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Wood stove hack-secondary burn tubes added

Cowboy Billy

Cowboy Billy

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Awesome Job. I have been thinking of putting a secondary air on mine. I was going to buy air tubes from a newer stove. But yours is a much better idea. Thanks for posting it. I am not going to do mine until summer but I will let you know how it turns out.

Billy
 
volks-man

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If this is correct, could the preheat pipe actually be counter productive?

Ian
how big a factor is "cooler" intake air temps?

doesn't some wood stoves route intake air to pre-heat?
I think "cooler" is a relative term. The gases at the top of the firebox are anything but "cool" when a hot fire is going. The idea of preheating is so that the secondary air you introduce into the firebox is hot enough as to not self-extinguish the hot gases at the top of the firebox, and to promote a vigorous burn.
honestly guys, i have no idea how or why this stuff works. :) i likened it to the air injection system on older gm vehicles. cooler oxygen rich air was injected in to the exhaust manifold just outside the head. this resulted in the unburned hydrocarbons burning. why? how? i dunno! also reminds me of the 'bessemer process' in making steel. if you inject molten steel with cool oxygen rich air the contaminents burn off giving you purer steel.
what is it that lakeside's sig says? ;)
 
Techstuf

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If he doesn't want to do the door mod, he could maybe go to a used appliance dealer or junkyard. See if he could score a piece of oven glass or other tempered glass to fit over the stove doorway. Put a bead of silicone around the edge and press it up against the doorway to occasionally inspect or show off the results of his conflagration creation.


TS
 
Haywire Haywood

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he could install a piece of stove gasket on the glass and just hold it against the opening using an oven mitt to watch the flame... get a thick oven mitt though LOL.

Ian
 
farmermike

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This is an interesting post. Question, does the secondary burn only work if its a good hot fire? What about when its just idling along? My stove in the barn has a manual damper in the door and most of the time its almost closed. This is a great idea and I could install one easily.
 
volks-man

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my stove must be hot, with a good bed of coals. i must shut the regular draft for best effect. the hotter the stove the better it draws through the tubes when you shut the draft down.
oh yeah, no damper whatsoever. if you had one and closed it i would think you would slow the draft too much.

i can throw a peice of wood in on top of just coals and shut the door with draft shut....
and the wood starts charring with flames burning above it. it almost looks like gas flames shooting out of the holes! neat stuff!
 
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volks-man

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for those unfamiliar

here is a fast vid of the effect. not the best example but sufficient. looks a lot like a backdraft. the flames are moving toward the camera and away from the holes in the top of the firebox. notice the bottom of the wood is barely burning. this is an example of wood gasification.
<embed width="448" height="361" type="application/x-shockwave-flash" wmode="transparent" src="http://i263.photobucket.com/player.swf?file=http://vid263.photobucket.com/albums/ii150/volks-man/PICT0794.flv">
 
chainsaway

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hot oxygen-rich air

it is guesstimated that about one third of the wood's potential heat is lost in smoke in older wood stoves. so an "easy" way to get more bang for the same buck woulb be to find a way to ignite that smoke. thats where the air tubes come in play; they provide oxygen-rich air right where the smoke is (after the flammes) therefore in the O2-poor area. the snag is that this new air has to be as hot as possible so the smoke self-ignites on contact to get the full bang benefit. this is why there is quit a bit of metal ducting exposed to the fire before this new air exists the holes, the last thing you want to do is cool the smoke...hope this helps!
cheers alain
 
Moddoo

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itn

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Yes, pre-heat your over-fire air as much as you can. Then continue to deprive your intitial combustion air. It will vary depending on your wood and moisture.
 
husky455rancher

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i can throw a peice of wood in on top of just coals and shut the door with draft shut....
and the wood starts charring with flames burning above it. it almost looks like gas flames shooting out of the holes! neat stuff![/QUOTE]

ok i get it now. i always wondered how why the fire looked like it was comming from inside the tubes on my insert. i just thought the fire was kinda over the wood alot of the time .cuse the firebox is so damn small and it had no where to go.


so essentially the tubes just hold the heat above the fire and allow the gasses to burn more? would you leave both valves all the way open all the time or do you gotta play with them?
 
sesmith

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I was thinking gate valves must have been used because they are often all brass where ball valves usually have seals that may not take high temperatures.

??
You've got it right. Also, I used the gate valves cause initially I thought I'd be regulating them partly closed, not leaving them wide open.
 
sesmith

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The idea of the small holes is to introduce good mixing of air with the volatile gases to get the combustion as soon as possible near the back of the stove; just dumping it out of the open ended pipes close to the baffle would likely burn it but that would happen further along toward the exit pipe so less heat gained.
Yea, that's what I had in mind. The long run of pipe from the valves up the side of the stove and to the back before going into the burn tubes was to get as much preheating as possible. More preheating of the secondary air is better. The secondary combustion won't occur until around 1100 degrees. Cooler air will just quench the fire. I also aimed the jets toward the center of the stove instead of down for this reason though I doubt it would have mattered much which way they point. Most of the air probably exits toward the rear of the stove (at the beginning of the burn tubes). FWIW, I made the last 6 holes (the ones at the front of the stove) a little larger cause they were at the end of the run and would get less air. Also, since most of the air is going to leave the pipe at the beginning of the run, It has more time to mix and burn if the beginning of the run is at the rear of the stove.

No guys, I'm not getting rid of my cast iron door cause I kind of like it :) I really wanted to see the tubes work in the worst way tho. Unfortunately, when the door is opened, the secondary draft stops and you can't see a thing. So I had the bright idea to open the door and add some air...like with a vacuum cleaner blowing in reverse. Yesterday this worked and I got a real nice blue blast furnace thing going. Today I tried to take a couple of pictures and didn't have a lot of luck cause I didn't get the stove quite hot enough. But the pics below at least give you the idea. The 1st is with the door open and no secondary air. The 2nd is with secondary air added to the right side tube only. I got a little flare, but since things weren't hot enough I didn't get the nice blue flames I saw yesterday.

Also, I noticed a major improvement in emissions when I left for work today shortly after starting a fire. Normally, I'm embarrassed to say, I'd have been fogging the road outside my house when I leave for work. Today, there was very little smoke. (I'll try to post some pics when I get some time to take them) I built a fire at about 7 am and set it on a slow burn. Got home tonite at 6. The house was only 2 degrees colder and I had a good bed of coals to work with. Normally, I would have been playing boy scout with about 2 coals and scraps on the floor to start the fire after 11 hours. I'm loving this thing. I'd have been happy if I bought a new stove and it worked this well. The fact that I was able to do it for less than a hundred bucks is icing on the cake.
 
sesmith

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I think the biggest reason shutting down the secondary air to burn down a large bed of coals works is because the primary air is usually enters right at coal level. Secondary air most likely just cools down the fire box in coal burn down mode.

Just rambling off theories! Don't quote me.
I wondered about this too. Yesterday I measured temps at the same place on the stove pipe (my point and shoot IR thermometer doesn't go much over 500 degrees) at various times during the burn with the secondaries opened and closed. At no time did leaving them open decrease the temp. When they're working, the temp. increases with them open. When they're not working at the end of the burn, they have no effect on the flue temp.
 
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