Backpuffing VC Defiant insulated 8 inch vs 6 inch chimney question

We have a VC Defiant non-cat stove in a room with a 10 foot ceiling. The chimney is an uninsulated 8 inch round steel pipe up to the 10 foot ceiling and then it goes through the ceiling and roof with an insulated double (triple?) walled ss pipe another 8-10 feet. The chimney cap is not above the roof ridge, but it is probably 12 feet or so away from the ridge. We are in West TN.

The stove burns well and draws good until it has been throttled down for the night by closing the air intake lever. Sometimes, an hour or two after the fire has been put to bed for the night the stove will begin getting little dancing wisps of fire in the firebox as if it is getting only enough oxygen for a tiny fire. It is almost certain that he stove will have a little explosion and puff smoke out of the griddle/lid. I am sure that the firebox in the stove is operating correctly, BUT, I am not sure if I have enough draft when the stovepipe cools down and/or the stovepipe is too large to keep a good enough draft going. The problem seems to be much worse if we fill the stove with new wood right before bed. It seems that the heat cooks some volatile gases out of the new wood and then it has a tiny explosion. It does not seem to backpuff if there is pre-charred wood in the stove.

My friend down the road has a VC Encore non-cat with a 6 inch insulated pipe exiting its entire length up through a masonry chimney with about 25 feet total length. He never has any drafting or backpuffing issues no matter how he adjusts his air intake lever.

I would like to correct the backpuffing issue. It seems as if I have the following choices:
1) extend the 8 inch insulated pipe another 3 or 4 feet.
2) build a wooden "fake" chimney around the existing pipe in order to insulate it better.
3) Reduce the inside diameter of the 8 inch pipe by putting another liner inside of the current pipe and reducing the inside diameter to 6 inches in order to create more velocity. I am not sure if that would be "code friendly".
4) replace the entire 8 inch system with a 6 inch system to try and create more velocity.
5) install an outside air kit (OAK) in case there is some sort of negative pressure issue that I am not aware of.

My goal is to be able to close the air intake lever completely down when we go to bed and not have to worry about backpuffing.

I did a search of the forum and never found this exact problem discussed. I did see some discussion about larger pipes not drawing as well as smaller pipes and insulation being very important. The VC defiant is approved for a 6 inch pipe unless you are burning with the doors open, which we never do.

Any help would be greatly appreciated.

I will try to chime in with some thoughts:

1- Extending the flue pipe may help with your draft issue
2- The question that will come your way (and a question I have) is how is your wood? Is it dry?
3- How long have you had the stove? Is it possible that the primary air feed may be partially blocked?

Just some questions that come to mind. Good luck.

Try giving it more air. For my Encore cat stove the temps have very little swing to it. I have no fear in leaving the stove with the air controls opened up a bit when I go to bed.

Does your Defiant vary greatly on stove temps throughout the burn cycle? If not, give it a little more air and don't close it down all the way.

Assuming the stove is tight, tyring to increase the draft is not going to do anything about backpuffing. Backpuffing is directly related to a hot stove too much fuel and not enough air. When you throttle down the stove too quickly with a full fuel load you are generating large volumes of potentially flammable gases, if there in adequate O2 in the stove, the gases will enter the chimney and at some point it will get access to enough air that it will ignite in the stack. At that point a pressure wave builds up and pressurizes the chimney and the stove which causes a puff. When the puff occurs ,the air damper opens which allows more air in the stove potentially causing a secondary puff.

What it comes down to, is that backpuffing is the "nature of the beast", unless you modify the stove so that you cant shut down to air (like an EPA stove with hidden secondary air ports), learn to cut back on fuel slowly and dont try to "bank" up the stove at night.

Remkel said:
I will try to chime in with some thoughts:

1- Extending the flue pipe may help with your draft issue
2- The question that will come your way (and a question I have) is how is your wood? Is it dry?

Yes the wood is dry. It is two years old and kept outside under sheets of roofing.

3- How long have you had the stove? Is it possible that the primary air feed may be partially blocked?

We have had the stove two seasons. The air feeds are open, including the little 1/4 inch holes through the bottom of the ceramic? secondary system.

BrowningBAR said:

Try giving it more air. For my Encore cat stove the temps have very little swing to it. I have no fear in leaving the stove with the air controls opened up a bit when I go to bed.

Does your Defiant vary greatly on stove temps throughout the burn cycle? If not, give it a little more air and don’t close it down all the way.

We were trying to reduce the air in order to make the wood last all night. We were not particularly worried about the stove overfiring.

Peakbagger said:

Assuming the stove is tight, trying to increase the draft is not going to do anything about backpuffing. Backpuffing is directly related to a hot stove too much fuel and not enough air. When you throttle down the stove too quickly with a full fuel load you are generating large volumes of potentially flammable gases, if there in adequate O2 in the stove, the gases will enter the chimney and at some point it will get access to enough air that it will ignite in the stack. At that point a pressure wave builds up and pressurizes the chimney and the stove which causes a puff. When the puff occurs ,the air damper opens which allows more air in the stove potentially causing a secondary puff.

What it comes down to, is that backpuffing is the “nature of the beastâ€, unless you modify the stove so that you cant shut down to air (like an EPA stove with hidden secondary air ports), learn to cut back on fuel slowly and don't try to “bank†up the stove at night.

Peakbagger, that is about the clearest explanation of what I have observed with our stove. I have considered installing an outside air kit and putting a butterfly valve in it so that we can get full control of the secondary air supply. I just don't know if that will help. Maybe it is just the nature of the beast.

One of my theories is that when the stove gets throttled down for the night and is not putting out as much heat up the chimney that the chimney cools off and the cooler gases in the chimney become too heavy for the lighter gases to push out. Thus, the thought of a smaller chimney or a better insulated chimney. The smaller chimney would seem to give me move velocity so that the gases stay warm enough to exit properly. The better insulated chimney would seem to keep the gases warm enough to continue rising.

Any more thoughts?

The Encore easily goes over night with an 8 hour burn. How long are you getting out of that Defiant. It should be getting at least 8 hours without choking it down.

I'm not sure what vintage your Defiant is, but in the classic non-cat Defiants the "little explosions" you're having can become a big explosion with enough power to blast the griddle plate right off and create really impressive shooting flames from the opening. Smoldering wood releases combustible gases. The sudden introduction of oxygen, by opening the loading door, for example, can make a large boom.

The desire for overnight burns leads many people to stuff the stove full and then shut down the air. A better method, in my opinion, is shutting down the air when you have a good, deep bed of coals. The volatile gases have already been released and you'll have enough coals left in the morning to quickly get the stove back up to operating temperature. The stuff it then shut it down method creates a lot of creosote and can be dangerous in a stove like the Defiant in which a part like the griddle is held in place only by its own weight.

I disagree on the backpuffing and tight stove. Increasing the draft can indeed improve the situation, especially if the wood is less than idea. We had a situation where a fellow was burning, shall we say, less than ideal fuel and had lots of backpuffing. Extending the chimney did nothing. However, burning better wood and leaving the draft open further did stop almost all the back puffing. He still has some but not as violent and he got them only very occasionally.

Installing an OAK and then putting a valve in it seems to be contrary. Install the kit but don't block it, otherwise, don't install it at all. You need more air so don't try to block it.

Another thought on backpuffing and this goes along with the draft. If a stove is dampered down too much and especially if you have lots of smoke, that smoke can and will go up the chimney too slow and the chimney will cool. This will effectively stop or almost stop the flow of the smoke and gasses out of the chimney until you suddenly get the little explosion and backpuff. Keeping the flow going in the chimney is the priority and if closing the draft too much, you are causing some of the problem.

Good luck and I hope you get this figured out. After any backpuffing if the smoke is too bad in the house, light a couple candles to get rid of the smoke.

I will stick to my contention that it is backpuffing. I have a 32' high interior chimney that rarely if ever needs cleaning and has more than adequate draft. If I want my Defiant to backpuff, I can predictably get it puff quite easily by making sure its good and hot, stuffing it full of wood and then cranking the damper down. Give it a few minutes to gasify and get ready for the griddle to levitate. Once I gave up on all night burns and watched when I did my final load of wood, I didnt have any backpuffing.

Technically every flammable gas has a flame propogation speed which is the speed a flame travels down a pipe. I donT have a value for wood gas which is predominately CO, but its usually in the 2 to 7 feet per section range. In order to avoid backpuffing, the speed of the exhaust gases would need to exceed the flame propogation speed, and I dont believe that natural draft is ever going to reach that speed. The only possibility is that you could keep the O2 out of the pipe until after the CO exits the flue but all that would do is create a fire ball at the top of the exhaust stack. This is a very simplified version of the flame dynamics, I used to have to size and design where to install rupture disks on a hazardous flammable gas system and it was quite complex.

As said before, with well seasoned wood and air closed 3/4 way
you should not experience any puffing with the set up you described.
I had the same stove and it was a beast (tempermental I must add!!).
With full box of dry fuel you should easily get 8 hr burn from this unit!
You should read up on "everburn" or "neverburn" here on the forum, loads
of info on how to properly operate this stove!

I agree with Backwoods and peakbagger on the backpuffing, a better drawing chimney can help with the problem and you can make some good drafting stoves back puff with bad operating practices, so not much help just pointing out the obvious that YRMV.

If it calls for a 6" you need a 6". In your manual it talk about back puffing and how the problem gets going. Going from a 6" to a 8" the gases cool and become heavy and draft stops working gases fall, fill the burner and puff.

I always try to find the cheapest solution possible first, to any problem.
Start with adjusting your burning style and reading up on things here on the
forum!

Thanks all! The problem seems to always occur when we stuff the stove late in the evening trying to get an all night burn. We hardly ever get 8 hours at night. There is however usually always enough coals burning so that the stove starts right up. I guess it must be the human condition that causes to want to fill up the stove and shut down the air.

We can easily change our burning habits and see what that does for us.

What is more likely to cause backpuffing, wood a little too wet or a little too dry?

What is class A pipe? My pipe is double or triple walled and insulated.

I have a vermont castings encore 2040 cat with a 6 inch pipe. I have backpuffing issues. Thus, I do not feel extending your pipe or downgrading from 8 inches is going to resolve anything. I agree with previous posts mentioning increasing the air supply. I have effectively resolved my backpuffing by closing the draft and then slightly opening it (maybe an 1/8 of the way). The downside of this resolution is that the stove reaches 600-700 degrees at some point during the burn time and can barely get an 8 hour burn time. I have a 1200 square foot house and try to keep the stove around 500 degrees. But as I see it, this is the only solution. I have researched everything else and have changed all of my other burn habits. This was my last resort.

I would like to try to get a nice bed of goals and bank the stove down as the gentleman from Maine mentioned earlier. I sincerely doubt I will get an 8 hour burn, but that's worth trying. I understand why loading the stove up and banking it down is a bad practice. The science that he explains makes sense. I just am afraid I will have dead coals when I come out in the morning : (

In my experience, wood that is wetter can cause more backpuffing. For us, we really have to make sure we have a heavy bed of coals and a 300+ degree pipe before closing down the stove for the night and cutting back the draft. If I go to bed at 9pm, I'll load the stove down at 8 pm and let the temps soar for an hour with the flue open. This burns off all "live" parts of the wood and allows those gases to leave the firebox. Then, at 9, I'll fit 2 more logs in the top and choke her down. This usually prevents the backpuffing and gives me a 10 hour+ burn.

Good luck! It's a mad science!

We spent nearly two seasons with this stove. We (well, I) went crazy attempting to remedy the backpuffing (mini-explosions that farted smoke into the house, not just a bit of smoke escaping when the griddle was open). It seems to me that this is a result of gasses building up in the stove and igniting. I don't think that decreasing your pipe to 6" would resolve this issue, because it's not going to move the gasses out of the stove any faster - in fact, it may complicate things more.

In our situation our wood was fine, we had 20 ft. of 8" pipe, and every piece of info. from this site and other avenues in how to run the stove - including everything mentioned above. Nothing we did made it any better. We had the fountain assembly replaced and that helped for a short time, but within a month we were back to random farts of smoke from the stove and it got progressively worse as the second season wore on.

The only thing that made it better was when we replaced the stove with an f600. No backpuffing since.