Wood stove hack-secondary burn tubes added

A week or 2 ago I put a question out on this forum here:
http://www.arboristsite.com/showthread.php?t=85750
Basically, I wanted to know if replacing my stove with a more modern EPA stove would save me much wood. Since I cut my own wood and our stove is in the cellar (where it's not part of the furniture in the living room), it would be pretty hard to justify the $2500+ for a new one, and the one I've got heats the house pretty well. One of the replies was from KSWoodsMan, who said he had put secondary burn tubes in his older stove with good results. Now why didn't I think of that! So instead of cutting wood in the cold yesterday, I worked on the stove. The 1st picture below is of the tube assemblies as they were going to look in the stove. The 2nd is of the tubes mounted in the stove and the 3rd is of the outside. The tubes run up the sides of the stove. Secondary air can be controlled from the gate valves and shut off if there were ever a chimney fire. Then it enters the front of the stove over the fire, is routed through solid pipe to the rear of the stove for preheating, and then is dumped into the top of the burn area just under the baffle through the holes drilled down the length of the burn tubes. I didn't weld the tubes in place in case I ever have to change them, so I added a couple extra 1" washers so the coupling inside the stove and elbow outside the stove could be tightened against the stove wall and sealed with high temperature masonry caulk.

So far, I'm kind of impressed with my cob job. After a lot of experimenting today, I'm finding that I can get nice long hot burns by leaving the secondary valves wide open and closing the primary controls way more closed than I would normally run them. The secondary air doesn't just make the wood burn faster. In fact, it looks like I may be getting longer burns than I would normally get and put out the same or more heat. When the fire is burning at a good rate, opening up the secondaries from the closed position without changing the primary controls gets me about a 100 degree gain measured on the stovepipe. So I guess they're working. Unfortunately, since I don't have a glass door I can't see them working like you guys with the new stoves can So if my stove burns a little cleaner and I burn a little less for the same heat, I'll have it made.

So now for the disclaimer. I'm not recommending anyone try this on their own stove. If you try this on your old stove and trash the stove or burn your house down, I don't want to hear about it If you do try it and it works, or you improve on my design, let me know so we can compare notes.

nice job!

Nice work! + rep here.

All New pipe You must be rich Nice job hope it works great!

im new to wood stoves. what do secondary air tubes actually do? i see i have them in the insert i have but not in my woodstove. i would be inclined to hack up my stove if it is worth the effort. great job bt looks real nice. what type pipe did you use? anything special or just plain steel pipe?

husky455rancher said:

im new to wood stoves. what do secondary air tubes actually do? i see i have them in the insert i have but not in my woodstove. i would be inclined to hack up my stove if it is worth the effort. great job bt looks real nice. what type pipe did you use? anything special or just plain steel pipe?

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cooler oxygen rich air hits hot rising wood gases and whal-la, the fumes ignite rather than going up the chimney! do it right and your firewood will cook on top of a coal bed rather than burn. the flames will actually be burning above the wood rather than on it!

husky455rancher said:

what type pipe did you use? anything special or just plain steel pipe?

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3/4 id (1" od) black pipe.

Nice work! Now ya gotta do what that other fellow did and hack in a window.

Ian

Mighty fine install!I wonder why you need a fancy tube with holes to do this job?Why not just set of pipes termenating at the baffle mixing air at the baffles edge You could still run the preheat tube and delete the hole filled tube and still have the same effect could you not??Just a question in my mind?If you ever study your fire with the door open the flames tend to follow around the edge of the baffle.

volks-man said:

cooler oxygen rich air hits hot rising wood gases and whal-la, the fumes ignite rather than going up the chimney!

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

Ian

Good job!...rep deserved. :newbie:

Looks good and alot higher tech than what I was thinking. Be interesting too see if you infact use less wood and get longer , hotter burns over time. In the old Tempwood stoves that I replied about in the first thread, they used two tubes perpendicular too the stove top pointed down (downdraft) with sliding air controls (just a flat plates). Very simple but quite effective. Here's a pix of how those worked.

Great job on your overfire air project. This is part of what 20yr old coal plants are doing to retrofit and reduce NOx emissions.

Nice job & rep for having the stones to just "do it". I have also been thinking of modding my shop stove. It has a really large opening straight to the flue. I put in a vintage cast iron 6" flue damper & it helped keep more heat in the box, but it still screams for more mods.
Will that schedule 40 pipe sag when it gets really hot? Be the guinea pig & stoke it up.......... Please keep us posted on it's performance.

RD

Nice job.

MS,
No, black pipe should hold up for many years.

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

doesn't some wood stoves route intake air to pre-heat?

volks-man said:

cooler oxygen rich air hits hot rising wood gases and whal-la, the fumes ignite rather than going up the chimney! do it right and your firewood will cook on top of a coal bed rather than burn. the flames will actually be burning above the wood rather than on it!

Click to expand...

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.

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.

The air flow through the pipes should keep them from getting hot enough to sag or scale up too badly. The thinner stainless tubes in the patent heaters have less mass and likely are a bit quicker to heat up to light off temperatures. I like watching the fire through the glass door when the flame is more blue than yellow; almost like watching a natural gas flame.

How much heat you gain by good secondary combustion of the volatile gasses depends a lot on the type of wood; my guess is it would be a lot higher savings on on pine or fir than on the hard woods.

Nice retro fit project Sesmith!

That is an interesting point Crofler. I wonder if one could soak some wood in copper sulfate. The out gassing should be blue/green. You should be able to see the difference in the secondary combustion rates?

i understand the burn tubes but what are the taps on the front for? sorry maybe a dumb question but i want all the info i can get before trying on my stove.