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First build - a few questions  RSS feed

 
Michael Graham
Posts: 4
Location: Georgia
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Greetings all. I have currently completed the construction of my MHRS at a semi permanent campsite. It has performed well during test fires and I have just mounted the barrel and am ready to install exhaust piping and seek the collective wisdom. Specifications: 6 inch metal duct - 16 inch feed tube, 20 inch burn chamber and 40 inch updraft with 2 inches of vermeculite/ fire clay/ portland cement/masonry lime in a 9 inch sonotube. In ground portion is layer of heavy foil then bottles with vermiculite/fire clay masonry mix in which duct was embedded. Outer perimeter of stove base is granite and field stone. Barrel is heavy steel 55 gallon with 1 3/4 inch gap. I will top this with a mixture of portland cement/ fire clay/ masonry lime and sand and will also use this around the barrel half way up supported by metal lath.
My primary purpose for building this was to heat the floor in my tent. Heated area is 8 feet x 8 feet. From stove outflow the run is 7 feet to a 90 degree elbow and 1 foot into tent area. My original plan is to snake 6 inch flexible duct in the floor area. I will excavate approximately 7 inches, lay down welders blanket, install pipe and cover with granite crusher run. I am concerned the flex duct will have to many sharp turns. Is it possible to, say, run two 3 inch ducts from the six and then back to six and out to chimney? Also, since I would conceivably like to use the stove portion during hot weather for cooking purposes without heating the tent and I was thinking of using a tee in lieu of the 90 degree elbow and installing two dampers as cut offs and adding another straight run to a second chimney. Would simply blocking one chimney serve this purpose? How much crusher run should be above the heated floor duct to avoid too much heat? ( sleeping on floor ) Also, I have an abundant supply of high quality kaolin nearby. Can this be used instead of fire clay. Any insight/ suggestions are warmly appreciated.
 
Glenn Herbert
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Location: Upstate NY, zone 5
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Kaolin would not have the dimensional stability on heating that fireclay has, but for your purposes it would be very strong and work fine.

I don't think your flex duct idea will work though. You are right about the bends, and flex duct has much higher friction per foot than smooth duct. Two 3" ducts would have half the cross sectional area of one 6", not to mention the greater proportional surface area and friction. In your case, I would try several 6" straight ducts in a ladder-like arrangement, with the entrance being at say the lower left leg and the exit at the top right. You will not get perfectly even flow, but if you can get access to adjust the flows after you experience the heat distribution you may be alright.

I would run the duct back near the barrel after the tent, so you can put in a bypass for just cooking use. You are probably going to be nearing the maximum equivalent length for a 6" system accounting for the elbows, so I would put the barrel as near as you safely can to the tent. A dry stone wall between barrel and tent would help, and it could be a nice backstop for chilly evenings too.
 
Glenn Herbert
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Location: Upstate NY, zone 5
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The portland/lime part of your heat riser composition will disintegrate at interior riser temperatures, so hopefully they are minor components and will be compensated by the fireclay. Pure vermiculite/perlite and fireclay would be durable in the riser, and get stronger with firing.
 
Michael Graham
Posts: 4
Location: Georgia
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Thank you kindly sir for your thoughts. A ladder configuration hadn't occurred to me; do you think just a loop with entrance on one side and outflow on the other achieve the same result? I'm curious to find out just how hot the floor will be with a couple of inches of crusher run ( i.e. melt a ground tarp ). The total length from stove out flow to chimney rise is approximately 20 feet. If I added a second chimney for warm weather use it would amount to a straight run of about 15 feet. Do you think dampers are needed or could I just cap off one chimney when not needed? I don't have the option of running the out flow back towards the barrel, plus it would add about an additional twelve feet or so but I like the thought. I have a fire pit/ cooking area about four feet from the MHRS and it is backed by a granite drystacked wall four feet tall. My plan was to continue this behind and around the stove although I may use hypertufa over metal lath for that area. I was aware of the heat deteriorating the portland. As an old mason it just did not feel right to leave it out and I am fairly confident with the ratios I used the fire clay/ vermeculite will stand on its own. I will examine after a time and evaluate. I've worked on a couple of historical log cabins and was always amazed at how well the lime and aggregate mortar of yesteryear held up in the fire boxes of the chimney but am aware the relative temperatures were much lower. I will use the kaolin as the coating for the barrel over lath and am interested to see how well it performs. I would enjoy being able to use a local resource as part of the build. So far it has been a fun project and I am keeping my fingers crossed it will all come together. I will be down there soon and will take some photos to share. Namaste.
 
Glenn Herbert
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Location: Upstate NY, zone 5
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I believe a loop would only get about half of the 8' x 8' floor heated. A four-rung ladder 6' wide would heat the whole floor pretty evenly.
 
Michael Graham
Posts: 4
Location: Georgia
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Got it. The whole mathematical deciphering leaves my head spinning but if a six inch duct is 28.27 inches and a three inch duct is 7.06 inches wouldn't the air flow be more even with a four rung ladder in three inch as opposed to six?
 
Satamax Antone
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Location: Southern alps, on the French side of the french /italian border 5000ft high Southern alpine climate.
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Another option.

 
Glenn Herbert
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Location: Upstate NY, zone 5
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A bell-style channel/chamber would give the most even heating with the least friction, but would require significant height off the ground or digging down a bunch (which may not be practical). If a foot-high platform with stone edging is practical, a half-barrel bell with four half-barrels connected would be an excellent alternative. That would easily allow the exhaust to run back near the barrel without being too long.
Having a raised platform would be good for keeping the tent floor dry in all weather as a side benefit.

The gross cross section of four 3" ducts may equal one 6", but the surface friction effectively robs a half inch or an inch from the useful diameter. Four 2" circles are much smaller than one 5" circle. You want minimum friction for easy flow, so 6" ducts as the ladder rungs would allow the air to flow easily.
 
Michael Graham
Posts: 4
Location: Georgia
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Glenn. Thanks so much for taking the time to share your guidance. Appreciate it.
 
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