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Post by pinhead on Nov 29, 2012 11:18:34 GMT -8
I've been delaying starting a thread about my RMH since until last night it's been pretty temperamental. It was hard to start and smoked back unless the fuel was positioned perfectly. It burned decently warm once it was going, but wouldn't run continually without repositioning the wood to effectively stuff more fuel into the throat of the feed; otherwise it would either smoke back or burn out.
This changed last night when I made a very small modification that made a HUGE difference.
I had originally built the burn tunnel and heat riser with two six-inch HVAC duct Tees attached end-to-end. The heat riser went on one of them and the fuel was fed into the other.
The 6-inch ducting is surrounded by eight-inch ducting with a slightly insulative vermiculite-furnace cement mixture between the two. This whole contraption was surrounded by a much more insulative clay-grass-perlite mixture.
The problem I was having was due to the open "chamber" that was created by the tee directly beneath the fuel feed. This allowed heat to build up and air to swirl around, eventually finding it's way out the top and into the room.
I filled this chamber (see the accompanied drawing) with a clay-ash mixture which seems to be a pretty good insulator thus far. This eliminated all of the problems that i was having.
I think I should have went with an 8-inch system, though.
I haven't taken any pics of the thing yet since I haven't yet added the mass to the half-barrel bells.
Hopefully once I get the stove finished up it'll work better and I'll be able to begin sealing the old rock shop up.
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Post by peterberg on Nov 29, 2012 12:00:03 GMT -8
Everybody is warning against extra air, horizontally brought in at burn tunnel level. Ianto Evans' book doesn't show such an opening, so it shouldn't be there. But on the other hand, maybe you plugged the ends with end caps and filled the dead end with clay/whatever mix.
At the other end of the burn tunnel there's still another dead end, where there's a small vacuum created by the streaming gases. This will severely limit the speed of these gases.
I'd suggest you plug this hole with a suitable clay/whatever mix. On top of that, you could round off the bottom transition from tunnel to riser, obeying the system size c.s.a. in that particular spot. This will make your stove draw better.
Check the transition from barrel to first bell, this has to be large enough in order to avoid a restriction. When you think it's the perfect size, make it twice as large, this will help a lot too.
Probably it will run so hot the fire will eat the metal of the HVAC-duct T's.
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Post by matthewwalker on Nov 29, 2012 12:52:35 GMT -8
In the interest of sharing info, my home system has that open front with a door and window. It's about 12" long, from feed to opening. The seal on the door is terrible, so there's probably 4" sq. of air inlet there. I run it as a traditional top feed, and only open the little door for cleaning or baking small bread. The stove runs amazingly well, and pretty much never smokes back. Not saying you aren't correct in your trouble shooting Pinhead, just that there has been a bunch of talk about this and I guess I'm the exception to the rule. It can be done successfully.
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Post by pinhead on Nov 29, 2012 12:55:46 GMT -8
Both ends of the T's were capped and the outside was covered with the clay mixture -- there was no secondary air feed. Last night I completely filled the void under the feed tunnel with ash-clay. Next time I pull the barrel I intend on filling the chamber under the heat riser, too.
The metal of the inside duct has already started to burn out. That's why I used furnace cement and vermiculite between the six-inch and eight-inch ducting; I can pull the metal shards out, leaving the concrete form intact.
The bell is made of two halves of a fifty-five gallon drum.
The transition from the barrel to the bell is as wide as the half-barrel and goes all the way around the eight-inch ducting. I'll measure it when I pull the barrel, but I would guess each side of the burn tunnel transition, when viewed from the top, is 3"x16" or so for a total area of 96in^2 feeding into the entire width and height of an open half-barrel.
Viewing the transition from the half-barrel side, you'd see the open end of the half-barrel with an eight-inch circle blocked. The rest of it is open. This would calculate out to (226in^2 - 3.14*4*4), or about 175.75in^2 or slightly less.
I'll try to get some pics and more accurate measurements.
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Post by peterberg on Nov 29, 2012 13:41:48 GMT -8
The transition from the barrel to the bell is as wide as the half-barrel and goes all the way around the eight-inch ducting. I'll measure it when I pull the barrel, but I would guess each side of the burn tunnel transition, when viewed from the top, is 3"x16" or so for a total area of 96in^2 feeding into the entire width and height of an open half-barrel. That's about twice the minimum size, that should be perfectly adequate. No restrictions there, I would say.
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Post by grizbach on Nov 29, 2012 17:46:33 GMT -8
pinhead, I have a grate in my build for ash and ember removal. If you get a lot of coals in your feed tube, all that radiating heat ignites the rest of the fuel supply.
Matt, 4 sq. inches of horizontal air feed is massive! I usually run ½-3/4”. Is your house build piped into an existing chimney? Mine is and it draws wonderfully. I think this might be the reason I can have a small air intake at the bottom of my feed tube.
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Post by matthewwalker on Nov 29, 2012 18:22:08 GMT -8
Yeah, it is quite a bit of horizontal air, but I've likewise choked my feed down to about 4"x6", and yep, you got it. Existing masonry chimney. I agree, I think it makes all the difference in draft.
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Post by pinhead on Nov 30, 2012 6:31:47 GMT -8
pinhead, I have a grate in my build for ash and ember removal. If you get a lot of coals in your feed tube, all that radiating heat ignites the rest of the fuel supply. That's what I'm thinking is the "problem" -- I can feel the radiating heat all the way up to the roof directly above the feed tunnel even after all of the wood has burned down to coals. I did notice, though, that the problem is greatly reduced when burning dense wood such as hedge or even cedar. Burn-back is also reduced if i burn a big log -- maybe three inches across in a six-inch system -- surrounded by the smaller sticks to take up the extra space. This keeps the air velocity high enough to cool the wood that isn't yet down in the feed throat, while also burning enough fuel to keep the BTUs up. I got a window sealed up last night (was a series of slats with three-inch gaps between each of them so for all intents and purposes, the window was "open") and it looks like I may not need a bigger system, after all. There's still a lot of leaks in the crumbling rock walls and I was able to work without a jacket last night. This thing may work out yet!
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Post by grizbach on Nov 30, 2012 10:16:46 GMT -8
Burn-back is also reduced if i burn a big log -- maybe three inches across in a six-inch system -- surrounded by the smaller sticks to take up the extra space. This keeps the air velocity high enough to cool the wood that isn't yet down in the feed throat, while also burning enough fuel to keep the BTUs up. Yes, it is a balancing act. I run with my damper about 1/2 to 1/8 closed and depending on the wood, sometimes the heat gets sucked out of the feed tube. I then open my bottom intake a little more to heat up the coals that are on the grate and sticks.
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Post by jjonesmx on Nov 30, 2012 20:09:38 GMT -8
I too have a problem with smoke backinto the house.so what I did was put a vertical divider into my burn tunnel.I wanted to create more airspeed in my burn tunnel. now I have 2 small burn tunnels that feed my 6 inch heat riser....same concept as a carburetor on a car and it works quite well most days. I beveled both ends of the 1 inch fire brickso it had good transition.I think now I have a problem with my house being sealed very tight so I have to leave a window cracked for it to draw like I think it should....when I crack the window open you can hear the rocket sound that it should make and that is good news in my book.... next I will try to seal my clean outdoor with some insulating rope and i think that will Finnish it off..... thank all you all for the inspiration to build this cool stove blessings to all...... JJONESMX
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Post by pinhead on Dec 5, 2012 14:14:26 GMT -8
Anyone interested in a math problem?
I have a 6" diameter feed tube and am trying to decide how to get the required 5% area for the P-channel...
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Post by peterberg on Dec 6, 2012 6:59:44 GMT -8
I have a 6" diameter feed tube and am trying to decide how to get the required 5% area for the P-channel... The 6" diameter gives a c.s.a. of 28 sq.in., 5% of that is 1.4 sq.in.. You're quite close using a plate of 3.56" or 3 9/16", the segment behind will be about the right c.s.a.. I'm not good at maths, but I can use SketchUp to draw it for me...
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Post by pinhead on Dec 6, 2012 11:16:01 GMT -8
Thanks!
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Post by pinhead on Dec 10, 2012 7:49:08 GMT -8
Only one pic per post? And a 1024k limit? I guess I'll upload them elsewhere and link here.
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Post by Donkey on Dec 10, 2012 8:10:59 GMT -8
1024 K is too big for one image. Most people's screen size is 1024x768.. Why would anyone need an image larger than that?? Unless it's meant for print, you don't need an image much over 800x600 (or so) which means your images will be LESS than 800k Also, some of us have SLOW Internet connections and large images are a TOTAL pain in the neck!! Be kind and keep it small.. Ok?
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