seeking feedback on a double flue hybrid approach

[patamos]

Hi All,

I am getting ready to build a 6" J-feed rocket system for friends.  It will be in a light clay and log post and beam house we built with year-round microhydro that feeds all off peak surplus into a boiler that supplies heat to the bottom and near the top of a 7ft deep sub floor sand bed...  So am hoping this heater will be the icing on the cake.

The cabin is 900sqft on the main floor and 500 on the upper floor.  Very well insulated with a long flywheel radiant heating system that should take care of most heating needs.  The RMH will be there as a more tweakable secondary heat source, a back up cooking system, conductive (bench) heat source and aesthetic comfort.  

More than anything i want to make sure it pulls well and is generally easy to operate with as little maintenance/repair as possible.
Thinking of high temp firebrick splits for the feed tube, burn tunnel and riser.  Maybe a circular cast refractory riser.  Question: i have seen and done circular risers with smaller CSAs than burn tubes, eg 6" square burn tube and 6" round riser - does this work because some of the CSA in a 6" square heat riser is redundant?
Also, are the 1-2-4 ratios primarily a minimum guideline for riser height?  In other words, if i add another 10" vertical to my riser i am assuming i do not need to boost the dimensions of the feed and burn tubes...

 Optimizing fast radiant heat delivery is not important so i am looking to skip the barrel and have the flue gasses go from heat riser to a cast iron grittle about 4" above.  From there gasses will drop down under 1 or 2 cast refractory joists (supporting the grittle).  One route will be up into a primary flue that runs 20+ feet vertically through the rooftop.  This will be for a good start up, and possibly for quick cooking tasks.   

Sliding a cast refractory damper closed will divert gasses down one side of the stove body with a relatively thin cobb outer wall so that some heat can radiate quickly into the living room.  As this small wall surface (2'x3' approx?) will be one of the more thermally dynamic layers of mass i am wondering how best to construct it.  The idea taking shape so far is firebrick splits on edge glued with refractory cement.  Then 1/2" of cobb plaster.  let it dry, check for cracking then another 1/2" cobb plaster with loose weave burlap or fiberglass mesh embedded for mechanical strength.  Suggestions/cautions are most welcome.

From there gasses will drop into a zig zag zig zag bench flue with 4 runs.  We want the bench for a sitting/lounging/quick conductive warming space... but have only about 7' horizontal area to splay it out.  So i am thinking of a run and back in the bench then another 2 in the back rest to tie back into the vertical flue above the damper.  I have read that i can expect 30' to 45' of horizontal push from a 6" system.  That a circular heat riser will push better than a square.  That each crimped 90 elbow reduces that by 3'.  But then, the tall vertical flue (mostly uninsulated) run will enhance the pull so long as it still has a fair bit of heat in it.  My sense is i will be going for about 30 feet of horizontal.
I understand that i need to keep some heat in such a long vertical run otherwise condensation could become an issue.  MHA guidelines saying 90c at exit, but that may be overkill.  I am planning to wrap the uninsulated flue in 3" of cobb (except where going through floor and roof - insulated double flue there) to make it safe to touch, and to keep drawing heat that can radiate where it is 360 exposed in the upper floor area.

So all in all i have a whole bunch of variables at play.
I am planning to make my transition from riser to up and down draft chambers much bigger than riser CSA.  I am also thinking about widening out the CSA in the 180 bends to even out the flow rates and cut down on eddying.  Also making sure the tie ins to the vertical flue are on a 45degree angle.  Planning to use 6" sacrificial galvanized pipe so there will be some slicing taping and riveting there...
Cleanouts in recesses at bottom of first downdraft chamber, 180 bends at end of bench, and bottom of vertical flue. 

The questions most in mind at this stage are:

How much push will i lose/gain by not including the fast cooling downdraft of large radiating (barrel) surface immediately after the heat riser?

What is the best kind of glass for a window in the burn tunnel?  
Is there a maximal size and optimal placement of this window so that i do not rob too much heat heading into the riser core?

How low can i hope to bring my exit temperature without fear of condensation creating resistance?

What might be the simplest ways to tweak the system if i find my exit temps too high/low?  One thought is to place the first horizontal flue run about 3" from the front vertical surface of the bench.  If gasses are too cold on exit then replace those 3" of cobb with clay perlite.  Maybe do the same with areas of cobb that surround the vertical flue upstairs.  
If gasses are too on exit... then, maybe some sort of mini bell near the the first 180 return.


I realize this has made for an epic read with many questions at play.  I had hoped to read/absorb as much of this forum as i could before launching into this next build with not much experience.  But man, the rabbit holes are everywhere!!!!

So, with the deepest respect and gratitude to all of you who share your knowledge so openly... i cast my wishes into the ethers.


well being

pat 



 

[matthewwalker]

Dang, lots to cover there. Here's a shot at a couple answers:

Glass: I've had great success with Robax, which is a brand of ceramic glass. I've sourced it off ebay, found some in old stoves, and have also purchased direct from the manufacturer. If you need a custom size, I can probably help, but they are pretty slow and rather expensive direct. Best to source it locally if possible.

I recently did some experiments with flue dampers controlling two different paths on one heater, with pretty great success. Your plan sounds good, but a bit complicated to me. I just used 6" stove dampers to control the flow through different zones. It was nice to be able to adjust them in small increments to keep the exhaust temps where I wanted them as the heater came up to temp. A very simple set up can be made by plumbing your bypass as the main flue with 6" flue pipe. Install a pair of T's and use that as a send/return loop for the additional mass runs. A single damper between the T's will give you good control over the flow, and you can have partial bypass, and so on.

I have found that a system that makes far too cool exhaust when cold can be a very well balanced system once the mass is warm for the season. So, rather than permanent solutions for one state or the other, I believe that a variable flue path via mass/bypass will give you great control. A simple cooking thermometer stuck in the flue above the junctions will show you what's going on there and should be very simple for anyone to run, once they learn the proper temps for smooth operation.

[patamos]

Thanks Mathew.


I hear you on keeping things simple. So your are suggesting sending all gasses from the heat riser into 6" flue to start with, then T the longer secondary flue off of (and back onto) that. My only thought here would be to cast it all around a sacrificial pipe and use a sliding refractory damper for longevity's sake.

So with the thermometer feedback from higher up the flue stack, the operators can tweak the gas paths as necessary. With the longflywheel sub floor heating system also in the building i don't think things will get too cool. But there will be some seasonal and situational fluctuations. Good to know this is an option in keeping it dialed.

I might look for robax and the like in an old woodstove door...

btw, had fun viewing a bunch of your you tube posts last night. The chickens sure know where it is at...

[matthewwalker]

Jan 7, 2014 16:52:01 GMT -8 patamos said:

So your are suggesting sending all gasses from the heat riser into 6" flue to start with, then T the longer secondary flue off of (and back onto) that.

Sort of. I would start the bypass run down low, after the downdraft section can shed some heat. Right off the riser is always going to be way too hot for comfort, in my opinion. Too hot in the chimney, too hot for longevity of moving parts, dangerous if forgotten for a few minutes, etc. I know you are planning something other than the barrel, but, to use a normal system as an example: I would pull the "bypass" exhaust from the bottom of the barrel, where the normal start of the mass run would be. Then you would have a "mass-less" rocket stove for quick radiant heat and short burns to quickly warm the space, with the option of bringing the mass online to store heat when burning for longer times.

I did something sort of interesting, I'll try to describe it here. I set up a normal system, with a mass flue run starting off the bottom of the barrel as normal. For the bypass, I installed a chimney right into the barrel just a bit higher off the bottom than where the mass flue run was connected.

The mass flue run then returned to a T in the bypass chimney, and there was one damper between the T and barrel. It worked very well, I have a Testo graph of a preliminary test run using the system that shows the control over the chimney temps. It's interesting, I'll see if I can get it uploaded shortly.

Oh, yeah, the chickens....that was a fun time. Took forever to train them out of the yard! They sure loved the stove, that's for sure. I wasn't kidding about animals loving these things, my cat and dogs all pester me to light the dang stove in the morning, and the cat will lay right on the feed until things really get going.

[patamos]

Thanks Matthew,

That makes sense.  With the intended configuration of things i will have to think of a way to T the two flues from a common down draft.  ONe benefit of this idea is that the damper can be further away from the hot gasses, so i can feel more at ease about a standard in-flue unit.
The testo numbers from your experiment would be good to see.

I still have a burning question about how best to construct a durable and/but quickly heat shedding non-metal/barrel down draft chamber.  And another about whether widening the 180 turns really matters...  I think i will load these as separate threads so the time/energy in reading and responding is not so daunting.
 
Many thanks for your help on this.  I'll send pics as it takes shape.


p

[matthewwalker]

Pat, I finally got the data off of the laptop for you.

A few details on this graph. First, I was playing with the Testo and the heater, and well, everything. I didn't intend to publish this or even use if for anything. This is a "broken riser" 6" J tube going into (2) single barrels bells upright and inline. The short exhaust path is from about 1' off the bottom of the second barrel, and the longer path comes out from the same barrel but all the way at the bottom, then through about 6' of cobbed 6" flue, then another 6' of uncobbed flue, then to the stack. I was totally playing around.

Anyway, you can see there at 11:20 or so stack temps are getting high so I start bringing the lower mass run online. Temps drop, efficiency goes up. There's a hiccup in the CO there, I can't remember if I loaded more wood or just stalled it with the bypass, but it recovers pretty quickly. This graph shows the stove clearly warming up to where it's starting to consume the CO. Once there, other than the hiccup, it's pretty stable at around 60ppm CO, with 85% or so efficiency. Pretty good for a horizontal "broken riser" test. Unfortunately, I disassembled this heater the next day to do some more pressing remodeling in that space before I permanently stuck myself with a big heater in the way of an unfinished project. I hope to get back to it soon, it can go together and be burning in a few hours.

I think the main take away here for you is to know that you can monitor stack temps and fairly easily control the heater with a system like this. You can see the "steps" there as I bring the mass run online incrementally.

[patamos]

Thanks Matthew,

Ya i'm seeing the story there.
That spike in CO looks like the ones Peter VDB was getting whenever his fuel stack dropped down in the feed tube. I'm tuning way into the thread RObert recently started about J-feed trip wires. Looking to sort out the P-channel proximal to fuel feed.
Still, if CO of 1100ppm is acceptable in Europe, things are well in hand.

Today i pencilled my layout on the floor. I will drystack the footprint and set some of the ducting in its likely location tomorrow and send some pics for feedback on the 'widening flue turn' thread.

I have the ear of 'some' of the regional building inspectors in the Cowchan and am preparing to give a talk/presentation to them next month about the basic process. So this makes me think about picking up some testo gear. Can you recommend a better set of gear than others?

Stove building is thoroughly becoming my favourite aspect of Nat Building. Lots of variety mixed with head time and wild sculpting.
Very cool crowd on this forum to boot...


well being

pat

[patamos]

Hi Y'all

Looks like the rest of the story is posting at:

donkey32.proboards.com/thread/1065/non-metal-barrel-downdraft-chambers

many a question and quandry to go...

and many thinks for the interest and help

p