc:
    Brookhaven, Long Island
    Here's a questions for you: Why do you CARE when the cat lights up? Really, it doesn't matter, get the stove/cat hot enough, and turn the bypass, then let it do it's thing. Why are you so concerned with at exact what point the cat lights, and if it lights exactly when you close the bypass or 5 minutes later?

    If I waited for my soapstone top to get up to temp (250-300), it can be 1 to 2 hours! So I just wait for the single wall pipe temp, about 10" above the stove to reach 250 to 300 degree's. Turn the bypass and start closing the air down (in 2 or 3 steps about 5 minutes apart). Sometimes the I can tell the cat lit right up cause the pipe temp goes down 50 degree's. Sometimes it doesn't change for 10 or 15 minutes. So the question is, who cares? My point is, I think you are to worried about it, just get the stove hot, and turn it.
  11. Waulie

    Waulie Minister of Fire

    Joined:
    Aug 31, 2011
    Messages:
    1,010
    Loc:
    Nothern Lower Michigan
    A 600 degree pipe is one hot pipe! I kind of agree with Machria in that you may be searching for something you don't need. I would think if your cat doesn't light off right away at 500 or even lower stove pipe temps, it should take off after a couple minutes. You should need about 500F at the cat for it to light. If you're sending 1200F air through it, it'll light pretty quick. At 1000F it might be a little slower but it should still definitely light.

    Does your stove keep burning pretty hot if you close the damper and the cat doesn't light right off, or does it start to smolder? Do you keep the air wide open until after the cat lights off?

    If it was me, I'd try to engage at lower pipe temps and see if the cat will light within a couple minutes. I wouldn't want to constantly run my stovepipe up over 500F degrees external temps.
  12. charly

    charly Guest

    I'm wondering if you are running too much draft through your cat with that high pipe temp? What I'm getting at,, with my Fireview I have watched my cat not doing anything and actually watched it start to glow by slowing my draft.. Maybe too much turbulence going on with your high temps..
  13. rideau

    rideau Minister of Fire

    Joined:
    Jan 12, 2012
    Messages:
    1,955
    Loc:
    southern ontario
    I have a Woodstock PH.

    Use flue temp to engage cat....cat lights off at 500, so used to use stovetop of 250 which correlated to internal stove temp of 500. Can't do that succesfully anymore because of the cooktop under the soapstone. So use an internal flue probe. The cat will always light off once the temp in the flue is over 500. Problem is, judgement is needed. On a reload, the flue temp on my stove is very often 500...or gets there very quickly, as my wood is burning before my door is closed on reload. SO; do I engage cat right away and dial down air? This works, and I get a less vigorous seconday fire. But, my wood has not yet charred,>any moisture goes through the cat.
    Woodstock showed on video that the cat can handle the moisture just fine, so sometimes I do this, especially if it is really cold out, because of my vigorous draft, just let the fire burn a few minutes so there is no cold air hitting a warm cat, If the temps aren't crazy low out, I close the air down to about 1/4 after reload, let the fire burn for about fifteen minutes, then engage the cat and close the air. By this time, I will have a very active fire in the firebox, which usually does not close down to a cat burn, but gives a substantial secondary burn., or secondary/cat burn.

    Cats engage at 500, while secondaries start at closer to 1000. So, you are wasting a lot of potential heat (at least in a Woodstock stove) if you wait until your internal flue temp is 1000 degrees (500 external on single wall).
    Woodstock wants the flue temp to drop into the 400's once the cat is engaged, and stove temps to rise accordingly, which will happen if you haven't too strong a draft. . Photo is of a fire on a cold day, flue 700, stove top 350. I have to get a damper, so my stovepipe is hotter than I want, and I can't close the stove down more than this, so heat is being lost up the chimney. Air is completely closed. Stove top got to 450 about ten minutes later and stayed there for hours, as did the flue temp stay at 700. P1020527.JPG
  14. Machria

    Machria Minister of Fire

    Joined:
    Nov 6, 2012
    Messages:
    1,117
    Loc:
    Brookhaven, Long Island
    That's a good point! I also notice sometimes it will give off more heat faster (aka cat is lit up!) when I have a big fire/fully loaded stove IF I shut down the air quicker, instead of having a raging fire in there.... I think the reason is exactly that, the fire is raging, and flames are going right thru and up the screen, and probably clear across the stove to the entrance of the Cat in the back. It's probably moving too fast and too much flame in it to actually burn in the cat. I slow it down by shutting down the air, and a few minutes later I get alot more heat out of it.
    charly likes this.
  15. Woody Stover

    Woody Stover Minister of Fire

    Joined:
    Dec 25, 2010
    Messages:
    5,779
    Loc:
    Southern IN
    I myself want the cat to light off as soon as I start running smoke through it. As I see it, unburned smoke through the cat is depositing crap that has to be burned off later, when the cat lights. Then the resulting ash stays on the cat surface and may mask it to some degree, making it not as efficient. Or I could be full o' crap, too... ;lol
    I've got the cat probe in the Fv but it doesn't reach back far enough to be over the cat, so I use it as an indicator of conditions in the fire box. On a moderate ramp-up, that probe and the surface thermo lying on the tee snout seem to run in parallel. When I reload, the stove top may still be at 250 but I want the probe at 900-1000 for several minutes before I close the bypass; Then I get the immediate light-off. I will probably end up putting in a digital cat probe. That'll help for light-off, and to see how hot the cat is actually getting. I've just got to eyeball the brightness of the glow now to get a feel for how hot the cat is.
  16. rideau

    rideau Minister of Fire

    Joined:
    Jan 12, 2012
    Messages:
    1,955
    Loc:
    southern ontario
    Glass looks dirty in the photo above...isn't. Will try attaching another to show. This photo was taken about seven minutes later, stovetop at cruising speed.

    Attached Files:

  17. charly

    charly Guest

    I've been surprised a few times by thinking I didn't char the new load long enough, only letting it burn for 2 minutes before engaging the cat mode,,, no sir,,, it burned great and the cat did it's job... So I learned that I was wasting some heat and wood by waiting too long... To me it's all just fun seeing and watching what happens..sometimes you win:). As Woodstock told me,, just because you can't see the cat glowing doesn't mean it's not doing it's job..
  18. Machria

    Machria Minister of Fire

    Joined:
    Nov 6, 2012
    Messages:
    1,117
    Loc:
    Brookhaven, Long Island
    Funny you mentione that, I have been playing a bit with the "charing of the wood" before shutting it down. I'm noticing the stove does better, and I get longer and hotter burns if I do NOT char the wood. So lately I have been reloading, letting the fire just start up, then shutting it down in 2 or 3 steps about 5 minutes apart (sometimes even quicker, 2 minutes apart). I've been getting great results this way. I just have to be carefull how far I shut it down. If I go 100% closed, the flames go out, and the wood smolders, but every 5 or 10 minutes I'll get a puff of flames in the box. Most of the times you just see the flames puff up, and go back out after a minute or two. But sometimes you hear it "POOOOF". Not enought to blow smoke out or rattle anything, but enough to make you hear it sitting in the room. So I instead open the air just a smidge, which will allow a very small lazy flame to stay lit somewhere in the stove, ussually in the front/bottom split. After an hour or so, the secondaries will come on, and rage for an hour or 2 at the top of the stove. But there will be very little toi no flame at the bottom of the box. This is my sweet spot. Had a 22 hour burn yesterday without trying (see the New PH install thread).
  19. rideau

    rideau Minister of Fire

    Joined:
    Jan 12, 2012
    Messages:
    1,955
    Loc:
    southern ontario
    Yes, I do that too, when it is warm enough out. I can't get a really low burn when it is cold out because of my draft. The one thing I do always is make sure the stove has a minutes or two to warm the air and wood before I engage the cat.
  20. mellow

    mellow Resident Stove Connoisseur

    Joined:
    Jan 19, 2008
    Messages:
    3,512
    Loc:
    Salisbury, MD
    As long as you have DRY wood you can engage the cat soon after a load on a hot coal bed.
  21. Machria

    Machria Minister of Fire

    Joined:
    Nov 6, 2012
    Messages:
    1,117
    Loc:
    Brookhaven, Long Island
    Yes, of course. I always give it a minute or 2 to make sure "cold" air I let in the door heats up.
  22. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    You've clearly not burned a downdraft stove! If I engage too early, the stove stalls. Cat will hit somewhere between 350 - 500F, hang there a while, and then start to fall. In the absolute extreme case (not common), I will come home to a cold stove full of half-charred wood, and a smelly house. I've only done that once in two years, but other downdraft cat owners have reported similar problems.
    Gark likes this.
  23. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    Geez... I thought I had the most photographed install in the history of this forum! Which photos do you want to see?
  24. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    Starts to smoulder, but this might be alleviated if my wood were more properly seasoned. I'm burning mixed 1-year CSS'd eastern hard woods, and there's plenty of oak in those piles.
  25. Joful

    Joful Minister of Fire

    Joined:
    Mar 7, 2012
    Messages:
    6,514
    Loc:
    Philadelphia
    I run the stove wide open, to somewhere between 400 - 500 on the stove top, depending on how vigorously it's taking off and how hot my flue gets. Then I turn it down to half throttle to continue charring the wood. I'll usually try to engage the cat around 600F on the stovetop, and sometimes it takes. If it stalls out around or below 500F, I go back into bypass, burning at half throttle or less. I'll sometimes have to try closing the bypass two or three times before the cat finally takes off.

    At a "stovetop" temp of 600F, measured on my top load door, most of the actual stove top is still at or below 250F.

Share This Page