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Post by getandistr on Jan 14, 2014 12:04:14 GMT -8
I am currently planning a build for this spring/summer of a TMH. After the polar vortex we are done with waking up every 2 hours to add wood to the wood burning stove and still wake up in the morning with frozen pipes. I have read the book and done some research but I figured asking the questions directly would make things simpler.
My plan is to get an 8” Dragon Burner core to save a lot of the guess work on getting it all setup just right. Also the wife has demanded that this look nice so using a metal barrel is out of the question since from what I read covering it with cob would lower some of the efficiency (please correct me if I am wrong on this). So my idea is to build it out of firebrick instead as my understanding is that firebrick absorbs heat fairly quickly so it should still keep enough of a difference in temperature to keep things running smoothly. Would this work?
My next question is to the location of the “barrel”. I’ve been seeing them placed off center of the heat riser and I read that you want to keep the temp lower on one side and higher on the other but I’m not sure why or what this actually does.
With the spacing between the “barrel” and the heat riser according to the book it should be 2-3” for an 8” system but more would be fine. What is the advantage/disadvantage of adding the extra headspace? Also my understanding is that you want the CSA of the heat riser to be the same as the CSA of the inside wall of the “barrel” to the outer wall of the heat riser (insulation) is this correct?
I am planning on building this system in my living room and I want to retain as much of the heat as possible, would a perlite/clay slip mix along the exterior wall and the floor the full length of the bench help with keeping the heat inside my house?
As for the bench itself, the book mentions using rock or some other material that can quickly absorb the heat from the exhaust tunnel then using cob the rest of the way but I have also read that you should use uniform material throughout the bench for best results. Which is it?
I am also planning on running the bench approximately 35-40’ but I have room to run it longer if I should. I have heard however that a 90° bend if the same as running the system 5 extra feet. Is this true and if so would a wide turn improve this instead of a sharper bend? Also, could I run it longer than that, besides the fact that it really won’t be providing much heat anymore is there any other negative?
Finally (for now) I have read that you need a vertical exhaust at the end of the system. If I run long enough of an exhaust tunnel that the temp is very low would I be able to just have a vent that goes out through my house similar to a clothes dryer or do I NEED the vertical exhaust? If I do need it can I use the chimney I currently have for my wood burning stove?
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Post by Dan (Upstate NY, USA) on Jan 18, 2014 6:42:39 GMT -8
Extra headspace makes a bigger "bell", the top is less hot but the more inner surface area your bell has the more heat it will get rid of before it goes into the bench, which could be a good or bad thing depending on what you are shooting for. Use your existing vertical exhaust and you'll do fine. This should take you to picture of my build: My build...I use a 8" system that goes into 30' of 8" horizontal pipe going to 27' of 6" vertical triple wall exterior pipe... PS tell your wife to suck it up, mine hated the barrel until she realized how warm the house was for less than 500$ of wood a year. But then again my wife is a saint...
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Post by satamax on Jan 18, 2014 9:08:43 GMT -8
Getandistr Well, keep at least some heat to power the chimney. Exhausting on the sde of the house is bad, as, if the wind changes, the stove smokes back. Instead of insulating your wall inside, i would lay the cob against it, with the pipes, and insulate on the outside = more mass Instead of a barrel, there's plenty of other options, only your mind can set the limit. For example, i got this lately donkey32.proboards.com/thread/1017/pure-geniusBut you can use prety much anything, ideas which i have published. Home heating fuel tank, tractor fuel tank, old cast iron boiler, an old stove (if big enough) or an old range. Welding cast iron range tops to make a rectangular box. Old stainless steel fridges. A barrel, or rectangular metal box, covered with brick lattice. And plenty of other ideas. Even some mad ones.
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Jan 19, 2014 15:30:12 GMT -8
Wolf, I will sure like to see the finished product!
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Post by Dan (Upstate NY, USA) on Jan 20, 2014 4:05:47 GMT -8
Ha! That is the finished product so far, actually I got to get off my ass and make it look pretty, I have been using it that way for two winters now, I should be starting the finish soon. I just got the ceiling painted in that room! I'll post some pics as I work it...
There are 84 pictures there, you have to click on "more" to see them all...
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Post by Dan (Upstate NY, USA) on Jan 20, 2014 4:11:53 GMT -8
I wouldn't put the cob right against the wall unless the wall is concrete or stone, if you have a wood wall leave a 2" gap and let that hot air move around. RMHs are all great for saving money and work but don't put fire safety at risk...
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Post by getandistr on Jan 20, 2014 6:06:27 GMT -8
Thanks for the info wolf. I mainly want heat over time so I will probably keep the bell low. How are you able to use an 8" system with a 6" exhaust? I thought the entire system needed to maintain a uniform size? I am actually looking into using 24" x 24" clay flue liners as the bell so it looks good but can still transfer the heat fairly quickly (hopefully). Also, here is what I am currently planning. It is obviously not to scale but should at least give a good idea.
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hpmer
Full Member
Posts: 240
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Post by hpmer on Jan 20, 2014 8:41:10 GMT -8
You'll want enough cleanouts to service the system. That last section along the wall before going outside could be a challenge to work with.
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Cramer
Junior Member
Posts: 129
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Post by Cramer on Jan 20, 2014 16:13:21 GMT -8
Ah, that is probably what I missed! I'll have another look! Quite Nice Saw the ones I missed, very nice!
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Post by Dan (Upstate NY, USA) on Jan 20, 2014 18:32:38 GMT -8
PV=nRT or the ideal gas law... I know it works because I am sitting next to it right now, its 18F outside and I burned 3 five gallon buckets of wood today, my living room is 91F at the ceiling, 80F at chest height, the two farthest bedrooms of the house are currently 70F each. When I get up in the morning the living room will be 75F and the two farthest bedrooms will be 65-62F each.
I tried out the math and it works well, p=pressure v=volume n=number of moles R is the ideal gas law constant and T in the temperature in Kelvin.
When you take out the P (the pressure changes are mild from the feed to the exhaust) and the N and R you get (real rough figures) V=T. Since I am going from in the burn chamber 1200F [922.0389K] (average) to 135F [330.372K] exhaust average I can get away with going from a 8" feed (50" squared) to and 6" exhaust (28"). The drag on a 6" feed is greater than and 8" feed so between the two the volume they can pass is halved. But my temperature is more the halved in Kelvin.
Now if my exhaust temperatures were 440F then this would be ~500K and my temperature would not have cooled enough to half my volume. With a exhaust temp of 440F my system would not work...
In reality my 30' of 8" pipe is too big and starts to act like a bell, slowing down the gases even more... and extracting even more heat to the concrete bench...
Not that I did the math to figure this out, I built the stupid thing in ignorance, and hooked it up to my 6" exhaust and it worked fine... later I roughed out the math and kinda proved the math worked...
Don't get me wrong if that last 27' of 6" pipe was horizontal and not vertical the pressure change would not be great enough to create a draft to get that relatively cool air (135F) out the chimney.
So pressure changes do drive this, once at the relative vacuum created by my 55 gallon drum cooling the 1200F gases and again by the vacuum created at the base of my vertical chimney. BUT in terms of volume all this flow can be passed due to the cooling of the gases and the corresponding shrink of volume as the exhaust loses heat...
It took me a while to figure this out... I am not rocket scientist and I am sure I got some things wrong but I think the general idea works...
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Post by Rinchen on Jan 23, 2014 4:58:48 GMT -8
I can confirm this, having had a 8" J-tube running on a 6" chimney. Peter has even done measurements on it and it ran well. Now I am running an 8" batchbox on the same bench and chimney and it also still runs well. My chimney temperatures get to about 212F(100C) currently (the weather doesn't demand a full batch yet) Last season (which was cold for our standards, two weeks below freezing with an occasional 14F(-10C)) the chimney would get to a maximum of 250F(120C)
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Post by getandistr on Jan 27, 2014 12:36:43 GMT -8
I still have a few more questions but the information so far has been great.
From what I read it is recommended to keep an air space between the mass and any walls or floor. For the walls this is simply done by just not building right up to the walls. But how do I achieve this for the floor?
Also I am still a little unclear on the clean outs. I know I need one right at the start of the horizontal run but where else should I put them and what's the best way to make them?
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Post by Dan (Upstate NY, USA) on Jan 28, 2014 13:13:55 GMT -8
This is my personal opinion, I like cleanouts avery 4 feet. My reach is 2' and that way I can clean them out even if the electricity is out by hand instead of using the shopvac.
I made the air space under my heat bench by laying down bricks and putting some used concrete board that I got off of craigslist on top of them. I used concrete put I think you could use cob on top of the concrete board too.
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Post by getandistr on Jan 28, 2014 16:19:31 GMT -8
Would regular brick work for that since the temps won't be too high? Also any input on spacing the bricks?
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Post by mannytheseacow on Jan 29, 2014 6:08:17 GMT -8
Extra headspace makes a bigger "bell", the top is less hot but the more inner surface area your bell has the more heat it will get rid of before it goes into the bench, which could be a good or bad thing depending on what you are shooting for. Use your existing vertical exhaust and you'll do fine. This should take you to picture of my build: My build...I use a 8" system that goes into 30' of 8" horizontal pipe going to 27' of 6" vertical triple wall exterior pipe... PS tell your wife to suck it up, mine hated the barrel until she realized how warm the house was for less than 500$ of wood a year. But then again my wife is a saint... Off topic, Wolf: I really like that ash catch from the lower level. Good thinking!
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