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rocket stove and butt warmer

tel jetson
steward

Joined: May 17, 2007
Posts: 3088
Location: woodland, washington
    
  53
ronie wrote:
Hey, Tel, I hope you post some pics of your stove.

shouldn't be a problem.  it won't be anything revolutionary, but I'll take some photos.  might get started this weekend, but I don't have all the materials just yet.


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tel jetson
steward

Joined: May 17, 2007
Posts: 3088
Location: woodland, washington
    
  53
the only thing that's still sort of intimidating to me about this project is cutting a hole in the wall for the exhaust.  can anybody speak to how that has worked out?  the wall in question is stick-framed with tongue and groove cedar on the inside and wooden siding on the outside.
Max Kennedy


Joined: Feb 16, 2010
Posts: 460
Location: Kirkland Lake, Ontario, Canada
Our new home has an engineered wood floor.  Any suggestions for putting an RMH on this floor?  An elevated base but made of what perhaps?


It can be done!
Len Ovens


Joined: Aug 26, 2010
Posts: 1278
Location: Vancouver Island
    
  15
mekennedy1313 wrote:
Our new home has an engineered wood floor.  Any suggestions for putting an RMH on this floor?  An elevated base but made of what perhaps?


Are you trying to save the floor? Not sure if that is possible... you could try felt covered with one or two layers of patio slabs which come as big as 36x36in. What is underneath? How much weight will that hold? Is new "new" or "new to us"?

If you have a crawl space, you may wish to put posts under that area. If you have a basement under, the posts will need to be taller. If you have a concrete pad, that is best... it is probably thick enough to support 5 tons of truck.

Your safest thing is to figure out the weight and area of your RMH and pay an engineer to tell you if you can put this real heavy furniture on your floor (yes people sit on it).
ronie dee


Joined: Mar 04, 2009
Posts: 586
Location: Cosby MO
    
    2
You could skip the mass and just make it a wood cook/heating stove. It wouldn't hold the heat like the mass heaters, but would still put off a lot of heat with just a few sticks.


Sometimes the answer is not to cross an old bridge, nor to burn it, but to build a better bridge.
Max Kennedy


Joined: Feb 16, 2010
Posts: 460
Location: Kirkland Lake, Ontario, Canada
Len wrote:
Are you trying to save the floor? Not sure if that is possible... you could try felt covered with one or two layers of patio slabs which come as big as 36x36in. What is underneath? How much weight will that hold? Is new "new" or "new to us"?

If you have a crawl space, you may wish to put posts under that area. If you have a basement under, the posts will need to be taller. If you have a concrete pad, that is best... it is probably thick enough to support 5 tons of truck.

Your safest thing is to figure out the weight and area of your RMH and pay an engineer to tell you if you can put this real heavy furniture on your floor (yes people sit on it).


It's new to us but it's not the weight that's a problem as it's slab on grade but the "combustible material".  Would stove board take the distributed weight?
Max Kennedy


Joined: Feb 16, 2010
Posts: 460
Location: Kirkland Lake, Ontario, Canada
ronie wrote:
You could skip the mass and just make it a wood cook/heating stove. It wouldn't hold the heat like the mass heaters, but would still put off a lot of heat with just a few sticks.


This is an option I'm also exploring for this winter as the snow is upon us but would like something better long term.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
we just built one in California, Pictures will follow as Erica gets time.


Need more info?
Ernie and Erica
Wood burning stoves, Rocket Mass Heaters, DIY,
Stove plans, Boat plans, General permiculture information, Arts and crafts, Fire science, Find it at www.ernieanderica.info


                          


Joined: Nov 20, 2010
Posts: 140
I read the first 4 pages of this thread, until my eyes started rolling back in my head.  I thought I would jump ahead and ask this question (which actually occurred to me while reading the thread).  I was at first thinking of doing a RMH in the living space of my home.  Several problems and concerns exist, namely the floor plan doesn't lend well,  nearby combustable building material, and weight. 

Then this occurred to me.  I have a large (mostly unused) walk out basement.  I also have a forced air central heating system.  It would be a much simpler task for me to build in the basement, and instead of making the system fit the area I could design the optimal system (plenty of room down there). 

If I incorporated ductwork into the mass and tied it into my forced air system I could heat the entire house evenly.

Go ahead and tell me the holes in this plan.
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
You have little or no experience building or running a RMH. As such your first plan as as grandiose as it can possibly be. I cant tell you of all the holes in it because you dont have enough background with RMH's for me to do so with out writing another book. 

Sorry you asked for it the above is the largest of holes.

build your mockup system outside and see how much heat you can get to an exchanger.
please do your first stove in your house till you have used and built a total mockup of your system a couple of times.
                          


Joined: Nov 20, 2010
Posts: 140
Ernie, to start with how would you handle the exhaust?  If it went straight out of the house it would be at ground level.  Would I be better off to go out the house and put an elbow and riser on it, or put the elbow and riser on the inside wall?  If I do it on the inside should I include this as part of the thermal mass, or just use a bare riser pipe?
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
that depends on the amount of mass and the temp as it leaves the house. in order to get the exhaust to rise you need at least 90 degrees F (this covers the widest range of outside temps). if you scrub to much heat from the exhaust it will fall so you might want to look at putting in a dry well to take the exhaust.

As an Edit to the last message: it was dont build your mock up in your house. ( sorry to be unclear with a typo.)
                          


Joined: Nov 20, 2010
Posts: 140
No problem Ernie, I knew what you meant.

What is a dry well?
                          


Joined: Nov 20, 2010
Posts: 140
I'm curious about the barrels used for the combustion chambers.  Is burn out a problem?  If anyone here has been regularly using one of these systems for a few years, how is the barrel holding up?
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
my definition is a hole in the ground filled with rocks. usually i put a bucket upside down in it with the duct running into a hole in the bottom. then punch a few holes in the side and cover with round rock. makes things work a little better to have open space and if the well does happen to fill with some rain water the stove still works. unless it fills all the way up to the duct.

we know a system thats been used for 15 years and the barrel is fine. I have yet to burn out a barrel. I try to do so deliberately so i can have a definitive answer to this question.
                          


Joined: Nov 20, 2010
Posts: 140
So would a "T" at the exhaust be a good idea? One going down to the dry well, and the other end to a riser?
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
wouldn't hurt.
                          


Joined: Nov 20, 2010
Posts: 140
Ernie, I have a question about your statement about 90 degrees being the range of temperature.  Are you talking about the temprature difference between indoor and outdoor temps?  Bear in mind here, we often see 30 below here and occasionally 40 below.

Next question, I have all these concrete silo staves.  They are about 12" x 24".  Could I make the horizontal flue out of these or are they too wide?  The flue would be about 12"x 12" inside dimensions.  Of course I could move them in so they would be say, 12" x 8".  any input?
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
Tinknal wrote:
Ernie, I have a question about your statement about 90 degrees being the range of temperature.  Are you talking about the temprature difference between indoor and outdoor temps?  Bear in mind here, we often see 30 below here and occasionally 40 below.

Next question, I have all these concrete silo staves.  They are about 12" x 24".  Could I make the horizontal flue out of these or are they too wide?  The flue would be about 12"x 12" inside dimensions.  Of course I could move them in so they would be say, 12" x 8".  any input?


That 90 degree figure was given to us by a masonry heater advocate.  To be honest, I'n not even sure whether it's 90 Fahrenheit or 90 Centigrade.  Of course the difference between outside temperature and inside (the heat riser, and the chimney) matters more than the absolute temperature of the chimney. 
Our heater smokes back like a demon if we try to light it in summer (unless we pre-heat it and actively manage the burn the whole time), but generally we don't want to heat our house in summer because we're using the thermal mass to keep ourselves cool. 

The biggest problems lighting a system come when the system is cooler than the outside air, and that is not likely to happen in an inhabited house during a 40-below winter.  If you're coming back from vacation, light your first fire at night or in the early morning, and you should still be warmer than the outside air at that point.

But the masons need a rule of thumb for what temperature you want the chimney in order to build the system, because outdoor temperatures change everywhere.  Apparently, below about 90 degrees water is very likely to condense in the chimney, making the exhaust get denser fast.  Exhaust denser than air will not rise in a chimney without some serious force pushing it.
With RMH's, we do sometimes let the exhaust get cooler than that.  We very often have foggy exhaust on the longer systems, which is why sometimes a horizontal exit is useful.  Also it pays to slope your last run or cleanout to allow for some drainage of that condensation.

About your concrete silo thingies:
You can build rocket stove 'intestines' with masonry, but the results may vary.  Remember you want the cross-section of the exhaust to be about the same as that of your burn tunnel and heat riser (re-reading the book carefully is a great idea before any new build). 
With a 12x12 flue gap you'd be looking at 144 square inches, or roughly a 14" diameter pipe.  Even a 12" diameter systems burns HOT, much hotter than an 8" system, so you may need better furnace bricks and more length and height in the combustion unit to take the heat.  12" x 8" or 96 "" would be more comparable to a  11" diameter pipe.  Still huge, as RMHs go.

Masonry (concrete, bricks, etc) generally offers more drag than round ducting, both from its square corners and its rougher surface.  So you may need to experiment with how large a system, or how short a system, you have to build in order to compensate for these differences.  Mock it up outside first, as always.

Might also be possible to line the ducts somehow and make them smoother and smaller, while still using the concrete forms for strength and uniformity.


Play with nature, make nifty stuff:
www.ErnieAndErica.info
                          


Joined: Nov 20, 2010
Posts: 140
Ernie, going with the existing floor plan, I'm dealing with about 30 feet total, with one 90 degree turn.  I'm guessing , what, 4 0r 5 feet for the burn tunnel and riser?  I'm not sure how you measure the system, overall, or the exhaust part.  The exhaust will be at the corner of the house so I could go straight out or add another 90.  Should I take prevailing winds into account?  If I go straight out it would be towards the west, into the prevailing winter wind, or I could put the 90 in and exhaust to the south, which is rare to get a winter wind.
                          


Joined: Nov 20, 2010
Posts: 140
I should add that I could go straight up through the roof for the exhaust.
Max Kennedy


Joined: Feb 16, 2010
Posts: 460
Location: Kirkland Lake, Ontario, Canada
Ernie Wisner wrote:
we just built one in California, Pictures will follow as Erica gets time.


Was that in reply to my inquiry about building on a wood floor?

Max
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
yes Max. We just built one in California a couple weeks ago.
                          


Joined: Nov 20, 2010
Posts: 140
Please forgive the machine gun nature of my questions, I'm asking them as they occur to me.  I notice that some of the risers are insulated and that some of them are just steel tubing.  Is there a "best way" here or do you use different set ups for different applications?
Max Kennedy


Joined: Feb 16, 2010
Posts: 460
Location: Kirkland Lake, Ontario, Canada
Ernie Wisner wrote:
yes Max. We just built one in California a couple weeks ago.


OK, eagerly awaiting the photo's and how to protect the floor.  May have access to a fair amount of clay bricks soon and am hoping I can use the info to build.  Have an outbuilding I can experiment in.  Currently -12C, a major snow storm forecast for later this week so clay for cob not available right now and I REALLY don't want to pay electric heating costs all winter.

Max
Ernie Wisner
volunteer

Joined: Oct 16, 2009
Posts: 788
Location: Tonasket washington
    
  23
Tin all of the heat risers are insulated. on any working stove i have seen. if the heat riser is not insulated the temps equalize (at times) and you get a space full of smoke. the system depends on a temperature difference.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
Tinknal wrote:
Please forgive the machine gun nature of my questions, I'm asking them as they occur to me.  I notice that some of the risers are insulated and that some of them are just steel tubing.  Is there a "best way" here or do you use different set ups for different applications?


We can get away with just a tube in a rough mock-up to demonstrate the thermosiphon effect, outside at workshops.  Just adding the barrel you already need the insulation or it won't work for more than about 20 minutes, and when you add the resistance of heat-exchange ducting, forget it.  Even for mock-ups, the uninsulated pipe has to be about twice as long as an insulated one because it loses heat faster.

We do sometimes use insulated steel pipe, 'metalbestos' insulation or multi-wall with perlite or mineraql wool, for short-term installations.  High heats can warp the inner pipe on these thinner insulated pipes, destroying them over time. 

So fireclay-and-Perlite filling between two metal liners is our favorite quickie heat riser, the ceramic can vitrify in place and outlast the steel liner if it gives up the ghost.  We've had this style last in place for several years or more, long-term testing still in progress.

Nice, plumb, brickwork (insulated with perlite-clay or sawdust-clay ceramic insulation), or a refractory insulant like kiln-brick, is probably the most durable heat riser for a permanent installation, but they are hard to build accurately in a weekend workshop.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
tel jetson wrote:
getting ready to install a rocket stove to heat a sauna in the next week or so.  there won't be a lot of thermal mass involved, so I'm expecting the exhaust to be rather warmer than a typical mass heater.  I'm wondering if I can preheat air to the feed tube using some of the heat in the exhaust.  I would run a six inch stove pipe from outside through a ten inch exhaust pipe then to the feed tube.  it's an eight inch system, so the area inside the ten inch pipe should be just right with a six inch pipe inside it.



is this a terrible idea?  just trying to salvage some of that energy that will be headed out the exhaust.


I can't speak to the draft differential, I've never done a RMH sauna.  I think the high indoor temperature might mean that your incoming air would still be a little cooler than the rest of the indoor air, but you'll find out when you build it I guess.

My concern about plumbing an intake into your exhaust for heat-transfer is 2-fold: 1) sealing it so you don't get exhaust leaks into the room, which given how steamy and odd people can feel in a sauna, CO might go un-detected until it was too late.  I'd probably get a CO detector so you can be warned if any part of the system is leaking into your space, just in case.

2) Volumes.  With a 10-in duct partially filled with a 6-in duct, you have areas of correct volume, and areas of high volume, and some weird corners that may interrupt gas flow.  RMH's work best with constant, smooth flow of the exhaust.  You've got some secondary, vertical draft on our exhaust at that point, but chilling it could create weird backflow issues in the large-volume sections or along the whole exhaust path.

I'd be inclined to just make your exhaust to the right size in the first place, it's safer and simpler.  Wrong-sized flues are a well-known problem for any combustion device.

You might could build a little box between it and the wall, with a slot for fresh air.  Put a little, lightweight damper at the bottom so if it starts drafting, it will suck the flap shut and stop the flow.  That way you get some heat-exchange potential.  Insulate the crap out of the wall, or when it gets hot enough to backdraft, it will shut off and get even hotter, possibly setting the wall on fire.

Or you could take Ronie and everybody's advice, and plumb the warm-air intake upward.  There's a sketch on p. 90 of the Rocket Mass Heaters book about one way to do pre-heated air intake, using the air to cool the wall behind your heater while moving that heat to the inside of the room elsewhere.

I've heard a lot about external air intakes backdrafting.  Mostly, when they come in high and drop down to deliver air into the combustion device, like you'd see if you were delivering air to a basement furnace or fireplace.  In these cases the intake frequently starts acting as a secondary chimney, drawing noxious gases backwards out of the device and into the room, or starting a fire in/near the air intake itself (usually lighter-weight ducting than the chimney). 

This happens in many cases because the air intake opens into or right next to the fire, but can also happen if they get warm enough to draft on their own.  They were adopted into building code before this problem was well known, so the idea has got more momentum than the associated problems as yet.

I'd be careful on this one, since the full extent of the problem may not become obvious until you get your stove, and space, up to full heat.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
tel jetson wrote:
the only thing that's still sort of intimidating to me about this project is cutting a hole in the wall for the exhaust.  can anybody speak to how that has worked out?  the wall in question is stick-framed with tongue and groove cedar on the inside and wooden siding on the outside.


Similar to cutting a hole for a window or dryer vent.  You cut through the siding and TG (or remove it in pieces if that seems easier), frame in a box in the wall itself to fit around the hole, and make sure you have enough thermal protection to keep hot pipe from setting dry wood on fire.  You can get triple-wall connections for ducting or stovepipe, or boxes designed to accept round stovepipe, to keep the pipe centered in the hole.  Or you can build your own with high-temp insulation and heat-resistant supports.  When we plumb these through cob walls, we sometimes stuff some perlite-cob around them.  We've used commercial triple-wall, or created simple flashing collars from scrap metal, for off-the-grid exhaust ports on RMHs.

I'm suggesting high-temp for your stove because  it's a big one with a short exhaust, so you may still be at stovepipe temperatures exiting the wall rather than the low temps that most RMH's achieve.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
Tinknal wrote:
Ernie, going with the existing floor plan, I'm dealing with about 30 feet total, with one 90 degree turn.  I'm guessing , what, 4 0r 5 feet for the burn tunnel and riser?  I'm not sure how you measure the system, overall, or the exhaust part.   The exhaust will be at the corner of the house so I could go straight out or add another 90.  Should I take prevailing winds into account?  If I go straight out it would be towards the west, into the prevailing winter wind, or I could put the 90 in and exhaust to the south, which is rare to get a winter wind.


You really might want to hire Ernie to help design your stove.  This level of detail is hard to answer accurately on an open forum like this one, with the info coming in bits and pieces.

With that caveat, I will say YES, take the prevailing wind into account.  Cold wind shoving itself into your stovepipe, and vacuum on the downwind sucking air out of your house, will almost certainly give draft problems.  Straight up, downwind, or sheltered-side exhaust are all better options than into the wind.
                          


Joined: Nov 20, 2010
Posts: 140
Erica, thanks for your responses and let me apologize for addressing you as Ernie!
                    


Joined: Oct 23, 2011
Posts: 0
you done yet Tel? I should be headed south next week. hope to stop in and see it. Your sis was chirpin'.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
Tinknal wrote:
Erica, thanks for your responses and let me apologize for addressing you as Ernie!


No worries - I didn't think you were! 

Our identities get confused sometimes, we log in from the same computer and answer questions addressed to each other.  Ernie has more hands-on practice, while I'm sometimes better at getting the explanation in writing.  Glad it's helpful!
                          


Joined: Nov 20, 2010
Posts: 140
Erica Wisner wrote:
No worries - I didn't think you were! 

Our identities get confused sometimes, we log in from the same computer and answer questions addressed to each other.  Ernie has more hands-on practice, while I'm sometimes better at getting the explanation in writing.  Glad it's helpful!


Maybe from now on I should just say Ernica, or maybe, "Hey, Wisner"............
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 734
Location: Okanogan Highlands, Washington
    
  86
Tinknal wrote:
Maybe from now on I should just say Ernica, or maybe, "Hey, Wisner"............


LOL
ERnie: heh, yeah ..
ERica: that'll work.
Kathleen Sanderson


Joined: Feb 28, 2009
Posts: 969
Location: Near Klamath Falls, Oregon
    
    1
LOL!  My youngest DD (the autistic one) when referring to her Uncle Mark and Aunt Sue, used to say "Unca Sue."  Ernica is better than that!

Kathleen
Clifford Reinke


Joined: Nov 26, 2010
Posts: 122
Location: Puget Sound
    
    4
Hello All,

OK, I’ve bought and read the book (and left it at my brothers house on thanksgiving..Grrr), read most of these forums, watched a ton of U-tube videos, acquired most of my supplies and tried a few mock ups.  In addition, I think I have redesigned my system about 12 times. 

My vision is an eight inch system for my small green house (12’ X 16’).  The riser is 35 inches high with an 8” interior diameter with a 13” exterior diameter (filled with a perlite/clay mix for insulation.  It will be covered by a barrel  with a 23” interior diameter.  I plan to have a 2” clearance between the top of the barrel and the riser (This will give me a 50.2 square inch opening off the riser).  The exhaust will run through a bench and use about 24’ of straight pipe, plus three 90 degree turns, and vent laterally outside to the downwind side of the green house.  The benches will be at least two feet wide to hold my soil block seed starter trays.  The benches will also be 18” high.

So here are my questions:

1. The book says the cross section area of the burn tunnel should be the smallest of the system.  But in reading this forum, I am getting the impression it should be the same as the rest of the system (50.2 square inches) say 8” wide by 6.25 high.  So I need to know if the burn tube cross section should be the same size or smaller than the rest of the system?

2.   Since this RMH is going to be in a green house, and my seedlings will be started on the bench, there will be lots of water spilled about.  Any suggestions on what use to protect the bench from water?

3. My mock ups worked fairly well until I would put the barrel on top, then the rocket action would stop.  The mock ups used old bricks with no mortar, so it was pretty leaky.  I also made the mock up using a smaller burn tunnel cross section than the riser.  Do you think this may have been the problem?

4. The cross section area of my 23” diameter barrel works out to about 415 square inches.  If I subtract the cross section area of the exterior diameter (13”) of the riser (133 square inches), I get a cross section diameter of the down draft portion of the barrel at 282 square inches.  That number is way bigger than the rest of the system (50.2 square inches).  Do I need to significantly increase the outside diameter of the riser to bring it into alignment with the rest of the system?

Sorry for all the questions, but I’m ready to build, and just have these last few questions in my way.

Thanks,

Cliff (Start a rEVOLution, grow a garden)

Cliff (Start a rEVOLution, grow a garden)
Elf Nori


Joined: Nov 16, 2010
Posts: 26
Location: Onalaska, WA
Ernie Wisner wrote:
Tin all of the heat risers are insulated. on any working stove i have seen. if the heat riser is not insulated the temps equalize (at times) and you get a space full of smoke. the system depends on a temperature difference.


Proof, proof, proof!  With pictures! http://norishouse.com/archives/date/2010/11/15


ElfN

Take a minute and make a donation to support RMH testing/certification.  Paypal your contribution to eawisner@gmail.com.

[url]http://www.norisstuff.com[/url]
[url]http://www.norishouse.com[/url]
Len Ovens


Joined: Aug 26, 2010
Posts: 1278
Location: Vancouver Island
    
  15
careinke wrote:
1. The book says the cross section area of the burn tunnel should be the smallest of the system.  But in reading this forum, I am getting the impression it should be the same as the rest of the system (50.2 square inches) say 8” wide by 6.25 high.  So I need to know if the burn tube cross section should be the same size or smaller than the rest of the system?

The inside of the riser should be no larger than the burn tunnel.... I would suggest the riser should be the smallest diameter in the whole system.... This doesn't really make sense as the gases are hottest there and so take up the most room, but that is what works.

2.   Since this RMH is going to be in a green house, and my seedlings will be started on the bench, there will be lots of water spilled about.  Any suggestions on what use to protect the bench from water?

Patio slabs, they come 18x18 to 36x36 (at least here) two layers would cover the cracks between them and they add mass right where you need it to keep your beds from over heating and to extend the time between burns.

3. My mock ups worked fairly well until I would put the barrel on top, then the rocket action would stop.  The mock ups used old bricks with no mortar, so it was pretty leaky.  I also made the mock up using a smaller burn tunnel cross section than the riser.  Do you think this may have been the problem?

Best if burn tunnel and riser are the same CSA

4. The cross section area of my 23” diameter barrel works out to about 415 square inches.  If I subtract the cross section area of the exterior diameter (13”) of the riser (133 square inches), I get a cross section diameter of the down draft portion of the barrel at 282 square inches.  That number is way bigger than the rest of the system (50.2 square inches).  Do I need to significantly increase the outside diameter of the riser to bring it into alignment with the rest of the system?

Try fixing 3 above first... I have seen systems with a big space between riser and barrel that still seemed to work.
                          


Joined: Nov 20, 2010
Posts: 140
I've seen concrete pipe used in sewer installations.  It comes in a variety of dimensions and I think I could find one the right dimensions for the heat riser.  Any opinions of this?
 
 
subject: rocket stove and butt warmer
 
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