Jotul Firelight 12 puffs smoke into house

[MaureenR]

After I've had the damper on (and the catalyst engaged) for a while, the stove starts to puff smoke into the room. There is a whoosh noise, a flash of firelight and then a puff of smoke is pushed through either the top door or front door. Sometimes the top lid actually is popped open as far as it can with the damper on.

I've read that the cause of my problem is that the damper was closed and the catalyst engaged before the stove has gotten hot enough for the catalyst to function properly, but mine happens long after I've engaged the catalyst and the stove has been burning for a while, and it also happens when it is windy outside.

This problem has been present for so long that I can't remember if it was this way when the stove was new or not.

 

[begreen]

Tell us about the wood you are burning. Do you split it up yourself or is it purchased and delivered?

 

[MaureenR]

Tell us about the wood you are burning. Do you split it up yourself or is it purchased and delivered?

I've purchased in the past, but for the past few years it's all been from our land. The puffing smoke has happened with both. Currently I'm using a mixture of black birch, tulip poplar, red oak and ash.

 

[begreen]

How long is the wood allowed to season after it has been split and stacked? Oak in particular needs a couple years.

 

[MaureenR]

Thanks for replying so fast. The wood I'm burning right now is left over from last year. It spent about a minimum of one year as felled logs, then split and stacked about a year ago - so about 2 years total. But the problem has persisted regardless of whether it was my wood or purchased wood, so I'm kind of doubtful about that as the cause, but maybe. Someone suggested I take out the catalyst and try to use the stove a few days without it and see if I still have the problem.

 

[Bluerubi]

After I've had the damper on (and the catalyst engaged) for a while, the stove starts to puff smoke into the room. There is a whoosh noise, a flash of firelight and then a puff of smoke is pushed through either the top door or front door. Sometimes the top lid actually is popped open as far as it can with the damper on.

I've read that the cause of my problem is that the damper was closed and the catalyst engaged before the stove has gotten hot enough for the catalyst to function properly, but mine happens long after I've engaged the catalyst and the stove has been burning for a while, and it also happens when it is windy outside.





This problem has been present for so long that I can't remember if it was this way when the stove was new or not.

That happened to me once, and it was 100% my fault. I opened up the ash drawer to provide some draft without opening up the front doors since it had a log near the glass. Flames start getting more intense, so I close the ash drawer, effectively starving the fire of air (air intake handle was essentially closed). Flames die out, fire smolders heavily, lots of smoke fills the firebox, then it gets hot enough in there to make the flue gases ignite and there was a big flash of fire and the top loading door flapped open. Woke my wife up two floors away since it sounded like an explosion coming out of the chimney. In my case the catalyst was not engaged.

As long as there is a visible flame in the firebox this shouldn't happen. Might want to open the air up a bit more so that such a high concentration of combustible gas can't build up.

 

[fire_man]

MaureenR:

You have described classic "Backpuffing" and it's caused by combustion gases not being drafted quickly enough. Do a search on "Backpuffing" and you will get lots of hits.
This was a good thread:

https://www.hearth.com/talk/wiki/prevent-puffback-aka-flashback/

 

[Ashful]

I have, unfortunately, become the resident expert on back-puffing Firelight 12's. I can say that my family room Firelight 12 does this very frequently, while my living room Firelight 12 never does it. The difference between these two rigs are:

Family room: 1993 vintage Firelight 12 on 15' of 6" insulated chimney liner
Living room: newer than 1993 Firelight 12 on 27' of 6" insulated chimney liner

The family room stove was back-puffing on me quite a bit, during my first year of learning to burn it. In contrast, the more recently added living room stove can be shut down tight under almost any conditions, and it will not back-puff. I always attributed this mostly to the stronger draft of that 27' chimney, but also wondered in the back of my head if Jotul made some hidden changes to the internals between the years these two stoves were built.

To deal with the back-puffing family room stove, I tried many different things, including some of the things you have already seen mentioned. One factor discovered early on was how long I toast the wood before engaging the catalyst, whether or not I successfully achieved cat light-off, and how well the cat heated up in the early part of the burn cycle. These items are very highly dependent on how much wood you load, and most importantly... the moisture content of your firewood. However, the biggest single factor turned out to be how low I set the air control.

On the stove that gives me the backpuffing troubles, I cannot lower the air control past the left edge of the minus symbol on the ash lip, or I am almost guaranteed some back-puffing. Under the weak draft condition of warmer outside weather, I push it even less, stopping around the middle of the minus sign. Since the speed at which your stove chews thru a load of wood is highly dependent on the amount of draft provided, it seems this stove is not chewing thru wood any more quickly than the other, which is completely shut down hard.

 

[MaureenR]

I have, unfortunately, become the resident expert on back-puffing Firelight 12's. I can say that my family room Firelight 12 does this very frequently, while my living room Firelight 12 never does it. The difference between these two rigs are:

Family room: 1993 vintage Firelight 12 on 15' of 6" insulated chimney liner
Living room: newer than 1993 Firelight 12 on 27' of 6" insulated chimney liner

The family room stove was back-puffing on me quite a bit, during my first year of learning to burn it. In contrast, the more recently added living room stove can be shut down tight under almost any conditions, and it will not back-puff. I always attributed this mostly to the stronger draft of that 27' chimney, but also wondered in the back of my head if Jotul made some hidden changes to the internals between the years these two stoves were built.

To deal with the back-puffing family room stove, I tried many different things, including some of the things you have already seen mentioned. One factor discovered early on was how long I toast the wood before engaging the catalyst, whether or not I successfully achieved cat light-off, and how well the cat heated up in the early part of the burn cycle. These items are very highly dependent on how much wood you load, and most importantly... the moisture content of your firewood. However, the biggest single factor turned out to be how low I set the air control.

On the stove that gives me the backpuffing troubles, I cannot lower the air control past the left edge of the minus symbol on the ash lip, or I am almost guaranteed some back-puffing. Under the weak draft condition of warmer outside weather, I push it even less, stopping around the middle of the minus sign. Since the speed at which your stove chews thru a load of wood is highly dependent on the amount of draft provided, it seems this stove is not chewing thru wood any more quickly than the other, which is completely shut down hard.

 

[MaureenR]

Everyone is being so helpful - thanks! I've read the recommended articles about back-puffing, so I'll try the fixes, starting with not closing the air vent so much. Unfortunately, I also have a concurrent problem (with another thread going) about overheating in my catalyst box. People there are telling me that maybe I have a leaky gasket causing the over-heating, but that wouldn't make sense if the back-puffing is caused by oxygen deprivation from a too tight air control, right?

 

[Ashful]

I didn't read your thread on catalyst overheating (please post link here, if you can), but the two actually can be related.

First, theory on back-puffing: Wood doesn't burn, but wood gas does. Pyrolization is the conversion of super-heated solid wood to flammable wood gas, and this gas must be combined with oxygen to combust. If oxygen is not supplied quickly enough to keep up with the rate of pyrolization, then the wood gas will just collect in the firebox, un-combusted. Eventually, the air slowly leaking into your firebox thru the tightly-closed primary air control, will provide sufficient oxygen for combustion, and (boom) you get a little explosion (back-puffing). This cycle continues, until the rate of pyrolization slows to the point where the rate of air coming into the firebox can keep up.

Your catalyst draws air thru a separate inlet, not controlled by your primary air level. So, you can shut down the air to your firebox, using that primary air control lever, but not to the catalyst. Now, going back to the wood gas, any gas which is not burned in the firebox gets burned in the catalyst. You can easily imagine a situation, under very low draft settings, where your firebox is cool but your catalyst is over temperature. In fact, the first step in cooling an overheated catalyst is usually to open the primary air control some, to promote more combustion in the firebox, leaving less fuel for the catalyst.

The challenge here is that, sometimes you can't open the primary air control without overheating the stove, if things are really ripping on a full load. This is where learning to load a smaller number of larger splits, or to choose species and stack things in an order that will help to slow combustion a bit, thus preventing either the firebox or catalyst from having to work too fast. This will also alleviate your backpuffing troubles.

It sounds like a lot to learn, but once you grasp the concepts, it's really quite simple. I really struggled with some of this in my first year, but the learning was quick. In my case, I have a lot of walnut in my wood stacks (enough to make any furniture maker cry), and have found that walnut really works the catalytic combuster hard. I avoid full loads of 100% walnut, finding that mixing in a little ash or something less resinous, really helps keep catalyst temperatures in check.

 

[mellow]

https://www.hearth.com/talk/threads/jotul-firelight-12-overheats-help.116851/#post-1563355

15 feet of chimney just like yours Joful.

 

[fire_man]

Joful hit on lots of really good points.

Woodstock added a small "antiburp" hole at the bottom of the firebox that always allows a tiny bit of air to help reduce the propensity to backpuff. You can often see a ribbon of flame shooting from it.

 

[MaureenR]

I didn't read your thread on catalyst overheating (please post link here, if you can), but the two actually can be related.

First, theory on back-puffing: Wood doesn't burn, but wood gas does. Pyrolization is the conversion of super-heated solid wood to flammable wood gas, and this gas must be combined with oxygen to combust. If oxygen is not supplied quickly enough to keep up with the rate of pyrolization, then the wood gas will just collect in the firebox, un-combusted. Eventually, the air slowly leaking into your firebox thru the tightly-closed primary air control, will provide sufficient oxygen for combustion, and (boom) you get a little explosion (back-puffing). This cycle continues, until the rate of pyrolization slows to the point where the rate of air coming into the firebox can keep up.

Your catalyst draws air thru a separate inlet, not controlled by your primary air level. So, you can shut down the air to your firebox, using that primary air control lever, but not to the catalyst. Now, going back to the wood gas, any gas which is not burned in the firebox gets burned in the catalyst. You can easily imagine a situation, under very low draft settings, where your firebox is cool but your catalyst is over temperature. In fact, the first step in cooling an overheated catalyst is usually to open the primary air control some, to promote more combustion in the firebox, leaving less fuel for the catalyst.

The challenge here is that, sometimes you can't open the primary air control without overheating the stove, if things are really ripping on a full load. This is where learning to load a smaller number of larger splits, or to choose species and stack things in an order that will help to slow combustion a bit, thus preventing either the firebox or catalyst from having to work too fast. This will also alleviate your backpuffing troubles.

It sounds like a lot to learn, but once you grasp the concepts, it's really quite simple. I really struggled with some of this in my first year, but the learning was quick. In my case, I have a lot of walnut in my wood stacks (enough to make any furniture maker cry), and have found that walnut really works the catalytic combuster hard. I avoid full loads of 100% walnut, finding that mixing in a little ash or something less resinous, really helps keep catalyst temperatures in check.

Ahh - two separate inlets! I never thought of that. Thank you so much for such a thorough explanation. I think I tend to cut and split my wood pretty small because I'm small myself and don't like lifting the heavy logs. Maybe I'll look into that idea of drilling a burping hole. And yes, I've had the problem of overheating my firebox while trying to cool the catalyst box. I've spent a few panicked nights alternating back and forth letting one box heat up to red while the other one cooled down and back and forth. My other thread is https://www.hearth.com/talk/threads/jotul-firelight-12-overheats-help.116851/#post-1563362.

 

[MaureenR]

I didn't read your thread on catalyst overheating (please post link here, if you can), but the two actually can be related.

First, theory on back-puffing: Wood doesn't burn, but wood gas does. Pyrolization is the conversion of super-heated solid wood to flammable wood gas, and this gas must be combined with oxygen to combust. If oxygen is not supplied quickly enough to keep up with the rate of pyrolization, then the wood gas will just collect in the firebox, un-combusted. Eventually, the air slowly leaking into your firebox thru the tightly-closed primary air control, will provide sufficient oxygen for combustion, and (boom) you get a little explosion (back-puffing). This cycle continues, until the rate of pyrolization slows to the point where the rate of air coming into the firebox can keep up.

Your catalyst draws air thru a separate inlet, not controlled by your primary air level. So, you can shut down the air to your firebox, using that primary air control lever, but not to the catalyst. Now, going back to the wood gas, any gas which is not burned in the firebox gets burned in the catalyst. You can easily imagine a situation, under very low draft settings, where your firebox is cool but your catalyst is over temperature. In fact, the first step in cooling an overheated catalyst is usually to open the primary air control some, to promote more combustion in the firebox, leaving less fuel for the catalyst.

The challenge here is that, sometimes you can't open the primary air control without overheating the stove, if things are really ripping on a full load. This is where learning to load a smaller number of larger splits, or to choose species and stack things in an order that will help to slow combustion a bit, thus preventing either the firebox or catalyst from having to work too fast. This will also alleviate your backpuffing troubles.

It sounds like a lot to learn, but once you grasp the concepts, it's really quite simple. I really struggled with some of this in my first year, but the learning was quick. In my case, I have a lot of walnut in my wood stacks (enough to make any furniture maker cry), and have found that walnut really works the catalytic combuster hard. I avoid full loads of 100% walnut, finding that mixing in a little ash or something less resinous, really helps keep catalyst temperatures in check.

I'm trying to understand your note more fully. So if I'm going to bed and would like to load up the stove so it might make it through the night, I should put in very large pieces of wood (to slow the combustion) and use wood that is less resinous (to slow the combustion), right? I bet poplar has lots of resin and I'm using too much of it.

 

[Ashful]

For overnight, yes, large pieces are the key. However, I always need to load a few smaller pieces under the large stuff, to kick the whole thing off. Poplar is a very light and poor wood, not that it doesn't burn well, but it doesn't last long. It's tough to get more than maybe 8 hours out of a load of poplar, whereas I can get more than 12 hours out of a load of hardwood. That said, burn what you got!

An ideal load, would be a few smaller splits of poplar right on the coal bed, followed by a few larger splits of hardwood on top. Let the small stuff toast the hardwood for 20 - 30 minutes, then close the bypass, let cat come up to temp, and then lower the air in three increments (1/2, 1/4, then almost closed).

I find that 6 splits burns real well in the stove, and any time I go over six splits, I have more trouble with backpuffing. It's a surface area per volume, sort of thing. Adjust your split size for your desired burn time, but don't be loading 10 small splits into the stove.

I don't like the idea of drilling a hole in the stove, since these F12's are so wonderfully controllable. I have just about zero concern with run-away, etc. But your post did give me a new idea. I might drill a small "anti-burp" hole in the air control shutter, an easily replaceable part. I do know that several Woodstock owners have had the same back-puffing troubles as us F12 owners, so I'm sure this is something Woodstock is studying / addressing.

edit: I'm going to ask Jeremy Harkin to join this thread. He runs a VC cat stove, of similar design to our F12's, and has many of the same problems with light-off, cat temp, etc.

 

[jharkin]

Ha ha hahaaaa

/evil laugh.


If you like 2 air inlets...
... try 4
.. and 2 bimetallic thermostats.
Such is the world of VC


In all seriousness, thanks for the invite Joful. I think you hit the nail on the head with your explanation of what causes backpuffing and the slow light offs - draft and wood quality.

Other than the first year with miserable wood I never had backpufifng bad enough to cause smoke into the room... mostly I just get the occasional light show if I'm burning too low. For me the real symptom of marginal draft is that I get smoke when I top load ifs over 45F outside. I think Ive adressed that by improving the stack insulation a bit and removing the unnecessary key damper.

The anti-burp hole.... I'm guessing that is just what woodstock uses to provide the minimum airflow with the primary closed, its what VC does with the two "epa holes" as folks like to call them behind the front legs that let a small amount of air directly through the bottom grate. Its the only air coming into the main firebox when the primary is closed.


The slow light offs... That has plagued me since day one, and I'm hoping to finally get it under control this year with a fresh rebuild and honest 3 year ~ 15% wood. The problem is big loads... Since we dont have a Blaze King we cant load the thing to the top with kindling wood without going nuclear, so we do big loads with the largest splits we can find. Problem with using large splits is two fold:

• They take longer to char, boil off the surface water and heat up enough to be feeding the cat hot,dry smoke
• The largest, densest splits that work so well for full loads also season the slowest so they are likely the wettest in your stack

The entire ordeal has taught me one of the main reasons that Dennis and others promote 3 year seasoning... When you let your wood dry that long you can leave the split size as big as you want, and bigger splits mean a more controllable fire. Those of us (me included) who dont have space or means to get 3 years ahead compensate by splitting small to speed up the dry time and the result can be out of control fires.

 

[Ashful]

I'm splitting big for several reasons, and the jury is still out on what's best, for my case. I figure bigger splits let me load up fuller with less surface area, and help avoid the dreaded back-puffing. However, they lead to longer and more painful start-ups, although I'm already working on a theory that might help that situation. Dry time is a big issue for me, but that is soon to be corrected:

Thanksgiving 2012: Split all 2013/2014 wood (1 year seasoned)
May - July 2013: Split all 2014/2015 wood (18 months seasoned)
Thanksgiving 2013: Will split all 2015/2016 wood (2 years seasoned)

... and thus the trend shall continue. With two big stoves, I anticipate burning 6 - 8 cords per year, so three years stacked means 18 - 24 cords on hand. I'm not sure I'll ever quite get into the 20's, but 18 is my goal right now.

 

[mellow]

18 cords is a mess load of Weekend/Holiday work. I suffer with backpuffing as well in my cat stove if I close it down to soon, usually in the shoulder seasons I have to leave the air open a slight bit more than usual to compensate for the weaker draft.

 

[begreen]

Ahh - two separate inlets! I never thought of that. Thank you so much for such a thorough explanation. I think I tend to cut and split my wood pretty small because I'm small myself and don't like lifting the heavy logs. Maybe I'll look into that idea of drilling a burping hole. And yes, I've had the problem of overheating my firebox while trying to cool the catalyst box. I've spent a few panicked nights alternating back and forth letting one box heat up to red while the other one cooled down and back and forth. My other thread is https://www.hearth.com/talk/threads/jotul-firelight-12-overheats-help.116851/#post-1563362.

Small wood pieces can be the reason for a too hot catalyst. They heat up and outgas too quickly. Try a reload of only 5-7" splits and see if that helps tame the cat temps.

 

[Ashful]

What's funny is that I got so frustrating dealing with shorter (not necessary smaller) splits last winter, not being able to fill the stove completely and watching my burn time suffer accordingly, that I started cutting and splitting everything much bigger. I'm now sitting on 13 cords cut and split to 20" long x 6" x 8" type size. They fit in the stove... in theory, but getting the third one thru the door with two already in the bottom of the stove is a bear, and you can just forget pieces 4 and 5, which would be required to fill the stove. Right now, I still have some shorter stuff on-hand, to put on top of the big'uns, and fill the firebox full. But, I see a LOT of re-cutting in my near future.