Optimum catalytic combustor burn temperature

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

Orerockon

Member
Oct 30, 2020
71
Oregon
This has probably been answered before but with my new combustors & probe I can finally see the temps inside the stove. Once I get the combustor to light off the probe temp goes up pretty fast and stops somewhere in the "ideal" temp range, usually right in the middle abt. 1000 F at the probe (1" or so from the combustor as specified in the instructions that came with it). If I keep adding wood I can get it to 1500 and one of the first times I tried it inched above that which scared me lol. (I opened the door until it went down so it wasn't up there for long) So my guess is "the hotter the better" but I wondered if there was a rule of thumb as to where you get the most heat for the amount of wood burned. I'm trying to keep it around 1000 because that's in the middle of the optimum temp range.
 
I don't think there is an optimum temperature. The reason is that the combustor is not being heated to combust the gases, it is heated *by* combustion of the gases. I.e. if you feed it more gases, it'll get hotter. If you feed it less gases, it'll get less hot. It's its processing of those gases that dictates the temperature it'll reach.

Just close the bypass when you reach the active temperature range (so that it can start combusting whatever it gets fed), and dial down your air supply (in not too large steps) to meet your heating needs, and all should be good.
 
Also, from your description I don't think that reading is "the temp inside the stove", it's the temp of the gases nearby (just after) the combustor.
 
I try to burn the cat in the middle range. I don't have a cat probe on the Keystone. Since I can see the cat through the window, I judge by how brightly the cat is glowing. I like to see a dull to medium red, not bright orange to where it gets hard to see the individual cells of the cat. On a stove where I both had a probe, and could also see the cat, I learned to judge how the glow relates to probe temps.
I've read that if you go much over 1600 for an extended period, you can damage that catalyst coating.
As far as the amount of heat from the wood, I'd assume it's most efficient to burn pretty slow so that the stove can extract more of the heat, rather than sending it up the flue.
 
I don't think there is an optimum temperature. The reason is that the combustor is not being heated to combust the gases, it is heated *by* combustion of the gases. I.e. if you feed it more gases, it'll get hotter. If you feed it less gases, it'll get less hot. It's its processing of those gases that dictates the temperature it'll reach.

Just close the bypass when you reach the active temperature range (so that it can start combusting whatever it gets fed), and dial down your air supply (in not too large steps) to meet your heating needs, and all should be good.
I've been running it with the air vent 1/4" open as FireCat recommends. Even if I close it all the way it still maintains the temp about 500. I'm sure the windows leak, there was never any seal between the panes. So higher temp at the cat = more heat = more wood to maintain it and that's that. Easy peasy thanks!
 
if I close it all the way it still maintains the temp about 500. I'm sure the windows leak, there was never any seal between the panes. So higher temp at the cat = more heat = more wood to maintain it and that's that. Easy peasy thanks!
Two panes of glass, you say? What stove make and model is it?
500 might be a little low. If you can see that it is still glowing, great. I doubt if it would glow at that temp, though. If not, go outside with it at 500, and see if the exhaust is still clear. A cat can still be burning smoke, even if it's not glowing. If the plume isn't clear, run up the temp until it is. You don't want to be sending a bunch of un-burned smoke through the cat and gunking it up.
 
Also, decreasing the air might get your cat hotter - because the firebox produces more smoke and half-burned gases, which are precisely the fuel that make the cat produce its heat.

Make sure your stove is tight, otherwise you might damage the cat.
 
Also, decreasing the air might get your cat hotter - because the firebox produces more smoke and half-burned gases, which are precisely the fuel that make the cat produce its heat.
Right, running a little flame in the box will burn up some of the smoke, leaving less for the cat which then doesn't burn as hot. You don't want so much flame in there that it can hit the cat, though. Damage will ensue.
 
Two panes of glass, you say? What stove make and model is it?
500 might be a little low. If you can see that it is still glowing, great. I doubt if it would glow at that temp, though. If not, go outside with it at 500, and see if the exhaust is still clear. A cat can still be burning smoke, even if it's not glowing. If the plume isn't clear, run up the temp until it is. You don't want to be sending a bunch of un-burned smoke through the cat and gunking it up.
1993 Appalachian 32-BW I know it ain't the greatest stove but was in my budget with the Fed tax credit & state rebate for replacing a crappy old stove. Has 3 panes, insulated aroudn the perimeter but not between the panes. I tried furnace cement and it just cracks and falls off. I don't leave it at 500, it does light off pretty easily at that temp though and I can usually get it up to 1000 fairly quickly. with the air vent slits and bottom damper shut all the way I can keep it right around 1000. Pretty sure it drafts through the ash pan too (what a stupid idea those are). I keep checking the smoke since I got the combustors switched out and I can't see anything except for the heat shimmers, and if the breeze is just right I can catch a whiff of it. We just had a nasty ice storm, 33-35 deg. temps and no power for 9 days, I was pleasantly surprised at how little wood I chewed through heating the whole house. I'm glad I can't see the combustors in it or I would get obsessed with it :D
 
I'm glad I can't see the combustors in it or I would get obsessed with it :D
If it's like the Buck 91 I ran, if I looked into the hole where the bypass rod went through the front of the stove, at just the right angle, I could see the cat glowing. No need to get obsessed, sounds like you have it running good. It was fun to look at the glowing cat every once it a while, though. >>
 
OK so one more question. I had the stove cranking at 1000, threw a few logs in the stove after it had cooled below 500 and closed it up just to keep it going another couple hours. I came back to turn off the fan and it had shot back up to 1000 in an hour or so. It isn't exactly airtight. So my question is what's the purpose of waiting until it's at 500 to engage the combustor? It probably kicked in somewhere around there but it was closed up somewhere around 300.