I need help with "Back Puffing"

[bmwbj]

I've owned my Intrepid II for 5 years now, and have always had an issue with the Cat. lighting off, until I got
a new Condor replacement. The cat lights off almost every time now --- I love it. But for the last several weeks,
every time I use the stove and get it hot enough to light the cat. it "back puffs" after about a 1/2 hour or so unless I open the primary air supply more. The only problem with that is that the stove temps will go over 700* and I end up having to shut it back again, than it will back puff again. I've never been able to load the stove up because the only way to prevent back puffing and not overfiring is to open the doors and let her cool down. I've never had a problem with creosote build up in the chimney, nor have I ever had a problen with the draft.
I've check the secondary air supply, and the flap works fine, I've checked the primary air supply and it
also looks fine. I just want to be able to operate the stove at around 500 to 550 with-out having a back puffing
problem, and also be able to load the stove up the proper way. Only burning a couple of logs at a time is really
a pain in the as*, loading every hour or so so I don't over-fire.

Is it possible that the temps here in Jersey just have not been cold enough to make everything work right?
I always open a window near the stove to make sure I have the right amount of supply air -- is this ok?
I'm burning well seasoned Maple -- Is this this "soft" a type of wood causing the stove to run hot?
Every now and then I can hear the Cat. roaring -- Is this normal?
I know I have a tight stove, I can turn it off by closing the Primary supply -- Does that sound correct?

Sorry for all the questions, but I'm getting very frustrated, and the Wife is going to kill me because I can't
leave the stove alone, and I told her 5 years ago that this was a "SET IT AND FORGET IT" stove, boy was I wrong...HELP PLEASE...

 

[swestall]

I had a similar problem with the Defiant CAT stove. It always stopped back puffing when I would open the air control just a little. I found that you really could not just close it all the way down as the gasses in the stove build up (the fire is smouldering in there) and then ignite. The air being closed down doesn't give the draft enough pull to get these fumes through the CAT and burned.
I'd suggest you try to find the sweet spot: damped down but not all the way. You might operate above 550 for a while, but not too long. 650-700 isn't a problem for that stove and the temp will drop as the burn progresses.

 

[bmwbj]

Thanks, swestall, I will give it a shot. I'm still alittle leary about loading up the stove, because the only
way I know to stop the overfiring is to open the doors completly, and this is not a good practice
with a fully loaded stove.

 

[swestall]

I'd never open a fully loaded stove. That CAT stove has to get pretty hot, 900+ for a while to overfire. I had mine (Defiant CAT) at 900+ a few times and no dammage or problem. It wasn't on purpose, just got distracted or my daughter did. But, burning at 750 or less on that stove should be no problem for short periods. The CAT is going to burn at over 1000 degrees anyway.
You might be surprised at loading it up; may not be a problem at all. Sometimes it is actually harder to get a full stove very hot versus one that is loaded a little less.
Also, make sure the gaskets have a good seal and check for leaks at the seams where furnace cement sometimes cracks and lets in air. There is a good thread about checking for air leaks.
And, Happy New Year!!

 

[webbie]

Maybe some hints here:
https://www.hearth.com/econtent/index.php/wiki/flashback/

 

[stoveguy2esw]

cat stoves are "burn cycle" stoves, ideal operation is to load , char preheat the cat and bypass the flue , allow the unit to burn down the load to coals and then repeat the procedure. also the catalyst may not be "happy" at 500-550F

here are some reasons for this statement
1. catalysts generally light off at 500F "+or-" so by choking the unit down to achieve that temp may cause the cat to briefly extinguish and relight , during the "out" stage , the secondary air can work back into the top of the priimary chamber and ignite the "smoke" there , this causes a brief overpressure in the primary box and can force out fumes through the primary draft control opening this is what causes catalytic "backpuffing" in many cases.

2. adding wood to an existing fire while in the midst of a cycle will cool the fire as the "char" stage sets into this addition of wood. uncharred wood does release a higher content of moisture which is allowed to bypass during the char stage, however , the ongoing combustion and active catalyst will then jump temps up rapidly and will cause even greater overpressure soon after the catalyst is re-engaged,this along with possible moisture left in the wood retarding catalytic function can slow the cat flow down and lead to more backpuffing.

3. and most important IMHO loading and firing on a cycle does give the unit the "fire and forget" function, remember , even when wood is at the end of its "visible flame" cycle and is down to log shaped coals , the catalyst is still functioning and burning the released gasses just as it did the heavier smoke from earlier in the fire, especially if the cat is allowed to run at a more "comfortable" range internally(600+) during the bulk of the cycle. a reload done when at this "coal" stage should allow relatively little smoke release and should go right back into char stage for the next cycle readily.

ive burned a cat unit for better than a decade in this manner and almost never had issues with smoke backpuffing, smoky reloads, and above all , would literally get cupfuls instead of bucketfuls when i swept the flue.

i hope you find this helpful

 

[jpl1nh]

Mike, thank you for a really nice explanation. I think that especially with the cat stoves, when you put in a large load, as the wood heats up and you start to get a lot of gassification, the cat is happy to work at a very low oxygen flow, but the rate of gas release by this large, ripe, new fuel source creates a very volatile condition prime for these mini explosions. I've come to believe that we all are inclined to ask too much of our stove in controling temperature. We want to put a large amount of BTU's into the firebox and then ask the stove not to burn it very quickly by trying to reduce the oxygen way down. We think "air control, air control, air control" instead of fuel supply, fuel supply. To prevent back puffing, you have to provide enough oxygen to combust the gasses as fast as they are given off. If you want less heat out of your stove, put less fuel into it or load the wood to minimize the surface exposure that provides the gassification or both. Use lesser BTU wood packed tightly (and in the direction that minimizes combustion in your stove, ie; load north-south instead of east-west), if you have high BTU wood, put less in and pack it tightly. A large part of the art or fun of learning to run your stove is learning what wood to use when and how to load it to get the results desired. Your stove is designed to burn wood and there is only so much that you can do with the stove itself to slow that process down.