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Post by petect on Oct 26, 2013 6:55:08 GMT -8
Hi all I haven’t posted for a while because I bought my retirement house and have been very busy moving and fixing up the new place – lots yet to do! But I have been lurking to keep up with the great work that is going on here. At the new house I have plenty of free wood, BUT unfortunately there is no chance of putting a RMH in the house. It would even be difficult and very expensive to put in a conventional stove. Around here outdoor furnaces en.wikipedia.org/wiki/Outdoor_Wood_Boiler www.centralboiler.com/Tech/C010.pdf are fairly common. These things are highly polluting, very inefficient, and really stink - even hundreds of feet downwind. So, I’m asking for help designing a reasonably easy to build, affordable alternative based on rocket stove principles. The ability to burn a big fuel load will probably be the biggest challenge. Since it will be located outdoors, adequate storage mass probably won’t be a major problem. Mechanical means of moving the heat into the house will probably be needed – sorry Donkey. Some combustion efficiency may be lost when scaling-up RMH designs, BUT I’m sure that the worst rocket-based design will be far superior to the best outdoor furnaces currently on the market. On the plus side, outdoor fireplaces, pizza ovens, etc are very common, and are completely ignored by building inspectors. I think an outdoor rocket furnace could be located fairly close to the house to minimize heat lost while being transferred to the house. ALSO most northeast (U S) states and many towns are considering banning, or putting restrictions on current outdoor furnaces, due to the pollution they create. SANDY could this be an opportunity for you? I’d like to think a clean burning furnace could be a winner. BTW your outdoor kitchen is BEAUTIFUL! Winter is coming, so a build will probably have to wait till spring, but some testing should be possible. So hit me with your thoughts. Thanks Pete |
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Post by Donkey on Oct 27, 2013 15:21:09 GMT -8
You probably want a batch box, really big, something you can shovel a lot of wood into and forget about for a while.. Buzzing out to the furnace every 10 minuets to shake down the J-tube isn't going to take you to your happy place.
What about building a relatively lightweight masonry bell inside with the firebox outside and an insulated flue that goes into the bell chamber, back to the building and out the chimney there.. ?? This way, you would eliminate moving parts (YAY) Keep the stove outside and fake out the building department, all at the same time..
'Course, if having ANY KIND of storage mass in the home is out, you could put the mass in your out-building and do some kind of anti-freeze liquid loop to a radiator inside or even just blow air through the mass.
What I like is a challenge..
You thought of doing a solar air heater too? Look that up on youtube. They're easy and ingenious.
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Post by satamax on Oct 27, 2013 23:24:49 GMT -8
You probably want a batch box, really big, something you can shovel a lot of wood into and forget about for a while.. Buzzing out to the furnace every 10 minuets to shake down the J-tube isn't going to take you to your happy place. What about building a relatively lightweight masonry bell inside with the firebox outside and an insulated flue that goes into the bell chamber, back to the building and out the chimney there.. ?? This way, you would eliminate moving parts (YAY) Keep the stove outside and fake out the building department, all at the same time.. 'Course, if having ANY KIND of storage mass in the home is out, you could put the mass in your out-building and do some kind of anti-freeze liquid loop to a radiator inside or even just blow air through the mass. What I like is a challenge.. You thought of doing a
solar air heater too? Look that up on youtube. They're easy and ingenious. Whenever i see this, t think seasonal mass storage  |
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Post by petect on Oct 28, 2013 6:04:40 GMT -8
Donkey
Your idea of putting a bell inside the house is a great one, but the layout of the house just doesn't permit that. Putting the heater and storage outside isn't all bad since supporting the weight of the burner and storage mass shouldn't be too much of an issue.
My main concerns are making the batch box as big as possible while keeping the temps in the riser high enough. My gut tells me that if I were to just scale up a proven stove design, at some point I think the wood in the box will just start to smoulder, and the combustion in the riser will suffer - maybe stop.
Also, I wonder if I try to run the exhaust through too much storage mass it might choak the flow.
Solar air is definatly going to be part of the mix. Unfortunatly some years it's very cloudy for the first half of the winter, so I want to build a rocket as another source of heat - along with satisfying my fascination with the beasts.
Pete
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Post by Donkey on Oct 28, 2013 17:04:37 GMT -8
There's almost certainly a limit to how much a stove can be scaled up. It may be that an 8 inch batch box is about as big as you would want to go for safety reasons. I can tune a n 8" J-tube to just about melt the barrel, I imagine that an 8" batch box can be EVER so much more aggressive..
It seems that it may be possible (based on other conversations here) to just about double the LENGTH of the batch-feed. Obviously, experiments are in order but it may be possible to almost double the load. I don't know of anyone building an 8" PvdB style batch-box. It might just be yer ticket..
Big heavy bell in the fire-house, some form of heat exchanger inside the bell, pipe THAT into the house.. I wonder what other folks think the best transfer medium would be. In my mind, since you will be heating the air in the house, I would move air through the system, though it would increase the size of piping, going to and from the fire-house. In my way of thinking, fewer conversions will be better, from mass to air instead of mass to liquid to air. Fewer conversions, simpler system, fewer moving parts, less entropy in the system..
Anyone else want to chime in on this??
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Post by matthewwalker on Oct 28, 2013 18:32:19 GMT -8
Well, I'm really not sure and don't want to make a negative presumption, but my gut feeling is that there is a finite limit to how big you can go with a passive system. At some point you are going to have a fuel rich situation and the system just won't be able to get itself enough air without more pyrolysis at the front end exacerbating the situation. There may be a way to design it to self adjust, but even in internal combustion at some point you need to ram the air in there to handle higher fuel loads, i.e. turbo, superchargers, etc.
The Richard Hill boiler from the late 70's does a pretty good job of being a forced air rocket capable of large fuel loads and fairly clean operation. Might be worth looking into Pete, if you haven't already. It does need external power inputs though.
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Post by Rinchen on Oct 29, 2013 1:32:19 GMT -8
There's almost certainly a limit to how much a stove can be scaled up. It may be that an 8 inch batch box is about as big as you would want to go for safety reasons. I can tune a n 8" J-tube to just about melt the barrel, I imagine that an 8" batch box can be EVER so much more aggressive.. It seems that it may be possible (based on other conversations here) to just about double the LENGTH of the batch-feed. Obviously, experiments are in order but it may be possible to almost double the load. I don't know of anyone building an 8" PvdB style batch-box. It might just be yer ticket.. Anyone else want to chime in on this?? Last heating season I have used an hybrid between a J-tube and batchbox since I had adapted my J-tube system to have a firebox. It has worked pretty well all season and the firebox is quite large, larger than what you would get for a 8" batchbox based on the measurements of Peter vdB and my burntunnel+riser measure 6.3" x 7.1" square, which is less than an eight inch system. I would sayin this configuration it did not burn as clean as the J-tube with P-plate, since I could notice some sooth each burn on the thermometer, nevertheless what came out the chimney was usually white steam. For this season I'm rebuilding it with a brick "barrel" and a new firebox based on the proper layout of a batchbox system, with the firebox not extended, but slightly wider compared to the standard system to accomodate for the firewood I buy here. The riser will be 8" round ceramic fiber tube. I choose to have a large firebox in order to have only one load of wood per heating session. Pictures will come when I'm finished :-) |
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Post by petect on Oct 29, 2013 6:18:42 GMT -8
Thanks for the comments guys
For those who have built working RMH's what is your guess as to the upper size limits of box? If I could get 3 or 4, 4" dia. x 16" logs into it and have it burn cleanly, I will be a happy camper.
Matt
The Hill boiler is very interesting. It seems to confirm my thoughts that if I try to go too big with this thing I will end up with a gasification unit and the fans and controls that go with it. Better to explore the upper limits of a conventional RMH.
Pete
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Post by grizbach on Oct 29, 2013 7:23:04 GMT -8
Hi Pete,
My 6" batch is mostly stock, except I made it slightly longer(20"). I place 17-18" logs in her. I didn't want to go too long since no one else had tried it.
I can fit seven 3" logs with plenty of room to spare. I think ALL 4"ers would not stack very well in this smallish box. but you could get about five in there. 3.5" is a good size though, with six logs.
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Post by Donkey on Oct 29, 2013 7:41:33 GMT -8
Well. According to the batchbox .xl file, a "regulation" sized 8 inch stove should have an 11.52" wide by 17.28" tall by 23.04" deep box.
Thats pretty big right there! If the box depth is half-again it would be 34.5"(or so) deep.
My stove here at home, the ancestor to the PvdB batch box, is an 8" stove. It's certainly nowhere near as efficient as the new models but through most of the burn cycle there is no visible smoke at the chimney.
The feed on this stove is an arched tunnel, about 12" wide by 10.5" tall by 24" deep. The spring-points for the arch are about 4.5" from the floor.. My guess at the CSA (very rough, it's a hand arched, cob tunnel so it varies all over the place) is 110.5 square inches.. About twice system CSA. A little more than half the CSA of an 8" PvdB batch box and roughly the same depth.
One full load on my stove will burn for (it's been a year since I've used it so, guessing here) about 45 minuets.. On a cold night, I'll need to fill it 2 or 3 times.
So.. Based on all that, if I replaced my current stove with a new-fangled 8" batch box, It would burn maybe half again as long per load and I'd need to load it twice on a cold night. The thermal mass on my stove here could be beefed up considerably, the exhaust temperature is higher than I'd like. If that were done, the mass wouldn't need to get as hot to heat my space well and I'd loose less of it out the chimney, if the mass were a bell it would do better again, etc.. Just guessing, but a TOTAL rebuild from absolute scratch and I bet that one load would heat this house effectively.. It's about 1600 square feet, hybrid strawbale/lightstraw/studwall construction..
Petect,
I don't know if any of that helped you.. It is meant to hopefully give a little perspective. Kinda worked for me.. I'm a little clearer on what could be done and how it relates to what I already use. (makes me want to rebuild my stove!! Boy, that would piss off the wife..)
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Post by 2tranceform on Oct 29, 2013 7:55:45 GMT -8
(makes me want to rebuild my stove!! Boy, that would piss off the wife..)  Its all in the name of increased efficiency!  |
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Post by matthewwalker on Oct 29, 2013 9:44:59 GMT -8
Pete, to answer your size question, my largest firebox on a 6" system is 8"x14"x20", I've since brought the height down to 12" but that one works great and can handle pretty large fuel. My 8" boxes, now that Donkey has done the numbers, aren't as big as they apparently could be, but my largest was something like 10"x14"x20" and frankly, I never had the guts to stuff it full. I was able to pretty easily over fuel it and end up smoking fairly badly. To me the big advantage to the larger firebox isn't that you can put in more fuel, since burn times are fairly consistent regardless of load size, but rather that you can use larger fuel. I am able to use my batch boxes to burn the stuff that won't fit in my J system, and there's less processing to do when using regular cord wood.
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Post by ronyon on Oct 29, 2013 11:10:36 GMT -8
I am fascinated with the outside or basement rockets.
There was even a design for an outside boiler that was basically a barrel stove wrapped in sand for thermal mass and with a PVC heat exchanger. The PVC did not last,failing under the constant heat, but the stove was burned hot and fast, to keep it clean and the system seemed sound other than the PVC grid failing.
Now I would not want to duplicate that system, but a Big bell with a large tank of water inside would be a good place for a copper or maybe a PEX heat exchanger to pick up heat.
I could see a series of 55 gal drums clamped end to end inside a bermed bell of cinderblocks with sodabottles and dry soil in the cavities. Put the batch box at one end and put a sheet metal lid on it. On the lid goes a layer of dry earth and then tarps and them more earth. Maybe a hoop house on top of that!
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Post by petect on Oct 30, 2013 6:31:30 GMT -8
Matt
An 8"x14"x20" box is very encouraging! And yes, being able to use bigger pieces of wood & spending less time cutting is a major consideration.
royon "Maybe a hoop house on top of that!" A RMH inside a hoop house would be the multimate for me! For now, if I could stretch the dimensions of Matt's burn box a bit and add a decent storage mass, I'll be happy. I think that air or water can be used to move the heat into the house. Each has its advantages and disadvantages.
On a side note, I was gretted by the smell of my neighbor's outdoor furnace when I got home last night. It's at least 500 feet away, and the smell was pretty bad - gives you an idea of how much wood is going up the stack.
Pete
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Post by satamax on Nov 1, 2013 20:38:31 GMT -8
Pete, to answer your size question, my largest firebox on a 6" system is 8"x14"x20", I've since brought the height down to 12" but that one works great and can handle pretty large fuel. My 8" boxes, now that Donkey has done the numbers, aren't as big as they apparently could be, but my largest was something like 10"x14"x20" and frankly, I never had the guts to stuff it full. I was able to pretty easily over fuel it and end up smoking fairly badly. To me the big advantage to the larger firebox isn't that you can put in more fuel, since burn times are fairly consistent regardless of load size, but rather that you can use larger fuel. I am able to use my batch boxes to burn the stuff that won't fit in my J system, and there's less processing to do when using regular cord wood. Matt, on my green machine, i have the firebox at about 6.7"x12"x20" Working lovely with the insulating firebricks. I had the whole end of the batchbox and heat riser refractory glowing orange the other day. |
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