OFN Forums

OFN Forums (http://www.offroadfabnet.com/forums/index.php)
-   General Fabrication (http://www.offroadfabnet.com/forums/forumdisplay.php?f=128)
-   -   Wood Stove w/ secondary combustion. (http://www.offroadfabnet.com/forums/showthread.php?t=9391)

kevtsi 01-24-2012 12:26 AM

Your stove is awesome Jaysin. Your craftsmanship is top notch as usual.

My factory made stove has a steel smoke shelf that draws the smoke to the front. But my air inlet is above the door. I can see the fresh air being pushed down back into the fire as it ignited with the smoke.

I'm not sure if that is considered a secondary burn but the stove seems to burn almost smoke free once it is up to temp.

I plan on building a new one some day as the one I have is too small to take the size of logs I tend to cut up.

JaysinSpaceman 01-24-2012 02:13 AM


Originally Posted by kevtsi (Post 117205)
Your stove is awesome Jaysin. Your craftsmanship is top notch as usual.

My factory made stove has a steel smoke shelf that draws the smoke to the front. But my air inlet is above the door. I can see the fresh air being pushed down back into the fire as it ignited with the smoke.

I'm not sure if that is considered a secondary burn but the stove seems to burn almost smoke free once it is up to temp.

I plan on building a new one some day as the one I have is too small to take the size of logs I tend to cut up.

I am glad you like the stove I built, thanx for the compliment.

Secondary combustion is actually the combustion or re-combustion of the hot smoke not simply the movement of the smoke in the fire box. If your stove has secondary air tubes on the underside of the smoke shelf you will see flames that almost appear to be coming out of them (similar to the burner of a gas cooking stove) once the fire box is up to temperature (min. temp. is about 700 degree smoke temp, stove has to be good and hot), the flames are actually the hot smoke re-combusting where the fresh oxygen rich secondary air is mixing with it. I hope that explains it.

Not to dissuade you or anyone else from building a stove but know a few things first. If your non-UL listed custom stove should cause your house to burn down your insurance would likely deny any claim you make. Second, there is a lot of science going on inside a secondary combustion stove, make sure you understand exactly how they work even before you start the cocktail napkin sketches, failure to build it correctly will lead to a lot of problems including smoking up your neighborhood, blacking up the glass, coking up the chimney, possible over-firing, and a bunch of other possibilities. But, building a custom wood stove is obviously able to be done, I did it and with the right understanding you can too, just be ready for a lot of research. Stove companies will not give you any help either, they spend big money to pass EPA testing and are not about to give away their secrets, so you are really going to have to ferret out the science. I will answer any questions that you might have if I can, but make sure you can't find the answers in what information I have already posted. :beer:

Thanx for playing along at home,

TheExpert 01-24-2012 05:04 AM

I have same thinking as 12husky.

ZTFab 01-24-2012 09:37 AM

Awesome project Jaysin.

That stove turned out great.

dselland 03-10-2013 01:21 PM

Looks like you have done very well on this stove

theboilerflue 02-03-2014 12:45 PM

Well information on homemade wood stoves is rare thing, hopefully you're still monitoring this.
My woodstove needs are a little off the off-road topic, so off I'm in the water, I live aboard a 36' steel sailboat in northern latitudes, it's well insulated and and burn beach wood in a homebuilt woodstove made out of stainless steel. I've built a few woodstoves, a couple for myself and a couple for other boaters. The Chimney for mine is a 4" stainless pipe running vertical for about a 14' rise through the deck, no insulation around the deck where the chinmey runs through and the deck plate has been replaced with a stainless plate to keep the paint around the chimney from burning and thus rusting. On a boat an interesting thing about woodstoves is the constant wind gusts (weather) and down drafts (from the sails) so the air intake is also taken form outside, so in theory the stove is completely airtight to prevent smoking out the cabin in every wind gust, it's a pretty dynamic environment the boat yaws, sways and pitches so backwinding is kinda inevitable in nasty weather even at anchor, a little different than a house for sure. Sealing the stove to the outside like this means that even if it does backwind, which is inevitable at times means it just back winds outside anyway. There have been times when I've been sailing upwind and the stove will run backwards, smoking out the air intake and sucking air, I imagine through the chimney, when I remove the chimney extension shortening the chimney rise to about 8' it runs just fine. I think this is because I've got high pressure spilling out the bottom of the main sail and forcing it down the chimney, removeing the extension puts the opening just far enough away form this high pressure off the sail. At least thats my theory.
So I want to build a different stove for myself, for the third time. The first one was real rough, a 8" pipe horizontal about 20" long with the chimney off the top of the stove and near the door the air intake was at the back and was a 1 1/2" ball valve, which by the way ball valves work awesome for air control, I don't know why you'd use anything else. Having the chimney near the door helped keep the smoke out when you opened the door as the in-rush of air would draw it up the chimney. The heat was concentrated at the back of the stove near the air intake. The stainless being the shitty heat distributer that it is would glow red hot in a small spot at the back and slowly move toward the chimney as the wood burned away from the air intake. It was a great stove, threw out a ton of heat the only problem was with the capacity, it wouldn't last the night.
The next stove I built was a copy of a Jodel coal/wood burning stove (Jodel is a Norwegian company that make small stoves) some friends had on their boat. The original is a cast iron six plate that smoked like a bastard out of all the joints, as the constant shifting of a boat never really kept the sealing cement in place. It was a vertical oriented stove with doors both at the bottom and the top, (well the front near the top not the top of the stove). There was a grate to allow air space for burning coal which I incorporated into the design of mine. The copy I built was again stainless pipe, split and flattened open to allow for a flat front, with two doors, top and bottom. The chimney came off the back of the top plate and I had a baffle welded in to form a bit of a shelf. The air intake came in the bottom under the grate and also a smaller pipe came up to dump air in above the shelf. It works well in that it heats the boat (not very hard) but from the amount of wood I've burned doesn't have the same efficiency as my first one, the nicest thing is the size (capacity) it'll burn for a very long time and is built of heavier plate which holds the heat better. The major drawback is that it's hard to fit the wood in this vertical thing unless I buck up the logs very short. Cutting up beach wood dulls a chainsaw quickly so I'd like to avoid having to cut short rounds this for the next one, that and it's a lot easier to gather random long bits of driftwood up when I'm too lazy to pull out the saw.
So I'm curious what you'd have to say about this secondary burn thing. It's my impression that I only get a clean burn when the stove is burning full out which quickly turns this small space into a sauna, and when I close the stove down to turn the heat down a bit then the smoke goes from invisible to dirty.

What do you think of this, is this just the way it is?
Am I stuck with the option of having a small stove that I can burn clean when fully open that won't last the night or a big stove that will last the night and won't have a complete burn?

JaysinSpaceman 02-05-2014 12:34 AM

I had to think about your question and situation for a bit and I think I've got where you're coming from.

First off,I love to see a picture of the stove you're currently using.

Now, the secondary combustion process needs to have two three things to work. One, Smoke. The secondary combustion is burning the unburned hydrocarbons from the actual combustion process, once the wood is turned to charcoal it is nothing but carbon burning so there is no smoke and the secondary combustion will cease. Two, Fresh incoming air not into the primary fire but directly above it and with this it really needs a smoke shelf that lengthens the path/time the smoke and heat stays in the firebox, it also helps if the secondary air is preheated. And third, Heat. The smoke needs to be at least 700 degrees F when it mixes with the secondary air supply to re-combust. This third thing is, I think, what your question is about. The fire doesn't need to be going full on for this process to take place, in fact it works better when the primary fire and air flow is slowed or damped down giving a the smoke a longer time to re-combust in the firebox. From what I have read a good rule of thumb it that as long as there is wood burning (as opposed to charcoal) there should be colorful flame in the primary fire, without visible flame the wood is just smoldering and the gasses wont be hot enough to re-combust. But that visible fire doesn't need to be much, doesn't need to fill the firebox. Just enough that there is some light emitted over and above the light that is coming from the secondary combustion. I personally have a very small Morso stove in my house (I took it apart to study while building the stove in this thread) and I can only get 3-4 15" splits of oak in it (really small stove) and while it wont be burning in the morning there will be enough coals to simply throw wood on and open the air supply to get it going again. It has to be really turned down but still needs that colorful burn. Small spaces are very easy to overheat so I do understand your conundrum. There used to exist an automatic thermostatically controlled mechanical air control for adding to wood stoves that you could set and forget but I didn't really spend much time looking for them. They were cool because they were set and forget and would keep the fire at just the right temperature, although I don't know if they are still available.

I hope this helps,

theboilerflue 02-05-2014 12:22 PM

The smoke shelf is there but relitvitly small unfourtunalty, any bigger and it would be even harder to get wood of any length in and turn the corner down into the stove. I tired to make it as much of an obsticle as I could or the smoke to go around and mix with the air but the whole vertical thing made it kinda difficult. For the next stove it'll be about the same size but horizontal and a rounded off triagular prism shape (to hug the hull) sitting fore and aft (lengthwise), this way the corners of the triangle I can block off for a baffle at the top and a grate thing at the bottom to allow air flow over the whole length under the coals. The main air intake and the chinmey will be at the same end, air intake under the grate and a baffle blocking the top corner to make the smoke travel all the way to the opposite end where the door will be, then it can turn the corner and run the length back to the chimney. Then I was thinking I'd run a smaller pipe up the back side and the along under the baffle and opening into an outlet to feed the hot air into the smoke right where it turns the corner to run along the top of the baffle. Maybe some ridges along the bottom of the baffle to make some turbulance in the smoke to force it towards the coals a bit. I also want to make an oven on the top of the stove, so we'll see ideally I'd like to wrap the chinmey around the oven somehow and also have a piece of thinner plate as the oven floor rather than the thicker 3/8" for the rest of the stovetop.

Here's a picture of my stove, for some reason the LEDs that I've got in the boat don't really get picked up by my camera on my phone, but the LED flashlight does, I appolgise for the bad lighting. The bolt near the bottom pushes the grate, which is two parts back and forth to drop ash down into the bottom to clean it out.

theboilerflue 02-05-2014 12:29 PM

They won't let me post photos yet, new member thing I imagine.

alwaysFlOoReD 02-05-2014 07:01 PM

You can post pics in two ways;
Buy a premium membership,
Host the pic on a photo hosting website and copy and paste a 'img' link into your post.

I'm interested in seeing your setup....


Boz 03-24-2014 03:55 AM

Shop stove
Putting together a shop from scratch so hope you breath some life into this post. I am at 9500' and stove is necessary first project. Some knowhow, have built a fireplace & reworked a steel (¼") 'Old Timer' stove. Currently using a Vermont Casting Intrepid, about 12" deep and 20" wide, in my little 1200 sq house. The VC will take a 14" or 15" split (too small). Works ok at full blast but never gets above 600 degrees. The Old Timer, 28" x 24" deep, would melt the ceiling fan when hot but smoked short of cherry red. No secondary air input, sm sheet metal shelf under flue. Shop will run about 20' x 24'. Like Goldilocks need a stove tween these.

Boz 03-24-2014 04:01 AM

And thanks to Jaysin for the word to admin so I can post.

realdeal 03-28-2014 06:35 PM

Beautiful work! I'm in the planning process for a similar setup. One question- How do you deal with ash removal? I want to incorporate an ash dump- maybe a pull out drawer designed to prevent ash from spilling out into the room. I recognize the importance of making the firebox air tight, so I haven't quite figured out how to do it. Any suggestions?

arcilla 04-10-2017 06:47 AM

Hello. Great job on the stove. And all the information has been very helpful.

But i have a couple more questions.Im planning to build a stove with a 6cm rammed earth shell to act as heat storage. In this case would you think the internal cement is still necessary. I want to allow a good heat transfer into the earth.

Also, you described the path of the primary air. It travels up the sides and then washes down the glass. How is this achieved? I can't make it out from the photos. Have you got a channel on each side to direct the air or how does it work?


alwaysFlOoReD 04-10-2017 11:26 AM

Unfortunately Jaysin died last year. He is missed. Maybe some other member can answer your question.

nweibel 06-05-2018 02:23 PM

Wood Stove w/ secondary combustion.
Mr. JaysinSpaceman,

I was wondering if you would answer a question about the stove you built. I would like to know where you found or made the front two air vents that are screw driven?


alwaysFlOoReD 06-05-2018 02:43 PM

Read the post directly above yours.

All times are GMT -4. The time now is 08:15 AM.

Powered by vBulletin® Version 3.8.0
Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.
Copyright ©2012-2020, Offroad Fabrication Network