Back Puffing

[AppalachianStan]

Can you please go though all the thing that can causes back puffing in a wood stove? Trying to find the problem in my stove set up. I find this http://firecatcombustors.blogspot.com/2013/10/what-causes-catalytic-stove-to-back-puff.html when my stove get up to 1000*- 1200* on the cat probe and the load is almost burned up it back puffing bad. I also saw if the stock is cool it will causes it to back puff so I not think a barometric draft regulator would work.Will the BDR keep the stock cool? The air inlet to the stove are clean. The chimney is clean. The cap is clean. The wood so so may be 20%. CSS Nov. 2012. The chimney class A 15' X 8". The stove gasket are good. Only thing left is to add more chimney hight or go to a 6" class A chimney. It will be Tuesday be for I can get the fire going it has warm up out side for a fire. Thank for all the help.

 

[Ashful]

Backpuffing is typically caused by combining weak draft with a very low primary air control setting. The weak draft is caused by a combination of warm outside temperatures and/or short chimney. The problem can be made better or worse by selection of wood (resinous woods and walnut are most likely to cause the problem, in my experience).

In an ideal world, you supply fresh air to the stove at the minimum rate required to support combustion of the volatile gasses coming off the wood. However, if the wood is off-gassing very quickly, and your primary air control set very low, you run into a situation where the gasses are building up in the firebox more quickly than they can combust (due to lack of oxygen from fresh air). These gasses build up, while fresh air is slowly leaking into the stove. Eventually, sufficient oxygen mix is achieved for combustion, and *whoosh*, backpuff. This immediately uses up the oxygen in the firebox, and the stove again stalls. Air slowly leaks in... repeat.

When this starts happening, I have to rely on experience to judge my course of action. Opening the air supply will make it stop, as you're now supplying sufficient make-up air to maintain constant combustion. In fact, I think Woodstock PH owner rideau once said, "if I don't see continuous flame, I can expect back-pufffing." Going back a year, this was always my course of action, too. However, then I'd sometimes find the stove running hotter than I like.

More recently, if I find the backpuffing start up in the 5 - 20 minutes after turning down the stove, I will sometimes just wait it out. The theory there is that the present burn rate is too high for the air I'm supplying, but that having just turned down the stove, the present burn rate should soon slow (and the back-puffing thus subside). If you find the stove backpuffing, and it's been more than 20 minutes since you made an air adjustment, then obviously the off-gassing is on the rise for your current setting, and it's time to give it a bit more air.

By "a bit more air", I find that VERY small adjustments of the air control help. Unless the reaction is so violent, to where you're worried it's going to blow the stove doors open, make your adjustments very small and wait a good couple minutes between adjustments. It will only take one or two small adjustments to make the back-puffing go away.

edit: almost forgot to add... I have run the same stove on three different chimneys:

1. 5' stove pipe + 10' of 8" clay tile flue: back-puffing was almost un-avoidable. Constant problem.

2. 5' stove pipe + 10' of insulated 6" flex liner: back-puffing is avoidable, as long as I don't shut air control to 0%. Leave at 5% - 10%, and it runs great.

3. 5' of stove pipe + 27' of insulated 6" flex liner: back-puffing never happens. I will occasionally get the, "die, then flare up," type behavior, but never strong enough to push any smoke out thru the inlets. I run this configuration with the air completely shut to 0%.

 

[AppalachianStan]

Joful, My primary are was open all the way up. and the back puffing did not start untill the wood was all burned up and temps was at 1000*.

 

[Charles1981]

Sometimes If I have let my stove build up too much ash and I have the secondary air control turned all the way down I will get what I believe is mild back-puffing: Just smoldering logs and then every 15-25 seconds a wave of flames goes across the logs from left to right and then goes out again. If I open the secondary a 1/4 turn usually flames appears and the process stops.

I know back puffing typically is a problem due to smoke leakage and I have a top loading stove, but smoke never leaks out, there is no rattle, no sound of explosion or huge WOOSH...it just is a lazy flame that pops up and slowly coarses over the logs typically from left to right and then goes out and repeats.

Is this even considered backpuffing and if so should I be terribly concerned? Usually i just empty the ash pan and it probably increases oxygen through the air intakes in the ask pan and is why the process stops, but I do like to run the stove with a very full ash-pan as I get much longer burn times and far better coals after a long burn with a full ash pan than after emptying it.

 

[Sprinter]

It could be a draft reversal problem which has similar results. But this statement:

Sometimes If I have let my stove build up too much ash and I have the secondary air control turned all the way down I will get what I believe is mild back-puffing: Just smoldering logs and then every 15-25 seconds a wave of flames goes across the logs from left to right and then goes out again. If I open the secondary a 1/4 turn usually flames appears and the process stops.

sounds like a backpuff. Your problem may be something related to the cat and I'm not a cat owner (well, I am, but not that kind of cat). But the only time I've experienced a true back puff is when I turned the air down too quickly on my non-cat. I also have a pretty short flue at 13' which I'm sure contributes.

If your fire is hot and needs O2 but you deprive it of enough, it will suck it in any way it can. Backpuff. It's gasping for air.

 

[Ashful]

...when my stove get up to 1000*- 1200* on the cat probe and the load is almost burned up it back puffing bad.

Joful, My primary are was open all the way up. and the back puffing did not start untill the wood was all burned up and temps was at 1000*.

I saw this statement the first time, but was confused. How is your cat at 1000F, if you're at the end of a burn cycle / load almost burned up? The cat will typically have fallen out of ignition at this point, as there's not much volatile left in the wood to feed the cat. Describe exactly what you mean by "back-puffing."

 

[Grisu]

Joful, My primary are was open all the way up. and the back puffing did not start untill the wood was all burned up and temps was at 1000*.

Plus, how can you have back-puffing if the primary air is all the way open? Back-puffing happens when the fire is starved for oxygen not when you give it plenty of air. Do you have a flue thermometer? Your cat may still be hot but your exhaust air may not which will result in insufficient draft. When was the last time you cleaned the cat? Maybe that one is plugged badly.

 

[Sprinter]

Back-puffing happens when the fire is starved for oxygen not when you give it plenty of air.

That's just what he said is happening in the following quote (post #4): I'm not sure what's going on.

Sometimes If I have let my stove build up too much ash and I have the secondary air control turned all the way down I will get what I believe is mild back-puffing: Just smoldering logs and then every 15-25 seconds a wave of flames goes across the logs from left to right and then goes out again. If I open the secondary a 1/4 turn usually flames appears and the process stops.

 

[AppalachianStan]

It will run for 2 hours at 800* then it will pick out at 1000- 1200*.It will run at 1000 - 1200* for about 30 mins to an hour. then there is no wood left to burn just coals and the temps will start to drop off and this will take about 2 hours to get back down to around 300* to reload. And the primary air is open from the start of a load to the end. The fire will go out then a big fire ball in the stove push out smoke. Now to get this to stop I can open the bypass by say 1/8" but the temps will rise.

 

[Ashful]

A plugged cat might be it, or maybe something plugging the primary air inlet. I remember one story of primary air blocked by a dead mouse carcase. I can't imagine running any stove at full open air, without self destructing, unless something was obstructing the air.

A gauge on stovetop temps would be interesting.

 

[AppalachianStan]

A plugged cat might be it, or maybe something plugging the primary air inlet.

No plugged up cats can see though them. Not a plug primary air inlet it is clean. and not a plugged chimney cap it is clean.

A gauge on stovetop temps would be interesting.

Stove top temps are at 400 when cat is running at 1000* and 500* when the cat is 1200* but flue temp are 250*

 

[AppalachianStan]

Here is the beast

 

[Ashful]

Weird. All your temps are perfect. Has this problem always existed with this setup, or was there some recent change?

 

[AppalachianStan]

recent change

Cats where put in in Feb.

Has this problem always existed with this setup

Draft has been a problem. But it have never Back puffing before.

 

[raybonz]

https://www.hearth.com/talk/threads/backpuffing-with-a-cat.75556/#post-956487

Ray

 

[Grisu]

Did you do any changes to your house recently? Like tightening up windows or doors? Or add a bathroom fan? With a poor draft that may have just pushed you over the edge.

I don't know your stove but is it normal for that model that it needs to be run with the air fully open? Mine would go nuclear in that case and I will send a lot of heat up the flue.

 

[Woody Stover]

I got some of that flash-over action last night with the Buck; Let too much of the load ignite at start-up, so a lot off wood was off-gassing. With 21' of stack, there's enough draw so that no smoke escapes the stove. My main problem, though, was that the cat wanted to go high with all the available smoke to burn. I didn't like doing it, but I ended up leaving the bypass a little open so I could leave.

Stan's shorter stack, and a warmer night would create good conditions for a back-puff. But I, too, don't see this happening when the wood is mostly burned up; Not enough gassing happening. Now if you have been burning wet wood (IIRC, Stan's wood could be drier) and have gooey creosote built up in the fire box, it can gas when the stove gets a little hotter. Don't ask me how I know. But even that shouldn't be a problem with the air open. So like Joful, I'd like to hear what exactly is happening when it 'back-puffs,' Stan?

Stan, seems like adding 3' of 8" chimney would be cheaper than buying a full-length 6" stack. The only thing is you would probably have to brace the additional chimney....

 

[Woody Stover]

Did you do any changes to your house recently? Like tightening up windows or doors? Or add a bathroom fan? With a poor draft that may have just pushed you over the edge.

I don't know your stove but is it normal for that model that it needs to be run with the air fully open? Mine would go nuclear in that case and I will send a lot of heat up the flue.

I think it's pretty similar to the Buck 91 that I run, and I cruise it with the air barely open. But like we've said, he should be having more of a problem at the start of the burn with low air, if it is a classic back-puff....

 

[AppalachianStan]

Did you do any changes to your house recently

No Changes.

Stan's shorter stack, and a warmer night would create good conditions for a back-puff

It was 29* the last time it back puffed.

I'd like to hear what exactly is happening when it 'back-puffs

If I start a cold stove I put in 2 splits with kindling in the mid to get some coals rack coals to front of firebox then put 4 to 5 split on the coals NS bring stove up to 700* on cat probe then close the door. the stove will level out at 800*. Stove has been running at 800* on the cat probe 250* on the flue and 350* to 400* on stove top with the by pass closed and the primary air open for about 2 hours with small flame in the firebox on 5 splits then the temps start to rise to 1000* the fire goes out, a second later the gases build up then it ignites with a fire ball and out comes smoke this will repeat until it hit 1200* and the log stops off gassing then the temp starts to drop for 2 hours until it time to reload.

 

[AppalachianStan]

adding 3' of 8" chimney would be cheaper

Yes it will be cheaper but it will run me about $300 to do this.

 

[AppalachianStan]

Are you guys sure by adding 3' of chimney will help my draft? Will 18' of flue be enough or should I go for 20'
I will be going by Lowe's tomorrow to order a chimney and the chimney brace.

 

[Sprinter]

Are you guys sure by adding 3' of chimney will help my draft? Will 18' of flue be enough or should I go for 20'
I will be going by Lowe's tomorrow to order a chimney and the chimney brace.

The way it works is that for every so many percent increase in length, the draft will increase by the same percentage. So 3 ft would increase draft 20% in this case. You'd have to figure out if that's enough. I think I'd try that first and see then you could add more later if necessary, especially if you have to add supports.

 

[AppalachianStan]

Order 3' Class A chimney and the chimney brace for $209.00 from Lowe's it will be here in 20 days. That will give me 18' of chimney.

 

[Sprinter]

Hope it works. 20% doesn't sound like much, but it may make the difference.

 

[Ashful]

Yeah, but I think this is the difference between being right on the recommended minimum vs. not.