0-600 in 30 minutes flat; then a smokey burnout

[Gridlock]

I have been experimenting with different ways to light my stove, a VC Defiant CAT, when completely cold. I have tried bottom-up, top-down, using starter blocks and different combinations of kindling and newspaper, cracking the ashpan or not, etc. Then I tried a technique that worked amazing well: a arranged the wood in diagonal pairs starting at the bottom and criss-crossing the pairs as the they make their way toward the top. The stove is not very deep, but there is just about enough room to be able to do this. I thought that arranging the wood in this way leaves a 'hole' in the middle of all the pairs which theoretically would create a chimney effect and allow the fire to rise quickly and light off as much as the wood as quickly as possible. Criss-crossing also leaves more surface area of the wood exposed to flame, versus stacking in one direction. I lit a small starter block and slid it in between the bottom pair of splits, slightly cracked the ashpan, and WOOF, in about 5 minutes, all the splits were burning!! I closed the ashpan, and in 30 minutes, the stovetop thermometer read 600 degrees! By far, this was the fastest way to get the fire going that I've tried.

Then, since I was feeling real cocky, I decided to experiment and engage the CAT which is much sooner than I normally would (I usually wait a bit over an hour until a good bed of coals has formed). My thinking was that since the stovetop temperature was already 600, I should be able to light off the CAT; and boy did it ever! The temperature of the CAT probe quickly rose to 1850 and the stovetop temperature rose to almost 800! Then smoke started forcing it's way past the gaskets in the top griddle, and it just started smelling like everything was burning. I opened the top to see what was going on at which point smoke poured out into the room. I think I just witnessed my first honest-to-god back-puffing! I then quickly opened the damper to allow the smoke to exit and to try cool down the CAT. After a few minutes both the CAT and stovetop temperature went back down to normal levels. I waited a little longer, closed the damper to reengage the CAT, and everything was fine after that.

So what happened? I thought of a few possibilities:

a) not enough hot coals which caused excessive smoke when closing damper to engage CAT
b) chimney not warmed up enough which caused poor draft
c) combination of a) and b)
d) some other problem

What do you think happened?

 

[Battenkiller]

Sounds like a draft problem for sure. Not enough info about your chimney, but it sounds like it may be partially blocked. What size flue diameter are you running with this stove? Do you have a screened in cap? When was the last time the chimney was cleaned?

 

[Gridlock]

Sounds like a draft problem for sure. Not enough info about your chimney, but it sounds like it may be partially blocked. What size flue diameter are you running with this stove? Do you have a screened in cap? When was the last time the chimney was cleaned?

I'm running a 6" flue. VC recommends 8" but says 6" is fine as long as I don't run it with the front doors open. Height of chimney is about 20-25 feet (two stories). I don't believe it is a draft problem in general as the stove works fine normally. This problem only happened this once and is now operating fine again.

 

[begreen]

Was the primary air shut off suddenly when you saw the stove top at 800?

Describe the entire flue setup, from stove to cap. Include elbows, single or double wall, etc.

 

[Gridlock]

Was the primary air shut off suddenly when you saw the stove top at 800?

Describe the entire flue setup, from stove to cap. Include elbows, single or double wall, etc.

Double wall and insulated 6" pipe
One 90 degree elbow; pipe exists out the back then turns up
Pipe goes up inside a two story stone and masonry chimney about 20-25 feet tall
Top has screened cap
Bottom of chimney is completely sealed around pipe to prevent any air from escaping up chimney

I don't think I shut off the primary air; in fact I think I might have opened it fully to try get get draft going and get the smoke out, then lowered it to reduce the temperature once the smoke cleared.

 

[Fsappo]

It sounds to me like the stove was trying to push more smoke thru the cat than the cat was able to burn off. Thats what happens with cat stoves a lot and causes backpuffing. It sounds like your draft is fine with the rapid start up of the stove and the quickness to 600 degrees. So I would guess a clogged cat (not likley if you got it to 1800 degrees) or just a case of engaging the bypass before the stove and air flow thru the cat had a chance to equalize. I has able to make the same thing happen with the encore cat I used to burn. If I added a bunch of dry splits to a hot bed of coals and slammed the bypass shut real fast, backpuffing. I'm sure if you build a a normal fire and don't rapid heat the wood by opening the ash pan, you should still be able to get the top of the stove to 5-600 degrees with 45 minutes or so, engage the bypass and have no drama.

 

[Gridlock]

It sounds to me like the stove was trying to push more smoke thru the cat than the cat was able to burn off. Thats what happens with cat stoves a lot and causes backpuffing. It sounds like your draft is fine with the rapid start up of the stove and the quickness to 600 degrees. So I would guess a clogged cat (not likley if you got it to 1800 degrees) or just a case of engaging the bypass before the stove and air flow thru the cat had a chance to equalize. I has able to make the same thing happen with the encore cat I used to burn. If I added a bunch of dry splits to a hot bed of coals and slammed the bypass shut real fast, backpuffing. I'm sure if you build a a normal fire and don't rapid heat the wood by opening the ash pan, you should still be able to get the top of the stove to 5-600 degrees with 45 minutes or so, engage the bypass and have no drama.

Thanks for the feedback. I had been cracking the ashpan to help with cold starts, but might not need to anymore using my new technique. My feeling is that if I closed the air supply a bit and let the fire settle down for a few minutes, I might have been able to engage the CAT without the problem I had. Since the fire had been blazing and consuming a lot of air, the closure of the damper at that point might have created an environment where there was too much smoke from the raging fire (as you had suggested) to get through the CAT.

Ah, it's an ongoing learning process! I really enjoy getting to know and master it though!

 

[hothead]

You mentioned in your last comment that you opened the ash pan drawer to help get your fire going. If you leave then pan drawer open for any extended period of time and then slam it shut, you will most definitely invite "back-puffing". There is no way your primary air intake can match the ash pan door. Your stove will try to maintain its oxygen supply from wherever it can get it (including sucking down your chimney). Using the ash pan door to get a fire going is very tempting but should only be used for few seconds and not minutes (though it is amazing what a little bit of extra air can do for a fire)...

 

[Gridlock]

You mentioned in your last comment that you opened the ash pan drawer to help get your fire going. If you leave then pan drawer open for any extended period of time and then slam it shut, you will most definitely invite "back-puffing". There is no way your primary air intake can match the ash pan door. Your stove will try to maintain its oxygen supply from wherever it can get it (including sucking down your chimney). Using the ash pan door to get a fire going is very tempting but should only be used for few seconds and not minutes (though it is amazing what a little bit of extra air can do for a fire)...

Thanks for the suggestion, however I closed the ashpan after only 5 minutes of lighting. The back-puffing occured after closing the damper 25 minutes later, so don't think the ashpan had anything to do it. Cracking the ashpan is very useful to get the fire started, believe me. I monitor it carefully; the time the pan is open seems to depend on how well the wood is seasoned, and the technique I'm using to build the fire. With the method I used this time, I could problably close the ashpan after just a minute or two; I'll see what happens the next time. My dealer mentioned that Vermont Castings used to have a start position on the air intake for some of its stoves, but no longer has that do to the fear of someone leaving it in that position and over-firing the stove, which could lead to warranty claims.

 

[CarbonNeutral]

My cause of poor draft (see pic). Maybe the extra high temps/gas velocity dislodged lots of soot.gunk that got caught on your screen?

 

[Todd]

It sounds to me like the stove was trying to push more smoke thru the cat than the cat was able to burn off. Thats what happens with cat stoves a lot and causes backpuffing. It sounds like your draft is fine with the rapid start up of the stove and the quickness to 600 degrees. So I would guess a clogged cat (not likley if you got it to 1800 degrees) or just a case of engaging the bypass before the stove and air flow thru the cat had a chance to equalize. I has able to make the same thing happen with the encore cat I used to burn. If I added a bunch of dry splits to a hot bed of coals and slammed the bypass shut real fast, backpuffing. I'm sure if you build a a normal fire and don't rapid heat the wood by opening the ash pan, you should still be able to get the top of the stove to 5-600 degrees with 45 minutes or so, engage the bypass and have no drama.

Thanks for the feedback. I had been cracking the ashpan to help with cold starts, but might not need to anymore using my new technique. My feeling is that if I closed the air supply a bit and let the fire settle down for a few minutes, I might have been able to engage the CAT without the problem I had. Since the fire had been blazing and consuming a lot of air, the closure of the damper at that point might have created an environment where there was too much smoke from the raging fire (as you had suggested) to get through the CAT.

Ah, it's an ongoing learning process! I really enjoy getting to know and master it though!

Yep, you have to remember when you engage the cat your reducing the draft and if you have a raging infirno in bypass mode then suddenly close the bypass the stove is going to be starved for air and burp smoke.