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Old 11-09-2011, 01:00 PM
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Wood Stove w/ secondary combustion.

Well this post is mostly for CarterKraft but it might help out others too.

First, I have to say that I am not a wood stove designer and the stove that I built is not certified by anyone but me. If you choose to build a wood stove do the first test firings out side in a fire safe area and build in fail safes.

Now that I am done with that crap. I was asked by a family mamber if I could build a wood stove for a very specific opening in a rammed earth (think adobe) wall with a built in chimney. The hole in the wall was about 20" tall 18" wide and 22" deep, there wasn't any insert on the market that was even close to fitting. So I said sure. I have taken an interest in wood burning technology ever since my wife and I bought a new EPA approved secondary combustion wood stove to heat our house, so this was a chance to test what I learned.

A little background on these stoves. As wood burns it does so incompletely, the smoke you see is essentially the fire burning rich (just like when you see black smoke coming from an old car in need of a tune up) and the smoke is flammable and carrying a lot of heat energy. The Idea with a secondary combustion wood stove is that by adding fresh air above the burning wood you can cause the smoke to combust and both release more energy and reduce particulate pollution.

Here's a side view drawing of the air/smoke path in my stove.


Primary air comes in through the front door goes up the sides and washes down the door glass (to keep it clean) and feeds the fire at the front bottom. The smoke rises and travels forward, up and around the smoke shelf and across the secondary combustion tubes finally going back along the roof of the stove to the chimney. Secondary air travels from the front lower corners through 1" x 2" rectangular tube along the bottom corners and up the back corners and feed out through 4- 1" stainless steel tubes with a series of 3/16 holes drilled down their length. The long path for the secondary air allows it to preheat before it enters the hot smoke. I hope that all makes sense.

To have secondary combustion occur the smoke and fresh air needs to be really hot (somewhere between 700 and 1200 degrees F, the info on the net is pretty vague on this) so the fire box needs to be lined with refractory insulation. The fire box is everything below the smoke shelf, the smoke shelf should also be refractory insulation.

Here you can see the insulation and the secondary combustion tubes.


The door needs to have a rope seal all the way around it so that primary air can have positive control, we don't want an uncontrolled over fire.

Here you can see the seal. My primary air control is in the door so I also needed to seal this to the air inlet in the front of the fire box.


And here is a video of the first test fire. I had 9 feet of chimney pipe, you have to have a chimney to make the stove draft.

First Fire

In the video you can see there is no visible smoke coming out of the chimney. This is the great thing about secondary combustion, the smoke is burned before it goes up the chimney giving you more heat and reducing the particulates your stove puts into the air we breathe.

I am sure there will be questions and things I forgot. I didn't really get construction photos as I just forgot to take them along the way.

Thanx for looking,
Jaysin
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Old 11-09-2011, 01:41 PM
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Stove looks great!

I work in the wood gasification industry building large systems and have been looking at building a house sized one for a while now. Our industrial sized ones are a 2 box system where the gasification (making smoke) happens in one box and the secondary air is added in a second box to create the fireball before it heads to the boiler.

I want to base mine on burning wood chips instead of logs or pellets. Chips are much cheaper than pellets and can be burnt green and autofed unlike logs. The material handling with chips is much trickier than pellets though. Thanks for rekindling the inspiration.
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Old 11-09-2011, 01:45 PM
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so is the first combustion chamber scavenging air to make to the second combustion chamber push air? Does not the smoke(escaping heat) create the most heat inside your chimney?

Last edited by smackdaddy; 11-09-2011 at 01:50 PM.
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Old 11-09-2011, 03:02 PM
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Quote:
Originally Posted by 12husky View Post
Stove looks great!

I work in the wood gasification industry building large systems and have been looking at building a house sized one for a while now. Our industrial sized ones are a 2 box system where the gasification (making smoke) happens in one box and the secondary air is added in a second box to create the fireball before it heads to the boiler.

I want to base mine on burning wood chips instead of logs or pellets. Chips are much cheaper than pellets and can be burnt green and autofed unlike logs. The material handling with chips is much trickier than pellets though. Thanks for rekindling the inspiration.
This is something that I have been studying on myself and I would love to hear more about how you are going to do it. My property is covered in manzanita and I am slowly chipping it up and it makes great firewood. I would love to figure out a way to burn the chips effectively.

Quote:
Originally Posted by smackdaddy View Post
so is the first combustion chamber scavenging air to make to the second combustion chamber push air? Does not the smoke(escaping heat) create the most heat inside your chimney?
The heat rising up the chimney causes a draft (and corresponding low pressure in the fire box) which will pull air into the fire box through both the primary air opening (which feeds the primary fire; burning wood) and the secondary air tubes (which feed fresh air to the hot smoke rising from the primary fire; burning the left over fuel in the visible smoke). As for the your second question, part of the reason for the long smoke path inside of the stove is so that the heat in the smoke/exhaust can be transfered to the top of the stove. When I did my testing I checked the temperature of the top surface of the stove and it runs between 650 and 800 degrees F and the chimney pipe is only running at about 300-450F so the exhaust is giving up a good deal of it's heat to the stove box before it heads up the chimney.

I hope that answers your questions.
Jaysin
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Old 11-09-2011, 03:41 PM
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hmm.you should have run for a congress position because I think you just said in a long ass sentence what I said in a short but brief one for the first half of my question. cheers.
Thanks for clearing that up for me. All kidding aside thanks for the input.

Last edited by smackdaddy; 11-09-2011 at 03:44 PM.
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Old 11-09-2011, 04:43 PM
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dig the phoenix music on the radio in the vid...
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Old 11-09-2011, 05:58 PM
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Awesome!!!

Thanks for taking the time to do that, I have some questions for sure.

I'll post them up later.
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Old 11-09-2011, 07:04 PM
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Very cool,nice job Jasin!
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Old 11-10-2011, 10:23 AM
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Thumbs up

Quote:
Originally Posted by JaysinSpaceman View Post
Well this post is mostly for CarterKraft but it might help out others too.


Primary air comes in through the front door goes up the sides and washes down the door glass (to keep it clean) and feeds the fire at the front bottom. The smoke rises and travels forward, up and around the smoke shelf and across the secondary combustion tubes finally going back along the roof of the stove to the chimney. Secondary air travels from the front lower corners through 1" x 2" rectangular tube along the bottom corners and up the back corners and feed out through 4- 1" stainless steel tubes with a series of 3/16 holes drilled down their length. The long path for the secondary air allows it to preheat before it enters the hot smoke. I hope that all makes sense.

To have secondary combustion occur the smoke and fresh air needs to be really hot (somewhere between 700 and 1200 degrees F, the info on the net is pretty vague on this) so the fire box needs to be lined with refractory insulation. The fire box is everything below the smoke shelf, the smoke shelf should also be refractory insulation.

Here you can see the insulation and the secondary combustion tubes.


The door needs to have a rope seal all the way around it so that primary air can have positive control, we don't want an uncontrolled over fire.

Here you can see the seal. My primary air control is in the door so I also needed to seal this to the air inlet in the front of the fire box.


Jaysin
Sorry for all the questions but you have built the finest example i have seen to date, this stove is on par with what I would like to build.

Ok so if you had to do this again since the air has to be sealed through the door might you keep the air controls out of the door? It would just make it easier to package I think but might not look near as clean.

Do you think there is a benefit to separating the secondary and primary air supplies for individual control of each air supply and tuning?

Glass.... Did you have it made, buy a premade piece for another stove or?
How is it attached, I see the blocks and what looks like bolts/studs but can't make out what is actually going on. Did you seal the glass to the door, high temp RTV or fireplace glue?

Refractory, those pieces look like one piece. Does that mean you made your own refractory cement?

Air tube holes, I was going to copy the air holes from a similar sized commercial stove I have downloaded the manual for, did you just guess or do something similar.

Smoke shelf, did you buy a ceramic plate for the top of the secondary tubes, or make a refractory piece or?

Again sorry to bomb you with these questions but I feel like you have a grasp on what you are doing and the theory behind it. Also don't worry about being to thorough, I am just curious what you did and if you would do something different. I have a design but would love to hear any thing you could have done differently.
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Old 11-10-2011, 11:43 AM
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Quote:
Originally Posted by CarterKraft View Post
Sorry for all the questions but you have built the finest example i have seen to date, this stove is on par with what I would like to build.

Thanx for the compliment.

Ok so if you had to do this again since the air has to be sealed through the door might you keep the air controls out of the door? It would just make it easier to package I think but might not look near as clean.

In the case of this stove I would still have the primary air control in the door as it really looks good, but I would try to stay away from it if I could just to make it simpler to seal the door.

Do you think there is a benefit to separating the secondary and primary air supplies for individual control of each air supply and tuning?

In reality you only control primary air, the secondary air should always be wide open letting the stove have as much as it wants, so you have to use two separate systems. However, because I was concerned with over fire and/or chimney fires I did put shutters on the two air inlets for the secondary air that could be closed easily in an emergency.

Glass.... Did you have it made, buy a premade piece for another stove or?
How is it attached, I see the blocks and what looks like bolts/studs but can't make out what is actually going on. Did you seal the glass to the door, high temp RTV or fireplace glue?

Most good stove shops will cut the glass for you. It is special glass though, has a ceramic in it that lowers the expansion to almost nil. Very expensive stuff too, I got it for $0.85 per square inch but I had to shop around (my glass was $104), I saw it as high as $1.25 per square inch. The glass hold downs are hard to explain so I will try to get you a close up picture, simple just hard to explain with out a good pic. The glass is sealed to the door with flat tape gasket material, same stuff the rope door seal is made of but a 1/8" x 3/8" flat weave and it has sticky stuff on one side to hold it in place while you set the glass in. Make sure the glass has some room to move, as the stove expands and contracts any metal to glass contact is not good, keep the metal to glass contact to a minimum.

Refractory, those pieces look like one piece. Does that mean you made your own refractory cement?

No, I didn't make the refractory. It is 1" thick sheet refractory material made for insulating ceramic kilns and held in with screw in tabs so it is replaceable, as it will get beat up by the firewood. The bottom is lined with 1 1/4" thick standard fire brick.

Air tube holes, I was going to copy the air holes from a similar sized commercial stove I have downloaded the manual for, did you just guess or do something similar.

Basically I looked at a number of different stoves and correlated their fire box size to the size and number (area) of secondary air holes they had (stoves vary some here so I don't think there is a fixed formula). Then I looked at the size of my fire box and calculated so that I had the same area of hole to cubic inch of fire box. I hope that makes sense. I have it written somewhere and when I find it I will post it up.

Smoke shelf, did you buy a ceramic plate for the top of the secondary tubes, or make a refractory piece or?

It is the same refractory insulation as the sides and back.

Again sorry to bomb you with these questions but I feel like you have a grasp on what you are doing and the theory behind it. Also don't worry about being to thorough, I am just curious what you did and if you would do something different. I have a design but would love to hear any thing you could have done differently.

Feel free to ask more if you want I like to share what I learned. But do remember I am not a wood stove engineer. That said I am very happy with how clean this stove seems to burn, I really wish I knew how the EPA tested stoves because I would love to know how clean it is actually burning.

Thanx
Jaysin
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Old 11-10-2011, 12:28 PM
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Hey thanks again for the response.

I see now that you do have the secondary air inlets on the side as shown in the first diagram. I didn't notice them first off and thought you where just pulling off the primary plenum in the front, your design makes sense to me now. My insert will be about a third larger than this one and I would like to add the shroud over the back and top to facilitate a forced air blower to pull the heat out of the alcove.

If I get the go ahead this year I will post up my progress, I may just get the plans built this year and have to start the project next summer due to way too many irons in the fire now HAHA.

Oh and what thickness material is that? You might have said but I couldn't find it.
I was going to do 1/4" initially but found allot of commercial stoves are 3/16" so i might follow the trend. That might be a case of bigger is not better as far as heat transfer goes.

Last edited by CarterKraft; 11-10-2011 at 12:31 PM.
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Old 11-10-2011, 02:07 PM
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Slylin' Jay... good job... now you need to make one with coils in the back to convect water ... hehe..

r'z

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Old 11-10-2011, 02:58 PM
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Quote:
Originally Posted by CarterKraft View Post
Hey thanks again for the response.

I see now that you do have the secondary air inlets on the side as shown in the first diagram. I didn't notice them first off and thought you where just pulling off the primary plenum in the front, your design makes sense to me now. My insert will be about a third larger than this one and I would like to add the shroud over the back and top to facilitate a forced air blower to pull the heat out of the alcove.

If I get the go ahead this year I will post up my progress, I may just get the plans built this year and have to start the project next summer due to way too many irons in the fire now HAHA.

Oh and what thickness material is that? You might have said but I couldn't find it.
I was going to do 1/4" initially but found allot of commercial stoves are 3/16" so i might follow the trend. That might be a case of bigger is not better as far as heat transfer goes.
I used 1/4" on this stove and I really don't think I would go any thinner.

This stove is a huge heater, it puts off massive radiated heat through the large front glass and top surface and when filled before bedtime will still have good coals to throw splits onto in the morning without a restart. Be careful how big you build your stove, in order for a stove to operate efficiently the fire box should be full or close to it, that means huge heat from a big stove. I wouldn't want this size stove in anything much smaller then a 1800sq.ft. house. Remember the secondary combustion not only cleans up the smoke but gives you about 1/3 more heat then a pre-EPA non-secondary combustion stove will. This is one place where bigger isn't always better. When you get your design worked out let me know if there is anything I can help you with. I am more then happy to help.

Jaysin

P.S. I like your Idea of a forced air blower.
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Old 11-10-2011, 02:58 PM
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Threads like this are why I am constantly on OFN. There's so much to learn. Jaysin thank you for graciously sharing your knowledge. This is really cool.
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Old 11-10-2011, 03:01 PM
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Quote:
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Slylin' Jay... good job... now you need to make one with coils in the back to convect water ... hehe..

r'z

Dog......
Thanx Dog.

Believe it or not the earth wall that stove is in has water coils around the chimney. Once we get that going the stove will help heat the water for a radiant slab floor. I'm going to get some pictures of where it is installed this afternoon.

Jaysin
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Old 11-10-2011, 03:08 PM
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Will do, I am thinking you are right with the 1/4" material.

Good point on the fire box size, I have a 2200 sq ft all electric house and the electric heat is a killer on the pocket book. The fireplace as is will increase the room temps easily 3* on one load of wood, but the burn time is not long, maybe 4 hrs or less. I would like that same load to go 8+ hours if possible.

Thanks for the tips.
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Old 11-10-2011, 03:18 PM
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Thermosyphon ?

I thought about doing that too but with my slab floor and plumbing where it is it would be a nightmare to connect to the water heater.

I have a similar idea for a thermosyphon forced air heater in my shop, any one care to critique it?
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Old 11-10-2011, 09:03 PM
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So a few more pictures for you.

The stove in action during one of the test fires. You can clearly see a bit of secondary combustion in the upper left corner of the glass.


The stove as it is installed in the earth wall. Next to the nearly useless open fireplace. I say nearly because it is ok for some ambiance.


A close up of the install.


For some of the nitty gritty. The handle and latch mechanism super simple quarter turn latch I added a brass washer so that when hot the two steel on steel parts wouldn't gall together, is it needed, I don't know but it can't hurt and it turns very nice and easy.


Here's the inside of the door and latch. Of coarse the latch is really hard to see.


Secondary air shut off, in case of an over fire. I doubt they will ever get used but you're better safe then sorry. Just slides back to close up the hole. One on each side of the stove.


And this is how the glass is held in. It is a threaded block (3/8" square bar) welded to the door. A stainless clip and screw, the clip is carefully bent so that is just compresses the gasket under the glass but not hard. I didn't want to bind the glass when the stove expands and contracts.


And for those that might be interested in the rammed earth construction of the house they had a blog while construction was going on. It never got updated with finished photos but it gives you a good idea. The House. And another House built the same way (the first picture is of a steel staircase, landing and railing that I built.

Let me know if you have more questions.
Jaysin
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Old 12-11-2011, 06:23 AM
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Wink

Just what type of material did you use for the thermal barrier?/

Where did you find it I have seen Fire bricks but nothing else the fire bricks look to run me about $ 200.00 for what I am doing .

i like the idea of the secondary air I think it can be added to my existing stove.

thanks again for a good write up.
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Old 12-11-2011, 04:43 PM
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nice stove man!!!
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