# Modifying a central wood furnace for secondary burn



## FixedGearFlyer

We're rolling into our first winter using a wood furnace as our primary heat source and Hearth.com has been an awesome resource. Thank you, all!

Our furnace is a new Vogelzang Norseman 2500 that was installed in the house last fall before we bought the property. If I'd had my choice, I likely would have gone with something more advanced and efficient like the Wood Chuck or a Yukon, but since we have the Norseman we'll stick with it. 

As a former woodstove user, my biggest complaint is that the 'secondary burn chamber' is simply an airspace separated from the firebox by two steel plate baffles. There's no secondary air supply and I'm sure that we're losing a huge amount of our potential energy through the flue and chimney. Granted, the few times we ran it in early spring we had fairly efficient burns based on the vapor-only chimney emissions, but those were hot, short fires for the shoulder season.

Does anyone have any experience with the risks and rewards of trying to plumb a secondary burn air supply into the top of a furnace like ours? I'm waffling back and forth between, "I don't even want to think about it" and "I don't want to waste a single drop of energy from the wood we put up this year".


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## geoxman

http://nepacrossroads.com/about2066.html#p13410

http://www.arboristsite.com/showthread.php?t=37370&highlight=forced+draft&page=2

http://www.arboristsite.com/showthread.php?t=77617&highlight=fire+chief

I did just like the links I posted. I used black cast that you can get at any local hardware store. It also really helps if you add firebrick above the tubes like in the 2nd thread. Good luck and you are on your own if there is a fire because of the modifications. Mine works just fine


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## FixedGearFlyer

Beautiful, Geoxman! The secondary air supply is almost exactly what I have sketched out.

I'll gather up the materials and give it a try. I'm thinking that I'll add firebrick to the upper side and rear walls, as well as to the baffle to handle the higher temps at the same time. Maybe I'll even figure out how to keep that steel baffle from rattling when the blowers are running . . .


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## freeburn

I've always toyed with this idea on the Englander Furnace that I and many others have. It just seems like it should have it, to get every last bit of energy out of the wood. I don't know of anyone who has tried it yet though.


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## jeff_t

The more I thought about it, the less I liked the idea. I would feel much safer with a furnace designed with secondary combustion, and I'm sure my insurance company would, too. The Hotblast, which is the same design as the Norseman but made by US Stove, fit my budget at the time. It does the job, and when I can afford it, I will upgrade to a more efficient unit. I'm comfortable with what I have, and peace of mind and my family's safety are worth more to me than a couple cords of wood. 

Just my thoughts.


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## laynes69

I had a furnace just like that was ran for over 20 years. I toyed with the idea of modifying the furnace for the secondary burn. What I found was I removed the plug in the back of the unit, which allowed for some extra oxygen below the baffle for better combustion. I mounted a thermometer above the loading door and when in the 500 range after a hot fire and closing it down, it would burn pretty hot and clean. With that said, I removed it last year and put a Caddy woodfurnace in its place. A full load with the Caddy is half the load of the old furnace. I always get secondary combustion with the new furnace, I lowered my wood usage and get every bit of heat from each charge of wood. I wouldn't modify the furnace too much. Its not made for the extreme heat thats involved with good secondary combustion. Couple that with a power outage, and your house will fill with smoke when the furnace burns off the paint of the jacket. Been there, done that. If you want to save wood and have a better unit, you can usually get most or all of your money back from the furnace if its only a couple of years old. Factor in the tax credit for a new efficient furnace, and get what you want. If you modify the furnace and you have a house fire, your insurance won't cover you.


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## vvvv

geoxman said:
			
		

> http://nepacrossroads.com/about2066.html#p13410
> 
> http://www.arboristsite.com/showthread.php?t=37370&highlight=forced+draft&page=2
> 
> http://www.arboristsite.com/showthread.php?t=77617&highlight=fire+chief
> 
> I did just like the links I posted. I used black cast that you can get at any local hardware store. It also really helps if you add firebrick above the tubes like in the 2nd thread. Good luck and you are on your own if there is a fire because of the modifications. Mine works just fine


gorgeous! how big the pipe, how big the holes in the pipes?


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## bigburner

got any pictures of the fan set up?? How are you controlling the ratio prime/second air ?? Any idea how hot the air is being heated by coal bed ??  This is a change I want to make on my unit, now we just blow a split air stream that does both primary & secondary, it's simple but it also using boiler room air.


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## geoxman

mine is regular black 1 inch cast iron pipe that you can find at any local hardware. I believe the holes were 1/32 and I did 64 holes to equal the 2 inches I drilled into the front of the unit. Mine is also fully fire brick lined, top, sides, bottom including most of the grate. I have a little space in front of the for scooping out ashes. It greatly reduced the firebox size but I can easily get 9-10 hour burns. I am not too worried about insurance as the unit is over built, it is an Energy Mate by EROC and it never had a UL to begin with. If I were you I would line the entire firebox as I did. I control the secondary air with two gate valves on the front of the furnace that stick out about 6 inches from the unit. Gate valves cost more than ball valves but there are no rubber parts in them. Late last winter we also modded my brothers Englander furnace the same way and it was night and day difference in burn times and heat output. We also added more firebrick to his as well, including the top. I think I have invested $60-$70 in the mod. good luck


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## bigburner

Fire brick is no problem, my burn chamber is made with 3 skids of it. I am confused about the burn times, you are saying that by adding the secondary air that the burn time is longer?? One of the reasons I am on the fence about this, is that I actualy shut off the fan [it does both prime & secondary Air] I can get into a low ox environment burn that does gassification. " I think" It goes into what I call Chernobyl it just gets way hot. top of fire box 1800F if I remember correctly. The wood last longer with more heat. If I leave the fan on the whole time it still gets hot, but it goes threw the wood faster, it seams anyway. I burn all flavors of wood and sizes. If you can get it on a two wheel cart,It will go threw the door, one draw back to this is getting all the wood in the game, it takes a while to get it burning good again and fills up the chamber effectively  cutting down the throat size which makes this thing work.


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## vvvv

geoxman said:
			
		

> mine is regular black 1 inch cast iron pipe that you can find at any local hardware. I believe the holes were 1/32 and I did 64 holes to equal the 2 inches I drilled into the front of the unit. Mine is also fully fire brick lined, top, sides, bottom including most of the grate. I have a little space in front of the for scooping out ashes. It greatly reduced the firebox size but I can easily get 9-10 hour burns. I am not too worried about insurance as the unit is over built, it is an Energy Mate by EROC and it never had a UL to begin with. If I were you I would line the entire firebox as I did. I control the secondary air with two gate valves on the front of the furnace that stick out about 6 inches from the unit. Gate valves cost more than ball valves but there are no rubber parts in them. Late last winter we also modded my brothers Englander furnace the same way and it was night and day difference in burn times and heat output. We also added more firebrick to his as well, including the top. I think I have invested $60-$70 in the mod. good luck


great design! bringing the pipes in thru the coals will cool the coals & avert burning the wood from the bottom up where it can overgassify & exceed the stochiometric requirement of combustion air from secondaries.


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## freeburn

geoxman said:
			
		

> mine is regular black 1 inch cast iron pipe that you can find at any local hardware. I believe the holes were 1/32 and I did 64 holes to equal the 2 inches I drilled into the front of the unit. Mine is also fully fire brick lined, top, sides, bottom including most of the grate. I have a little space in front of the for scooping out ashes. It greatly reduced the firebox size but I can easily get 9-10 hour burns. I am not too worried about insurance as the unit is over built, it is an Energy Mate by EROC and it never had a UL to begin with. If I were you I would line the entire firebox as I did. I control the secondary air with two gate valves on the front of the furnace that stick out about 6 inches from the unit. Gate valves cost more than ball valves but there are no rubber parts in them. Late last winter we also modded my brothers Englander furnace the same way and it was night and day difference in burn times and heat output. We also added more firebrick to his as well, including the top. I think I have invested $60-$70 in the mod. good luck



Geoxman - I PM'ed you.


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## geoxman

bigburner what furnace do you have? I drilled two holes in the front of the unit and it has nothing to do with my fan, which is sealed off now. I guess i am not understanding the question?


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## bigburner

Sorry for the confusion.what I have is a home made "laid up" fire box that is all fire brick, that has a very tall throat that allows the hot gases to collect at the top. Then air is blow on the fire "down and at the same time "up" into the area where the gas collects. for a secondary burn. Then all the flue gas is forced down into a fire brick chase [which also has a by pass] {use a magnahelic  to monitor draft} then it pass threw a wet back fire tube boiler. "My question is" I assumed that there was a fan on both the primary& secondary air supply and how were you controlling the input or what kind of adjustment is there. My original design had a pipe to supply super heated air to the secondary but that was lost along the way, for the lets get this thing finished so we can use it. Have heated with it for two years now and what to start tweaking it. Most of these guys are using down draft system and the ones that use green wood or similar work closer to mine design. But seeing the pictures , looks like you nailed it. Using less wood,would be appealing. another potential issue I have is there are no grates -- start with an empty fire box & burn on top of the last fire. I only clean it out about twice a season. my secondary tubes would get buried in the ashes not sure how hot they would get using your design. Thanks


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## geoxman

dude that sounds awesome! post some pics or email them to me and I might have a few ideas. Can you run the tubes on the right and left of the firebox to the rear and then up too the top and back to the front under your baffle?


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## bigburner

some night this week my friends are going to help me put up some cement board on the ceiling of the shed where this thing is located, I have scaffolding set up in there now,so pictures will need to wait a week or so. I promised some of the green wood guys some pictures too.


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## freeburn

OK, so I did it. Here are my results. http://www.youtube.com/watch?v=6D-zhJqL6JA
It was my first fire, the wood was a little wet, and the camera is old so the ticking you hear is the camera writing to the card, but you get the idea. The secondaries got more intense the longer the fire went. This was about 1,1.5 hours into it.


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## freeburn

Here are some pics of the outside and inside. It's not pretty on the outside, haven't painted it yet and the glass was dirty.


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## geoxman

SWEET! just like I told you! Did you use a bed frame or buy the angle iron? If you don't use the gate valves I would drill another hole above the holes and rig up some sort of air shut off damper with a piece of sliding metal on a screw. What was your total investment and did it take you more than two hours? If you have questions about the air damper I am trying to describe just give me a buzz. Great job


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## freeburn

I didn't buy anything. It all came out of junk that I had, and some metal pipe left over from a guy I know who was going to get rid of it for scrap anyway. I still owe him for the 90's That he had out of his stock. Other than that, I'd say with a whole lot of running back and forth probably 2 hours in the whole thing, maybe 3. Didn't take any time at all once I figured out what I was going to do. 

I don't know if I'm going to do anything as far as shut off, the caddy doesn't have anything built in for that. I'll just keep some fiberglass or non combustible material nearby in the meantime, otherwise, I thought of the sliders you described too (I know what you mean - screw above with sliding piece of metal to cover the hole). Do the secondaries suck more air, the more the outside temps drop? As of right now, they are pulling air, but not enough to make alot of noise. I can't really close the slide draft all the way either or they shut down, is that normal? Some of that has to be wet wood and lack of firebox temps too.


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## geoxman

When we did his it was very cold outside and remember I told you that we did his from the rear of the furnace because he had some PVC piping coming from his window getting all outside make up air and that was the direction the furnace was facing. His sounded like a roar when engaged and I am betting your wet wood his not helping. Keep playing with it, and not when it is 60 degrees outside, and you will figure it out. Great job!


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## FixedGearFlyer

Freeburn, can you give us a post-install report now that you've had it up and running for a few weeks?

What are the differences in your furnace and flue temps? Have you seen a change in your efficiency (amount of wood for a given amount of heat), burn times, or other markers of furnace performance?

How has the black iron pipe been holding up in the heat?

I have my parts list and am just trying to find time to head into town to pick up the odds and ends, then will give it a shot myself.


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## freeburn

FixedGearFlyer said:
			
		

> Freeburn, can you give us a post-install report now that you've had it up and running for a few weeks?
> 
> What are the differences in your furnace and flue temps? Have you seen a change in your efficiency (amount of wood for a given amount of heat), burn times, or other markers of furnace performance? The obvious difference is the evidence of secondary burn usually within 30 min of start up as well as a much better secondary continual burn once the wood is good and charred. My problem is that I want to load up the firebox right away because I have to leave the house. I ramble on. . . Flue temps are no higher than 400. In fact, I can't get it to go higher than 400 once the secondaries kick in unless I open the ash door. Otherwise, they hover between 350-400 respectively once the air is cut back. I found that if I leave my primary draft open one finger's width it burns much better. I also crack open the spin draft between 1/2 and 1 full turn. (I'm still debating whether that helps or hinders at this point).
> Keep in mind that I haven't really been burning all that much, and it warms up so quickly in the day that it's hard to gauge how that will affect the draft later on in winter. Oh, my outlet air temps I do notice are much higher and more constant. I think the firebrick helps with that.
> The only thing I don't really know yet is that I have too much smoke still coming out the chimney even when the secondaries are kicking in full force. I think my baffle is too short and some of the smoke goes out the chimney from the front of the firebox. By the way, my baffle is made out of firebrick only. I removed the metal baffle that came with it, and replaced it with firebrick. This gave me more heat and more room in the firebox too.
> 
> How has the black iron pipe been holding up in the heat? So far, in my small amt of fires (3-5) there is no evidence of any change in the pipe. I like though that I can change it out in the future for little to nothing. When I do however, I will change the configuration of holes on the pipes. I think the pipes cool the fire a bit too much. My theory on that is that the air should almost be directed sideways not down at the fire, I think that is why the secondaries have a hard time staying lit sometimes. One other thing I did, and this helped me out alot, was add more holes in the front of the firebox nearer the door to burn up some more of those gases before they escape into the chimney. If you look at my pics I added holes in the 90's that are in the middle/front of the the firebox nearest the door.
> 
> I have my parts list and am just trying to find time to head into town to pick up the odds and ends, then will give it a shot myself.


 What furnace do you have again?


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## vvvv

freeburn said:
			
		

> OK, so I did it. Here are my results. http://www.youtube.com/watch?v=6D-zhJqL6JA
> It was my first fire, the wood was a little wet, and the camera is old so the ticking you hear is the camera writing to the card, but you get the idea. The secondaries got more intense the longer the fire went. This was about 1,1.5 hours into it.


sweet! sorry u voided your UL listing but life can be so silly.....


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## FixedGearFlyer

freeburn said:
			
		

> What furnace do you have again?



We have a Vogelzang Norseman 2500. It's the same as the Johnson Energy J9000/US Stove Hotblast/etc., under a different branding.

Thank you for the follow up! I'll make my modifications this weekend, put a few fires through it, make any required adjustments, and get some pictures and descriptions up ASAP. I have a slightly different plan for the air tube layout that will give greater coverage and am trying to track down some ceramic blanket for the baffle insulation.


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## freeburn

Oh, and I added approximately two more hours of burn time to the loads I tried.


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## sparke

This comment is pure speculation for discussion only.  Take my opinion lightly : )  I am just thinking that the secondary tubes at the top would work better  if they ran parallel to the smoke shelf.  Then the holes should be approx 45* angle.  This is assuming the smoke exits the stove in the front by the door.  If the smoke path is on the sides then that is different.  The only thing that makes me think the pipes should be running in the opposite direction of your design is the way my wood stove is designed.  The pipes all run parallel to the smoke shelf.  Either way nice project.  I love seeing retrofits that work, it is a nice accomplishment.  Well done!


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## freeburn

sparke - 
My thoughts exactly and when my pipes are exhausted or when I get antsy to try something different I'm going do that with my next "try". Actually the way the pipes are in the 30NC they are all turned at a slightly different angle to follow the path of smoke and burn up the most possible on its way out.


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## FixedGearFlyer

That's what I have drawn up - dual inlet pipes entering on each side of the door, running just above the coals, then up the back wall (so far, just like what's posted above), then into a 'ladder' that comes back toward the front. Each 'rung' will come off the supply tubes with T-fittings and be connected to both sides of the ladder. I'm hoping to squeeze in 4 rungs with the front most one just behind the front edge of the baffle. I only have 19 inches of front to back space, so I may need to thread a few pipe slugs to make it all fit. I doubt they'll have the right sizes available at Ace or Lowes . . .


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## freeburn

FixedGearFlyer said:
			
		

> That's what I have drawn up - dual inlet pipes entering on each side of the door, running just above the coals, then up the back wall (so far, just like what's posted above), then into a 'ladder' that comes back toward the front. Each 'rung' will come off the supply tubes with T-fittings and be connected to both sides of the ladder. I'm hoping to squeeze in 4 rungs with the front most one just behind the front edge of the baffle. I only have 19 inches of front to back space, so I may need to thread a few pipe slugs to make it all fit. I doubt they'll have the right sizes available at Ace or Lowes . . .



Yes exactly! That's the way I was going to do it and didn't have the right size pipe at the time. I should have done it right the first time. Oh well, there is always next time. Don't forget to add one or two dielectric unions so you can remove the "ladder" when it goes bad. It will also be easier to install. That's how I'm going to redo mine, when/if I do. Keep us posted.

If you have a home depot, they'll thread you whatever size for free if you buy the pipe from them.


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## freeburn

OK, another update, I lit a fire tonight b/c the temps are dropping. So happy with the results. I have huge secondaries, see the pics, sorry it's so blurry, old dig camera. (The pic doesn't do it justice.) Flue temps are 300+ and the best part, no smoke from the chimney! The difference was the operator. I waited before adding wood and got it coaled up all over enough that it wasn't smoking. The pic is about an hour into it.


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## FixedGearFlyer

That's awesome! 

I bought the parts this evening and had to change the layout a bit. The only way to thread together a ladder section was to put unions in the middle of each rung, and that took up too much space. I now have three pipes running N-S coming off a manifold in the back.

I took a picture of the secondary tubes assembled outside the furnace, but can't upload directly from my iPhone, so I'll post it in the morning. Our toddler is also in bed, so I can't fire up the bimetal hole saw and start the install. Guess I'll have to wait for morning ...


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## FixedGearFlyer

Here it is. I just need three pipe caps for the ends and two holes drilled in the front of the furnace. 

The problem with the ladder arrangement was that there was no way to get it all threaded together in a ladder layout without big unions in the middle of each rung. This layout should work, although I don't have the coverage I was hoping for. The baffle is 19 inches back-to-front, and this will give me about 15 inches, so the tips of the secondary tubes will be about 4 inches shy of the baffle edge. I'll cut down the long pipes once it's all assembled so that they extend about an inch and a half past the front of the furnace, thread the ends, and put caps on them to control the airflow.

I went with 3/4-inch pipe for the manifold and 1/2-inch for the secondary burn tubes. I may find that it's undersized and rebuild it with 1-inch and 3/4-inch pipe, but I'd rather not have to worry about run away furnaces, especially when I'm travelling for work and it's just my wife and toddler at home. Until I know the design and sizes are dialed in, I'll just use furnace cement to seal the holes around the tubes, then will weld it once I have everything working correctly.


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## freeburn

Yes, I remember that that is why I opted not to do that. The one time i did try it, I ended up threading one side all the way in and then unscrewing it as I threaded into the other side working my way through the ladder that way. But then there was the problem of getting it through the door. Is that going to be a problem with your nice design? 

My other thought would be to use some 1 1/4-1 1/2 pipes on the side coming into the front of the furnace, then 90 up toward the top, then another 90 back toward the front of the stove, but instead of using "T's" drill into the round pipe and slide the 1/2-3/4" perpendicular pipes into the outside ones. Make them long enough so they will slide all the way into one side, but can be taken out easily enough. Then you could insert a pin through those pipes to hold them into place so they wouldn't fall out. That's the way some stoves do it with the stainless burn tubes. It would work better with 2" square/rectangular stock in place of one of the pipes returning toward the front of the stove. Is that clear as mud?!?@?! In any case keep us updated!


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## FixedGearFlyer

My original plan was also to thread all the rungs into one side, then thread them into the other while unthreading half way from the first, so that's clear as a bell.

Unfortunately, the execution was a bit tougher. My 6 3/4-3/4-1/2 reducing T's and 2 3/4-1/2 reducing 90's are all from an assortment of manufacturers and vary in length and depth by a few fractions of an inch. By the time I added up all those fractions, I couldn't get anything to line up and the pipe is too stiff to bend successfully. Even if I could, the idea of doing that INSIDE the stove was less than appealing!

The larger, drilled manifold with insert rungs is a great idea! Too late for this iteration, but I'll keep it in mind for the future. 

If I take off the two longer pipes, I can get the manifold through the door diagonally with some finesse. 

I'll be drilling holes in the furnace and installing it as soon as it cools down from this morning's fire.


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## FixedGearFlyer

Oh! Also forgot to add that I've completely lined the firebox and baffle with ceramic fire brick. Even now, my fires are hotter, cleaner, and last longer, but it dramatically increased the time it takes to heat the furnace up to the blower kick-on temp. I think I get that back on the other side of the burn, but it's a bummer when it's 56 on the first floor at 6:00 am and I want some quick heat. 

It used to take 5 minutes to get the fan to cycle and now it's 10 to 15.


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## freeburn

FixedGearFlyer said:
			
		

> Oh! Also forgot to add that I've completely lined the firebox and baffle with ceramic fire brick. Even now, my fires are hotter, cleaner, and last longer, but it dramatically increased the time it takes to heat the furnace up to the blower kick-on temp. I think I get that back on the other side of the burn, but it's a bummer when it's 56 on the first floor at 6:00 am and I want some quick heat.
> 
> It used to take 5 minutes to get the fan to cycle and now it's 10 to 15.



Yes, but you are heating up a thermal mass which will cause the fan to stay on longer and your heat will be much more even throughout the burn. That's what I've noticed too. In the end I think it's worth it.


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## FixedGearFlyer

Un-freakin'-believable!

I couldn't concentrate on my work, so I knocked off for a long lunch, put in the tubes, and lit a fire. 

I got a small bed of coals with a quick, hot burn using kindling and wrist-size rounds, and loaded it up with good splits. When it was burning nice and hot after the reload  (450 on the furnace front and about 500 on the flue), I knocked down the door air supply on the door, waited about 5 minutes, and reduced the spin draft on the ash pan door to about 15%. 

The furnace doesn't have a window, so I can't post pics, but I can crouch on the floor and peer through a couple of tiny holes in the door's flapper. What did I see? FLAMES DANCING ON THE CEILING Yeah

After it stabilized, my furnace front temps are around 500 and my exhaust temps are around 400. That's the first time I've ever seen my flue temps lower than my stove temps without smoldering the fire, and even then I've never seen it with either of them that high. 

After 1/2 a burn and very little concrete data, I'm already thinking that I should increase the air supply to the secondary tubes. I think that 3/4 inch  supply manifold pipe may be too small, or that the 1/16 inch holes I drilled in the secondary tubes should be greater in number or size, but I'm going to give it a few weeks before I make any more changes. If I make a change, the first thing I do will be to drill out the secondary tube holes to 3/32 and add a few more. Hopefully that will pull more air through the supply manifold. 

The important thing is that we're burning clean and closed down, which is unreal. I've burned clean, and I've burned closed down, but never at the same time!

I'll post pictures of the installed tubes after this fire dies down and try to figure out some way to get a video, but that's doubtful.


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## freeburn

It does that to you doesn't it?!? Same here. Can't concentrate until you figure the thing out. Sounds like you have success though. Nice Work! 

The holes in mine are just a bit larger than 3/32 (smallest bit I had) and I think I have too many of them. But what I do notice is that the secondaries tend to move around to different parts of the firebox (those areas that are offgassing the most, I suppose). So in short, I'm under the impression that the secondaries will move to different parts of the firebox and suck the air in as needed. Maybe someone else can chime in if that's incorrect.


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## laynes69

Secondaries will not suck out the air in a firebox. There will be a constant supply of secondary air entering the firebox at all stages of the burn. The only way those secondaries will burn is when your temperature is right, and there is the proper oxygen supply. With our furnace we don't need a huge firebox full of flames to burn cleanly. After a while the firebox will get hotter as the wood will outgass more and more into the burn. Then the furnace will go into the coal stage and produce heat. So keep experimenting when you close down and you will extend your burns as well as burn cleanly. What you guys are doing I wanted to do with our old furnace, but I just went the easy route and bought a furnace that had everything you guys are doing and then some. Good luck with your furnaces.


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## freeburn

laynes69 said:
			
		

> Secondaries will not suck out the air in a firebox. There will be a constant supply of secondary air entering the firebox at all stages of the burn. The only way those secondaries will burn is when your temperature is right, and there is the proper oxygen supply. With our furnace we don't need a huge firebox full of flames to burn cleanly. After a while the firebox will get hotter as the wood will outgass more and more into the burn. Then the furnace will go into the coal stage and produce heat. So keep experimenting when you close down and you will extend your burns as well as burn cleanly. What you guys are doing I wanted to do with our old furnace, but I just went the easy route and bought a furnace that had everything you guys are doing and then some. Good luck with your furnaces.



Laynes - I meant to say the secondaries suck the air into the firebox or understand as the air being sucked out of the secondary tubes by the fire. I guess I wasn't clear enough on that. After the firebox gets hot enough the secondaries take over and they just light up over the top of the glowing pile of charred/charring wood beneath. That's with the primary air turned down. Question for you, what are your avg temps on your flue? What are your temps of the firebox itself and is that a read with an infrared or surface mount thermometer?


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## laynes69

Can't tell you the firebox temperatures. There is no way to measure them. Basically almost the whole front of the firebox is a glass door, and the front of the furnace is a second layer, the firebox its self is behind the front of the furnace. My flue temps are normally 175 to 200 on the surface about 8" or so away from the furnace with the damper closed and a good hot fire burning. When our damper closes, the temperatures will increase in the plenum, and lower at the flue. But if the damper is open and there is a roaring fire, the temps will jump to 250-350 surface and the plenum will get hot. Our temps run lower because of the secondary heat exchanger. The old furnace would produce more heat at a time, but it was a waste of wood, and the burn times weren't there like now.


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## TCaldwell

pretty ingeneous, do you know a heating tech with a testo 327 or better portable fluegas analyser. The probe inserts into a small hole in the flue pipe and can be plugged or patched between tests. This will tell the residual o2 content of the fluegas [ stiometeric for wood is approx 7.5%] and a accurate flue temp and calculate a approx combustion effiency %. You could run a load with the secondary ports capped off [ stock] and then one modified [ ports open] to calculate actual gains. Beyond that you could experiment with modulating the secondary air to mainiain the 7.5% set point. try pressureizing the secondaries with a heat gun or hairdryer when at high burn, you probably will not need much air to see results. This is almost more fun than alot of other....
             tom


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## FixedGearFlyer

I'm as happy (and cozy!) as a bug in a rug!

Not only am I seeing longer burns with more heat transferred to the house rather than going up the chimney thanks to the secondary burn modification, but at 6:30am this morning, I still had coals left over from yesterday's 2:00 *PM* burn and was able to get a new fire going just by add some kindling and small splits. 

This morning I have the furnace completely closed down with active secondaries, a furnace front temp of 550 and flue temp of 400. Based on the stoves stock performance, I'm very content with the results so far. 

Tom, I'm now on a mission to find a flue gas analyzer! I would really like to have some hard evidence to determine the effects of the changes. It would also be beneficial to tune the system to a quantitative measure of efficiency rather than tuning to temps and a completely subjective evaluation of what 'looks right'.

Interestingly, I think that much of the gain in efficiency and the extended coaling time is due to the smaller, more insulated firebox created by the fire brick modifications. My completely unprofessional, unscientific evaluation is that the firebox on this thing was just too big to burn efficiently. It made up for inefficient and incomplete combustion through volume . . .


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## laynes69

FixedGearFlyer said:
			
		

> My completely unprofessional, unscientific evaluation is that the firebox on this thing was just too big to burn efficiently. It made up for inefficient and incomplete combustion through volume . . .



I think thats why so many furnaces couldn't even come close to becoming EPA Certified without huge modifications of the unit itself. I guess the EPA will consider the B415 standards when they put new emission requirements in place for central heaters. I know the firebox on the new furnace is half of the size of the old (Hotblast 1500). But the burns are longer and hotter. Alot of people say its impossible, but if they seen what I have seen and you guys who have done some mods, they would change their minds.


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## TCaldwell

fixed gear flyer, a little off context but i assume you are a track rider, I used to be a roadie.
  tom


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## freeburn

I wised up and put a mirror outside that reflects my chimney so I don't have to run outside and check for smoke the whole winter. That way I can stand inside and tune in the fire just right. My new motto is "no smoking". 

Fixed gear flyer - I also figured out how to get the pipes parallel to the door. Two pipes on each side attached to your in/out pipe and stagger them in between each other. (In with straight pipe, 90 up, short threaded stub, 90 toward the front of the furnace, T in as close as you can to the back, straight pipe, 90, same thing on the other side, but stagger the T and 90 in where the other side isn't to cover the dead spot and cap the ends). That way the pipes with the holes are removable and can be easily installed inside the furnace. You just have to weld in place or brace up with firebrick somehow. That will be my next mod when I get too bored with this. I'm also toying with the idea of extending the baffle toward the door to get more of that smoke burned up and retain the temps inside the "firebrick box".


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## FixedGearFlyer

Ha! A mirror outside sounds like an awesome idea, which proves to me that we're both obsessed! 

I just started my second burn of the day - mainly because I put some polyurethane down on a floor and had to open windows - and it's just amazing! It's like I bought a new furnace! 

Tom, my username has two meanings. One, I used to ride on the track and was pretty quick. Two, I'm a commercial pilot and flight instructor and used to work for Cirrus Design in the Flight Standards department. All of our aircraft were fixed-gear. Both of those references were current when I started using it on another forum many moons ago.


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## FixedGearFlyer

Fair warning to all who want to try this: your threaded iron pipe will not be easy to make changes to once it's installed and fired a few times. 

I just tried to pull mine to increase the size of a few secondary air tube holes and had to break one of them off with a pipe wrench and breaker bar, destroying it in the process. The cast iron fitting is fine, but the pipe's threads were destroyed. If I had wanted to take the whole thing out, it would require a torch or sawzall...

I had a spare (thankfully!) and am trying a burn with the new hole pattern now. Hoping for even more improvement!


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## freeburn

shoot, sorry to hear that I should have told you that earlier. I put my pipes together somewhat loosely to avoid the welding together effect. What's the new hole pattern all about ?


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## FixedGearFlyer

No worries, Freeburn! It's a lesson I'll never forget! So, assuming that I'd just threaded them by hand instead of cranking them in tight with a pipe wrench, they wouldn't have bonded and I could take it apart? That's good info and will be duly noted for the next version!

The change in hole patterns was driven by some calculations I did. When I figured the mass of air needed to completely combust the volatile gasses from my average size fire, I just didn't think my 3/4 inch pipe and 1/16 inch drilled holes (about 36 in each upper burn tube) were giving the fire anything close to the needed air supply.

On the other hand, I also calculated the BTU's needed to preheat that volume of air and the surface area of pipe required to transfer the heat to the moving air volume. Unless I want to put almost 13 feet of 1 inch pipe in my furnace, I'm just going to have to make due with a smaller air supply!

My goal last night was to start working toward the happy middle ground between not enough air and too much air for my current system to sufficiently preheat. The holes were in a set of 3, staggered rows per tube, with 12 holes per row. The middle row is at 6 o'clock and the side rows are at about 4 and 8. I left the side rows at 1/16 inch and drilled the center holes out to 1/8. I also added a 1/8 inch hole to the tip of the end cap on each burn tube to try and supply some air to the gasses that sneak by the baffle without contacting the burn tube area. BTW, each burn tube is a 1/2 inch iron pipe, 12 inches in length.

I immediately saw a dramatic increase the secondaries, a 75 degree increase in stove front temp and a 50 degree increase in flue gas compared to a nearly identical burn the night before. Actual burn time was identical. I'm now debating drilling out the side rows to 3/32. At the moment, it looks like the increase in volume was beneficial and didn't exceed the system's ability to preheat it sufficiently. If I increase the volume again, I risk reaching the point where the preheating isn't sufficient given the new increase in air mass needing heating.

I'll burn with it as is for a few weeks before making a decision.

I'll also try to remember to get some pics up tomorrow, including the burn tube patterns.


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## freeburn

quite intersting findings. I tried closing my middle section of holes or 6 oclock as you put it on my pipes and I think I didn't succeed in doing anything but having less flame. the reason I did this was that I thought the flame that shot straight down at the wood would cook it faster than the two shooting out at 4 and 8 oclock. (By the way, your description of hole placement sounds exactly the same as mine. My holes are all the same size though I can't remember how many holes I put in - maybe 32 per tube perhaps more) If you look at the EPA furnaces, and stoves, the holes they put in theirs are pretty substantial. 1/16 is a tiny hole! The NC30 holes are all almost 1/8 perhaps a tad smaller. I'm just wondering if it matter because the fire is only going to suck out of the pipes as much as it needs? I think. . .


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## FixedGearFlyer

I just looked at an Englander NC-30 and confirmed that the holes are much larger than what I have in my furnace. Each of the 4 tubes has 28 holes in it and they're 5/32 on the back two and 3/16 on the front two. I definitely think I could increase the sizes of mine even more . . . I'm sure that I'm not giving it enough air to get the most efficient combustion. 

Then again, I really don't want to exceed the 3/4 inch pipe's ability to pre-heat the air, either. 

I want to keep fiddling with it, but it's been too warm to burn for the past three days! Rats! :-D


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## freeburn

Does that mean there are a total of 112 (28 x 4 pipes) holes in it? Then I don't have enough air getting in either! I counted mine up and I have 36 holes on each pipe with 4 extra at the front. 80 holes is getting closer, but they are probably not large enough either. I'll try opening the center holes up to just over 1/8 and see what happens. And yeah, the temps are supposed to drop the end of this week.


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## FixedGearFlyer

That's exactly right! I found this document showing the sizes and arrangement of the holds on the Englander website. The last page gives the angle and size of the holes for each tube. It doesn't give the number of holes, but I just counted them on the stove I was looking at this morning.

That's a total of about 2.6 square inches of air supply, though I have no idea what velocities or volumes move through the Englander system, nor do I know how large the opening for the secondary air supply intake is. I guess I'll have to check that the next time I'm in town.

Granted, I don't have an Englander's firebox, but at 3.5 cu feet, it's fairly close to the size of mine. After the fire brick changes, I'm measuring out at 3.2 cu ft.


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## freeburn

FixedGearFlyer said:
			
		

> That's exactly right! I found this document showing the sizes and arrangement of the holds on the Englander website. The last page gives the angle and size of the holes for each tube. It doesn't give the number of holes, but I just counted them on the stove I was looking at this morning.
> 
> That's a total of about 2.6 square inches of air supply, though I have no idea what velocities or volumes move through the Englander system, nor do I know how large the opening for the secondary air supply intake is. I guess I'll have to check that the next time I'm in town.
> 
> Granted, I don't have an Englander's firebox, but at 3.5 cu feet, it's fairly close to the size of mine. After the fire brick changes, I'm measuring out at 3.2 cu ft.



That's probably about right considering the air moves through the rectangular stock that's about that size. 

Oh, and here's what I do when the weather is too nice to burn. I just had to. 

It's not completely finished, I have to cap off the ends, didn't have that supply. I haven't tried burning yet obviously, but it's gotta work. I also have to add some firebrick that I have to custom cut to size toward the front of the firebox (door side). The best part is, the firebrick is cheap and easily replaceable. The 1/2" pipes are removable and I just loosely put the 1" pipes in on the sides so they are easily removable too. I put 15 - 9/64" holes in each for a total of 60. I think that's over my limit, but we'll see what kind of results we have with this. I can always change it!


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## FixedGearFlyer

Now, that is just plain awesome. You realize what you're doing to me, right? Now I want to tear mine down and build a ladder!

I can't wait to see how that burns. 

And I know I still owe you photos of mine, too. Wish I could get a video, but my furnace doesn't have a window.


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## freeburn

Anyone know of anything that a person can get to act as a fiberboard baffle w/o the premium price and where can I get that locally (Ace, HD, Menards)? There is another substance that people use, and for the life of me, I can't remember what it's called. Ceramic fire blanket? is there such a thing??


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## FixedGearFlyer

I've been looking for the same thing and can't find it locally. There are a lot of online dealers that sell ceramic fire blanket, though. I wonder if a stove shop or mason would have something like that, too?

In my furnace, I have a steel baffle and layered fire brick on top of it, rather than replacing it. You might try the same if you have a piece of steel plate sitting around somewhere.


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## freeburn

I ended up calling my local fireplace store and they had some fiberboard leftover from an install. $20 for the whole section (16" X 36" with a half moon cut out of one side) everything I needed plus a whole bunch more. So it's cut and in now, looking beautiful and waiting for the first fire, when the temp comes down. I've never wanted it to be cold so bad!

The last pic is above the fiberboard. I had some left over and figured I'd put it in place of the old steel baffle above the other one. I figured it couldn't hurt to have one more way of kind of keeping that heat from escaping and reflecting back into the firebox.


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## FixedGearFlyer

That's fantastic! I already have my parts list for a laddered layout and plan to bump up to 1 inch supply pipes (from the 3/4 inch pipes I have now).

It dropped down to the low 30s last night, so we're burning this morning. I lit a small fire with kindling and two small rounds (wrist size) at 6:30am. It burned wide open and heated both the furnace front plate and external flue up to about 350 with a sufficient bed of coals by 7:00. I added two more small rounds diagonally on the coals, then two 6-inch splits on top of those. I let it burn for about 15 minutes wide open. When I went back to check on it, both the furnace front and the external flue temp were 550. I closed the loading door-mounted draft, waited 2 or three minutes, and reduced the ash pan spin draft so that it remained open about 1 turn. 

If I had done this pre-modification, the fire would have immediately smoldered and the temps on both the stove and flue would have dropped to about 275 or 300 within 10 or 15 minutes with lots of smoke from the chimney. 

What happened today, post-modification, is that the secondaries took off, the furnace remained at 500 or so, the external flue temp dropped to about 400, and there isn't even a hint of smoke from the chimney. The furnace is kicking out a LOT more heat than it used to because less is escaping up the chimney and we're getting 4 hour burns instead of 2 hour burns on a light load of wood. 

I think Freeburn's ladder system will work much better that what I currently have in place and is what I originally envisioned. As it is, about 1/2 the volatile gasses can slip out of the firebox without contacting a burn tube. Once I have the ladder installed, ALL of the smoke will contact a burn tube.

For the folks who were concerned about high secondary burn temps exceeding the limits of the furnace design, I'm not seeing any indications that the furnace is overheating. I think all of the high temp areas are contained by the fire brick that we added and the heat that is transferred to the metal furnace and flue is much more efficiently utilized in the house. 

Below are pictures of the furnace settings and temps when 'cruising' this morning on a light load of wood.

Edited to add: If I'd waited another 10 minutes to take the pictures, the flue temp would have dropped to about 350. That's where it stays during the majority of the burn. I shot these about 5 minutes after closing it down and the flue temp dropped quickly to 400, then slowly falls to 350.


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## geoxman

Congrats on the mod! It is very easy to do and the results are impressive. I might give that ladder setup a whirl next weekend and see if there are any improvements over my first setup. I am very happy in the modifications that I made to my furnace and I am sure you will be as well.


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## freeburn

My stupid fiberboard baffle broke in half. I'm going back to firebrick. It seems to work better anyway and you don't have to be all careful with loading your wood in and not hitting the board. I don't know why it cracked, I didn't even hit that section of it. . . oh well, whatever.


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## freeburn

For those interested in how the ladder works, here's a link. 
http://www.youtube.com/watch?v=6H2tZxDw8yg


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## laynes69

Sure its smoke? We run a baro and the moisture from the home shows in the stack. I've been on the roof and the exhaust is water vapor. I'm willing to bet thats what it is. I know our flue temps are low. Today I had a good hot burn and my flue temps were 150 external. Still the chimney stays clean.


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## freeburn

I'm not sure, although I took off my chimney cap yesterday and it was soaking wet inside the top. So maybe that's what it is, is condensation inside the masonry chimney. After a good bed of coals gets going and I throw another log on, I can burn without any smoke. So I bet that's what it is! Thanks for pointing that out!


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## sparke

I think it is really important to have some type of masonry refractory mass above your ladder.  Once it heats up it really helps with the secondary combustion. The firebricks are the same idea but I am not sure how good your coverage is.  You don't want any smoke going up the sides. If you decide to make your own refractory it is easy to do.  Buy a bag of castable refractory cement and put 1.5 the amount of SS needles as usual.  These pics are some refractory I made for my boiler:


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## freeburn

Above the firebrick I have the stainless baffle which has a layer of fiberboard on top of it. The problem with my furnace is that the ladder runs into the original angle iron that is welded inside the firebox for the removable baffle. I can't figure out where the smoke would be getting out. Perhaps it's escaping through a crack in the back somewhere. I try watching the flames inside the box when things are going hot, and I can't see anything getting sucked up anywhere else. 

This refractory thing is interesting. What is the "SS needles" you are talking about?


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## sparke

The Stainless Steel needles are meant to strengthen the refractory.  You should be able to find it from the same vendor that sells castable refractory.  If you do make something up make sure you let it cure for a few weeks and when you fire it,  you should make a few small fires to cure it even more.  If not the piece may crumble on you.  If you know someone with a kiln you could cure it that way also.  Not sure if it is worth the effort...


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## freeburn

Update - the pipes are holding up just fine. The firebrick and baffle are holding up too. All in all, happy with the mod. and much less wood use. I can only fit about 4 max 6 pieces of wood in at a time, but still get some nice long burn times and plenty of heat and secondaries.


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## FixedGearFlyer

My findings are the same as Freeburns. Initial worries about the pipes not holding up to the heat were definitely unfounded, though if you screw the fittings together REALLY tight - like I did - you'll never get them undone after the first hot burn. I also had to modify the bi-metal thermostat on the front of our Vogelzang Norseman 2500 furnace door. It was either wide open or closed - there really wasn't any adjustment. I took off the 'flapper' door that the bi-metal coil connected to and replaced it with a sliding gate door so we can fine tune the primary air. Now that we've done that, the ashpan spin draft stays closed all the time and we're getting great 4 and 5 hour burns that sit around 550 degrees on the stove face and 375 on the external flue pipe.

My neighbor has the same furnace and a very similar house in all respects. We both keep our homes in the upper 60's or lower 70's during the day. He's well into his 5th cord of wood and should start his 6th by the end of Jan. We, on the other hand, are just getting into the first pieces of our THIRD cord. Yup. We've burned half of what he has! Granted, his wood is green and ours was seasoned for 9 months (I know! Not enough, but we moved in in April.), but I think the stove mods play a big part in those numbers, too.

He's on track for an 8 or 9 cord year, which is what the previous owners of our house said they used, too. We're looking at about 5 cords, maybe 6 if Feb is wickedly cold, and we added three bedrooms and a bathroom to the insulated envelope of the house over the summer, doubling the square footage. 

My system's secondary supply tubes run front-to-back, not side-to-side, and only give sufficient coverage on the rear 3/4 of my upper baffle. I plan to change them to side-to-side full coverage baffles this summer and expect to see another small increase in performance next fall and winter.

Chimney-wise, I've been cleaning it from below about once every month and I've gotten about a cup of loose, flakey creosote and soot each time. We do use Rutland Creosote Remover powder about once per week and the chimney is a 32 foot tall, 7-inch square, claytile-lined masonry chimney.

The best news is that we put up 11 cords of wood over the spring and summer in 2010. If we end up burning 5 or 6, we're about a year ahead! Hurray


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## PassionForFire&Water

Some recommendations for future modifications:
I did not read all replies, so some can be duplications, ..please forgive me.
The secondary airtubes (ladder) are best made of stainless steel. They can withstand the heat better and the holes will not burn out over time. 
The thinner the better, because then the sec combustion air gets hotter. You probably need to TIG-weld them
I would recommend to use vermiculite board (like Skamol) or you can make your own boards with a 6/1 ratio vermiculite/refrac cement.
The firebrick will work, but firebrick is not a real insulator, while vermiculite baord is. So you will create a much higher temp up top.
An other method would be to cast the secondary combustion air supply in a vermiculite base. There are some Scandinavian wood stove that apply this concept. Like Scan for example.
Key is to not make the sec air to hot, so it's density is to low and it passes right-up the stack.
If it's to cold it will fall down and becomes prim air; result you will burn more wood.


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## TdiDave

Was thinking of making some mods to my stove.  Have a few questions: what is the benefit off having the 2 sides of the secondary air supply tied together?  Would the T style setup work as well, as this would be much easier for me to do with the supplies I have on have.
If using 1" supply do the burn tubes need reduced to 3/4" ?


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## brenndatomu

I found this to be an interesting read, should be for _anyone_ considering modding their smoke dragon!
Tdi Dave, You could try sending these guys a PM, I see they haven't been around for 6-8 weeks. This thread may be buried again by the time they sign back on.


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## TdiDave




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## M1sterM

Wow, nice layout TDIDave.  Pics or video of it in action?

And I have a 2002 TDI...


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