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Modifying a central wood furnace for secondary burn

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by FixedGearFlyer, Oct 8, 2010.

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  1. FixedGearFlyer

    FixedGearFlyer Member

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    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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  2. freeburn

    freeburn Feeling the Heat

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    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. . .
  3. FixedGearFlyer

    FixedGearFlyer Member

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    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
  4. freeburn

    freeburn Feeling the Heat

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    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.
  5. FixedGearFlyer

    FixedGearFlyer Member

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    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.
  6. freeburn

    freeburn Feeling the Heat

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    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!

    Attached Files:

  7. FixedGearFlyer

    FixedGearFlyer Member

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    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.
  8. freeburn

    freeburn Feeling the Heat

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    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??
  9. FixedGearFlyer

    FixedGearFlyer Member

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    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.
  10. freeburn

    freeburn Feeling the Heat

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    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.

    Attached Files:

  11. FixedGearFlyer

    FixedGearFlyer Member

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    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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  12. geoxman

    geoxman Feeling the Heat

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    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.
  13. freeburn

    freeburn Feeling the Heat

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    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.
  14. freeburn

    freeburn Feeling the Heat

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  15. laynes69

    laynes69 Minister of Fire

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    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.
  16. freeburn

    freeburn Feeling the Heat

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    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!
  17. sparke

    sparke Minister of Fire

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    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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  18. freeburn

    freeburn Feeling the Heat

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    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?
  19. sparke

    sparke Minister of Fire

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    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...
  20. freeburn

    freeburn Feeling the Heat

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    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.
  21. FixedGearFlyer

    FixedGearFlyer Member

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    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!!!
  22. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    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.
  23. TdiDave

    TdiDave New Member

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    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" ?
  24. brenndatomu

    brenndatomu Minister of Fire

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    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.
  25. TdiDave

    TdiDave New Member

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