Whuffing Kodiak Stove

[Eightball1313]

I posted about this before, seems like 75% of the time anymore when i load the stove it will "whuff" like a machine gun basically the whole time theres wood in it to burn. Once in a while if i dont control it right, smoke will whuff out of the air intake, but usually nothing happens other than the whuffing sound. But its annoying to me. I have never had this happen so frequently in all the years ive burned wood stoves so whats the deal all of a sudden? Chimney is clean as a whistle. The only thing that seems to work for managing the whuffing is to turn the air intake down and find a sweet spot where it will kind of stop, but thats nonsense to me. I should be able to burn the thing wide open if i want or burn it closed off if i want i shouldnt have to be sitting there every night for an hour or more making sure smoke doesnt start pouring out of the ait intakes because it just decides to start whuffing like crazy. Like i used to get the stove going, go do something for 5-10 minutes come back to a ripping fire and tune it down for a long even burn, but now i feel uncomfortable leaving the stove while it heats up because of the whuffing, like i said some times theres smoke and ive even seen flames come out of the air intakes already. I promise you all when i say this has only occured once or twice before this winter and ive lived in the same home with the same stove for a few years now, burning all wood 20% or lower that i have years ahead stock piled and what not. it has to be a draft issue all of a sudden idk.

My only ideas are:
1. We have had a very mild winter here in Pa so maybe the chimney just isnt drawing like it should, idk maybe i need to increase height.
2. MAYBE all of a sudden my stove doesnt like the whole 8" outlet reduced to 6" stove pipe right away to go into the thimble (yes i know this is wrong and shouldnt be done), but like i said ive been burning this stove like this the same way for so many years so idk why it hasnt done this before.

I attached a short video i took last night of it on my phone. This video doesnt do it justice, it has whuffed a lot more violently than that before but just for an idea take a look.

thanks

 

[Corey]

... The only thing that seems to work for managing the whuffing is to turn the air intake down and find a sweet spot where it will kind of stop, but thats nonsense to me. I should be able to burn the thing wide open if i want or burn it closed off if i want i shouldnt have to be sitting there every night for an hour or more making sure smoke doesnt start pouring out of the ait intakes because it just decides to start whuffing like crazy. ...

2. MAYBE all of a sudden my stove doesnt like the whole 8" outlet reduced to 6" stove pipe right away to go into the thimble (yes i know this is wrong and shouldnt be done), but like i said ive been burning this stove like this the same way for so many years so idk why it hasnt done this before.

thanks

I think you've sort of ask/answered your own question here. The air inlets / stove were designed to have an 8 inch flue. That is about 50 square inches of area on the outlet. You reduced that to 6 inch pipe. That is about 28 square inches of outlet. Nearly a 45% reduction in the stove outlet.

So you run the air wide open and the fire flares up, the smoke/flue gas hits that 6" restriction and can't exit the stove, so the fire dies down - choked by its own combustion products. Eventually the smoke clears and the fire flares up again, only to choke itself off because smoke can't exit through the restriction fast enough. Sounds like this might be happening a few times a second based on the video.

What makes it worse? Mild weather, humid weather both mean less draft, less ability to clear the smoke. Added air leaking into the fire - worn door seals, improperly closing door, poor fitting ash pan etc - all mean a bigger flare up. Combining any of these can make things worse still and yes - if you hit the exact right conditions / resonance point the whole thing can get pretty violent. Search for 'pulse jet engine' or 'buzz bomb' for the extreme examples.

How to fix it? Well, right off the bat, if you cut the outlet by 45%, you need to cut the inlet by 45% - so that would very roughly mean keeping one of the door inlets closed completely - or opening both only about half way. Though you might actually be surprised - generally stoves will run hotter as the air is closed down from 'full' because there is a longer time for the smoke to transfer heat out of the stove before it goes up the flue.

Hope this helps.

 

[Eightball1313]

I think you've sort of ask/answered your own question here. The air inlets / stove were designed to have an 8 inch flue. That is about 50 square inches of area on the outlet. You reduced that to 6 inch pipe. That is about 28 square inches of outlet. Nearly a 45% reduction in the stove outlet.

So you run the air wide open and the fire flares up, the smoke/flue gas hits that 6" restriction and can't exit the stove, so the fire dies down - choked by its own combustion products. Eventually the smoke clears and the fire flares up again, only to choke itself off because smoke can't exit through the restriction fast enough. Sounds like this might be happening a few times a second based on the video.

What makes it worse? Mild weather, humid weather both mean less draft, less ability to clear the smoke. Added air leaking into the fire - worn door seals, improperly closing door, poor fitting ash pan etc - all mean a bigger flare up. Combining any of these can make things worse still and yes - if you hit the exact right conditions / resonance point the whole thing can get pretty violent. Search for 'pulse jet engine' or 'buzz bomb' for the extreme examples.

How to fix it? Well, right off the bat, if you cut the outlet by 45%, you need to cut the inlet by 45% - so that would very roughly mean keeping one of the door inlets closed completely - or opening both only about half way. Though you might actually be surprised - generally stoves will run hotter as the air is closed down from 'full' because there is a longer time for the smoke to transfer heat out of the stove before it goes up the flue.

Hope this helps.

thanks for the reply.

I have been meaning to rebuild the chimney and put a bigger thimble in, or try a smaller stove i have that is only a 6 inch outlet just the kodiak is a bear to move lol.

I have tried various things with the intakes and basically never have them fully open, but i never tried closing one off like you said so ill have to try that.

I have learned over the years that it will burn plenty hot when you have the intakes closed down more, its like finding the sweet spot, can have too lean of burn and vice versa so i hear what your saying.

I think im going to try making halfway on the intakes the max and start the fire like that and adjust from there, i definintely think it has something to do with the mild winter too cuz like i said its done it once or twice before but this year it seems like almost every time i have a fire it does it, i like to burn it nice and hot since its an older stove it definitly puts out the smoke but the chimney has never been very dirty, mid season clean out was almost nothing, just a little powder.
thanks for the advice ill let you know how it turns out tonight, i figured the 6 inch pipe had something to do with it but i just never got around to putting a new thimble in, i have pour concrete walls in the basement so that will be a task and half to cut a new hole

 

[peakbagger]

Yup, a big stove with installation issues just will not be happy in a low load situation. Time to get a smaller stove for shoulder season.

 

[coaly]

What size is the chimney flue? You mention the stove has 8 inch outlet, reduced to 6, but what is the chimney flue? Round or square? And height from stove top to top Of chimney.

 

[Eightball1313]

What size is the chimney flue? You mention the stove has 8 inch outlet, reduced to 6, but what is the chimney flue? Round or square? And height from stove top to top Of chimney.

its a square flue that is 20-22' feet, forget exactly should run a tape down it one time and get the actual measurement, like i said ive been meaning to rip the chimney down and build a masonary or just run insualted pipe but just havent gotten around to it the last few summers.

 

[Eightball1313]

Yup, a big stove with installation issues just will not be happy in a low load situation. Time to get a smaller stove for shoulder season.

yeah when i bought the house years ago there was a small stove in it already, but i wanted my kodiak, it was passed along to me from my dad so i refinished it and what not. i dont even know what brand that other small stove is but its nothing new definitely and old smoke dragon stove lol. ive been meaning to try it out but seems like winter always gets here before ya know it and i just never feel like moving the stoves around, especially the kodiak that thing was a bear to get down into the basement, the little stove i can pick up myself and carry, still heavy but not like the kodiak lol

 

[Eightball1313]

and like i said, this whuffing ordeal really never gave me any problems until this winter all of a sudden. i really think a big thing has to do with the fact that this whole winter has just been so mild with some days being in the 50's and going into the 20's at night i just feel like it makes the stove not wanna draw right.

but last night i tried pretending that halfway on the air intakes was actually "full way" and started my fire like that and turned it down from there and had no whuffing at all and yesterday was like mid forties and raining all day so it seemed to be fine even tho that kinda weather usually calls for a crappy draw, seemed ok ill keep experimenting but ultimately ill end up redoing the chimeny one of these summers.

 

[coaly]

So the flue is 8 X 8 square?? I'm trying to figure your square inch area and cubic feet internally.

 

[Eightball1313]

So the flue is 8 X 8 square?? I'm trying to figure your square inch area and cubic feet internally.

yes it is 8x8

 

[coaly]

That's the problem, combined with the warmer temperatures this year. Fuel drying can have an influence too.
Legally you can't reduce anymore at all since PA is under the ICC family of codes allowing no reduction, but the NFPA 211 standard allowed up to 1 inch reduction. That said, the stove will physically work fine reduced with the proper chimney. It will not have the full BTU potential at wide open air, but most stoves are not operated in that range for long anyway. The larger chimney has more capacity, but also requires more fuel (heat) to make more capacity work.

So the problem is INCREASING from 6 inch round (28.26 sq in.) to 64 square inches. Increasing allows exhaust gasses to expand, cooling them. They must stay above 250 f to the top to prevent condensing of water vapor from combustion. Otherwise the flue becomes wet allowing smoke particles to stick forming creosote. So closing your air intakes reduces the flutter, but also reduces exhaust gas temps, creating more creosote. The cure is an insulated liner the same size as the reduced pipe. This will prevent expansion of gasses, keeping them much hotter to the top, increasing draft.
(We have a very good manufacturer of chimney liners both flex and solid locally in Gouldsboro)

The issue is caused by weak draft. Draft being the measurement of pressure differential caused by rising gasses due to the temperature differential between inside and outside of chimney. This year we have less differential temperature (we're close) and reducing the square inch area of flue increases velocity of rising gasses. The increase of velocity increases the NET draft measurement.

The dryer the wood, the faster it will gasify, so you get quicker ignition and becomes the flutter. Adding too much cardboard will cause the same effect. Slowing the air reduces the excess combustion.

 

[coaly]

If you check the surface temp of the 6 inch pipe before increaser, you will find it drops by about 1/2 the temp when expanded into the 8 going into chimney. That is a huge temperature reduction. The surface temp is about 1/2 the actual inside exhaust gas temp. Then figure how much heat loss to top and you have an idea how hot to keep the pipe. As an example 250* surface would be about 500 internal. Allowing that to expand over twice the internal area cools below the 250* needed before it even cools more rising to the top. That 250* surface temp in an insulated liner the same size would be 500* internal staying well above condensing point. Now you can add a baffle to increase stove surface temps, adding more heat to the home with the same fuel. The baffle adds internal resistance as a pipe damper would, slowing velocity, but you can do that with the smaller diameter liner. If you tried a baffle now it would further decrease draft. This stove may have a factory baffle, depending on age, but I'm referring to a custom baffle made for the size of your reduced chimney flue.

 

[Eightball1313]

If you check the surface temp of the 6 inch pipe before increaser, you will find it drops by about 1/2 the temp when expanded into the 8 going into chimney. That is a huge temperature reduction. The surface temp is about 1/2 the actual inside exhaust gas temp. Then figure how much heat loss to top and you have an idea how hot to keep the pipe. As an example 250* surface would be about 500 internal. Allowing that to expand over twice the internal area cools below the 250* needed before it even cools more rising to the top. That 250* surface temp in an insulated liner the same size would be 500* internal staying well above condensing point. Now you can add a baffle to increase stove surface temps, adding more heat to the home with the same fuel. The baffle adds internal resistance as a pipe damper would, slowing velocity, but you can do that with the smaller diameter liner. If you tried a baffle now it would further decrease draft. This stove may have a factory baffle, depending on age, but I'm referring to a custom baffle made for the size of your reduced chimney flue.

yeah i always keep an eye on the surface temp of the stove pipe, from what ive learned is if i keep it above 300 most of the time and really get it hot like around 400 degrees once every day that theres basically no creosote in the chimney, i never have too much at all.