I can speak from personal experience that a longer burn tunnel is a bad idea!
Here's what happened to me... Mid January suddenly I start having draft issues.
A look down my barrel showed the riser was in great shape.
A look into my transition area showed it was 80% full of ash! My 8" pipes had a couple of inches left before being plugged completely.
This is a Northern Montana greenhouse, there is no insulation. Without the RMH running all day it would freeze out at night.
I had to do a complete cleanout to get the stove functional again and a few week's later found me doing a mid winter core rebuild to shorten my burn tunnel...
Now a riser can certainly be taller , my J tubes averaged 50-54" from the burn tunnel floor.
Get to crazy tall and you might have a reduction in efficiency.
Hi Joe, Over the years there has been much study and refining of the J tube core that can't get much better in terms of complete combustion. Unless your chasing after half percents, making sure you burn dry wood, establish good draft, build it with low mass/ high heat resistant materials, keep an occasional eye on it while running etc. are going to be your best bets at making it as efficient as it was meant to be.
But don't take my word for it. Rocket science is all about experimenting for yourself. Playing with brick Legos in various fashions outside is fun and very educational so that you know from direct experience why the build ratios are as they are.
Thomas, at our 36 acres in northeast Arizona fuel for a RMH is an issue where a RS for cooking maybe more beneficial or even a large mass uninsulated masonry wood fired oven. No live trees are going to be cut and seasoned for firewood, only deadfall that is sun dried rotted so absent of volatile organic compounds. As such wood ash is going to be overly abundant as experienced with campfires and the 30 gallon 6” black stove pipe test RMH. (Again the test unit is heating a green house on another homestead.) With 36 acres of old growth Pinion Pine and Juniper (300-600 years old) and miles upon miles of uninhabited land otherwise we have decades of low quality fuel.
I’m not quit certain what your issue is with the longer burn tunnel, are you saying that the longer burn tunnel reduced velocity of the combusting gasses/solids(ash), causing (ash) to fall out sooner in the circuit that ended up being the transition area? Otherwise the ash would have been carried further down the heating runs? Seems that maybe that’s a good thing to facilitate mid-season ash clean out by simply gaining access via clean out for a quick vacuum of the bulk of ash.
Gerry, well aware of the brain power that has gone into this RMH movement specifically over the past decade, it’s been very interesting following along. BUT (had to do it) I’m not normal with some ideas that may well change or branch off the operational characteristics of RMH’s. Just what those ideas are will be held close for now. If you change the way you look at things, the things you look at change.
If you make a point of exploring and collecting recent deadfall or some standing deadwood, you can probably build up a reasonable stock of fuel. 36 acres in your area may equate to 5 or 10 acres in the Northeast for growth potential, but that seems to be adequate for modest climate heating. Pine may not be as energy-dense as hardwood, but it can serve well in the complete combustion environment of the RMH. Even if there is more ash from this fuel, there is still a lot less than in a conventional woodstove, and most of it stays in the feed tube and burn tunnel floor, easy to clean out on a regular basis.
The longer burn tunnel allows more of the combustion to occur in the horizontal leg, reducing the amount of combustion-induced draft generated in the vertical heat riser, and lowering the system draft. An excellent chimney may make up for the reduced core draft. Having the turbulence-generating sharp bend from tunnel to riser at the standard proportion may also affect combustion efficiency, I don't know if that has been investigated. A longer burn tunnel is not going to give you any benefit for the reduced draft risk. A longer riser as mentioned can be beneficial to a point; once there is sufficient length for combustion to finish in the core, there is likely to be no benefit to a longer riser.
No, Peter did not OK my burn tunnel being longer. I did that all on my own!
Originally I built an 8" J tube using perlite and clay. A Walker cast core. and a Walker style cast riser as well.
Worked beyond awesome! Barrel top temps of 1100F!
What did not hold up was the feed tube. By the end of the 2013 season, my feed tube had enlarged to the size of a 5 gallon bucket.
I decided to build a heavy brick core to replace it.
I located a source of free firebricks....
I made the burn tunnel roof apx. 13" - 15"" long. (Don't remember the exact number)
I did that because I did not have a clue... just like any first timer. Bigger must be better right?
And besides it made setting the barrel much easier...
Remember in 2013-14 information was not as easily available as it is now. There was one book by Ianto Evans and there was Permies!
I loved my new core, for the fact that wood had no effect on it. My feed tube started out at 7.5" and stayed there! Hooray we are rocketing now!!!
Long about the middle of winter I started having draft issues??? Why, I was keeping my burn tunnel cleaned out?
It quickly progressed to not burning worth a damn... smoke back was a daily problem! That was not acceptable!
My barrel of course has a removable lid, a quick look inside showed no issues in the riser.
As stated earlier, my transition area was almost full and my 8" horizontal pipes had maybe 2" left open... for the entire horizontal run! That's over 20' of pipe filled with ash!
That was when I discovered that a leaf blower can blast out your mass...BUT it make's one huge pucking mess!
A core rebuild happened in the middle of winter that year... let me tell you, it was very scary taking apart your only source of heat in a Montana winter.
That was when I learned all about WHY we use clay not refractory to mortar our bricks... A piece of cake to tear into that core. All the cob went in a bucket and was rehydrated to be reused. A scraper cleaned the bricks up in moments. Within 4 hours of walking in the greenhouse that morning I had my core rebuilt and was lighting a new fire!
Moral of the story here. The innovators, Peter Berg , Matt Walker , The Wisners , Ianto Evans. They tried all the configurations they could dream up.
They quickly learned what would work and what did not work. Eventually high dollar test equipment was utilized to fine tune their results.
They have openly shared this information with the world... we all owe them a huge thank you for telling us how they accomplished everything.
I do not believe there is a post showing that core rebuild. I may still have photo's on my computer.
Did you change from a J tube to a batch feed? Or is this a J tube that you were having problems with? I thought there was a post that someone had a wood supply that was 2 inches too long and then changed the batch burn area to accommodate the longer material.
Ahh now I understand we are talking years apart here.
All these discussion's have been about a J tube.
Yes last year I switched from a J tube to a batchbox in both my shop and our greenhouse.
Both my batches are "standard" size. After having my 6" batch for a while, I started wishing it could be longer.
After checking with Peter, he did indeed say that the length of a BB could be slightly increased as long as the other dimensions stayed the same.
My 7" batch is plenty long enough. My 6" batch would be much nicer if it was 3" longer.
No current plan on extending the 6" but... as soon as I think I have a reason... longer it will be!
A big difference in the J tube and the batch. In my plans I was NOT considering the J Tube and the replies here confused me. Now I see that the J Tube goes into a burn tunnel before entering the riser and the batch goes directly into the riser.
I just checked my transition area a few days ago.
I am happy to report that it was almost empty! Very little ash made it out of the core and into the system!
My 7" batch goes into a large brick bell so ash is not an issue at all with it.
There is a fair amount of fly ash that needs scooped from the box itself, if you burn nonstop all day like we do.
In an insulated house a batch might burn twice a day and fly ash would minimal.
I can see in a J tube that where the fire is burning at the bottom, the ash from the wood would be carried along with the heated air ( the rocket noise) as the wood settles, while the in the batch the heated air and gases are carried into the riser with gravity holding much of the ash in the burn chamber, not unlike a normal wood burner or burn pile. The batch will of course send up the small particulates that should be burned in the riser.
Been thinking... oops... given the location, purpose and fuel profile both the burn tunnel and riser will be extended to accommodate accumulation of ash within. I would rather provide means for daily cleaning of ash build up rather than depositing ash downstream. There will be means for an annual inspection and clean out of the system at the end of the heating season.
Certain design enhancements are going to be added to assure the system functions and is easy to start. It will be heating a 200 square foot floor once a day in the evening with a target peak release of heat for 3 in the morning and beyond. There is no reason to provide heat during the day as this is a working farmstead.
Ash downstream is an efficiency robber most likely when more heat will be needed during February and March. This is based on the available fuel that will be used: dead wood void of the majority of volatile compounds that support complete combustion that lends itself to ash production.
Live trees are not going to be harvested for fuel when there is so much dead wood available. Wood will be Pinion Pine and Juniper that 75% of is of a diameter that provides the surface area and spacing in the feed tube for good fuel air mixture.
Burning seasoned Pinion is a scary thought especially if done by an inexperienced operator. The wood is like turpentine: note the attached picture of an injured tree. Thinking the injury is from the Western Pine Bark Beetle that met the Pinion's natural defense of prolific sap production to encase the pest.
Pinion bark on dried wood falls off and will be used as mulch in the garden.
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