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Post by philippeelskens on Oct 4, 2019 2:51:11 GMT -8
I'm designing my first RMH. We live in Lisbon, Portugal, in a large 10m diameter tipi. Most of the year, ambient temperature is pleasant, but in winter, it drops just below freezing at times, so a heater is needed. The RMH will be in the center of the tipi, with a large cob bench. We have a separate bedroom in the tipi with plastered strawbale walls. Can we position the RMH against the strawbale wall, or is this too risky? A year ago or so I looked into RMH designs, but I've had so much work that I couldn't look into them any further. The following is based on what I remember: -I'd use the J-tube. I remember reading that this was a relatively simple, efficient model for heating. Correct? -I remember 6", 8", 10" diameters and that some said 6" was too small to be efficient. Should I go with 8"? Does this depend on our climate/our heating needs? -I have a 55gallon barrel. At what height should I cut it off? -What's the minimal/ideal length of pipe? -Do I need fire bricks or can I sculpt the burning chamber out of cob?
Any tips or references are more than welcome, of course! Many thanks!!
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Post by Orange on Oct 5, 2019 0:00:09 GMT -8
J is easier to make but it has way less power. I wouldn't build a J in less than 20-25cm system size. And of course everything depends on climate and volume of the space, you have calculator at batchrocket site. Firebox gets up to 500C on the outside so you can use thin firebricks and insulate with air-concrete. Temperature into chimney has to be around 75C so base your pipe length on that. You can also make bell instead of pipes.
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Post by gadget on Oct 6, 2019 11:08:56 GMT -8
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Post by philippeelskens on Oct 28, 2019 8:48:51 GMT -8
J is easier to make but it has way less power. I wouldn't build a J in less than 20-25cm system size. And of course everything depends on climate and volume of the space, you have calculator at batchrocket site. Firebox gets up to 500C on the outside so you can use thin firebricks and insulate with air-concrete. Temperature into chimney has to be around 75C so base your pipe length on that. You can also make bell instead of pipes. I've decided to go with a Batchbox with a bell into a cob bench into chimney. Couple of questions (mainly about sizing): -How big can I build the cob bench for a specific batchbox size? The calculator on Peterberg's website includes a number for how well insulated the building is. For our tipi, there is basically no insulation. Therefore, rather than going for immediate radiant heat, I'm going for storing heat in the bench. I understand that there has to be a 75 degrees temperature exiting the bench in order to have a proper chimney effect, but how do I calculate how big I can build the bench before this exit temperature is too low? -Related to the previous question: On Peterberg's site, there is a calculator for bell size. This is just the bell, without the inner surface area of the cob bench, correct? -What is the minimal/ideal length of the chimney? -The brick size that I've found here seems to be slightly different from what Peterberg uses. I could easily stack bricks so that the inner diameter of the riser is 190. I guess this gives me 2 options: take 10mm off and work with a 180 size; or keep the 190 and use a chimney of 200?
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Post by Orange on Oct 28, 2019 11:28:48 GMT -8
-make a sketchup of your system, ISA includes every heat absorbing surface except floor and firebox so bell and bench together make ISA: "The term we use for the total area available for heat absorption within the bell is ISA, short for Internal Surface Area. As noted, this does not include the floor area as that floor does not (directly) absorb heat." -chimney: round, insulated, above the highest point of the building -if it's easier, go with the bigger system
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Post by peterberg on Oct 28, 2019 12:45:05 GMT -8
-How big can I build the cob bench for a specific batchbox size? The calculator on Peterberg's website includes a number for how well insulated the building is. For our tipi, there is basically no insulation. Therefore, rather than going for immediate radiant heat, I'm going for storing heat in the bench. I understand that there has to be a 75 degrees temperature exiting the bench in order to have a proper chimney effect, but how do I calculate how big I can build the bench before this exit temperature is too low? The numbers on the batchrocket site are for bells and bell benches, as opposed to piped benches. I've built only one heater with a straight piped bench and that worked out quite well. But just one is definitely not enough to speak of reliable numbers. In case you want to build a bench out of adobe bricks or cooked bricks for that matter, just view the bell and bench as one bell system, the opening between the two parts as wide as possible. And I really mean as possible, the whole of the inside of the bench should be connected to the main bell. This way, the bell/bench combination works as if it was a single bell so the simple calculation of Benen Huntley applies. -Related to the previous question: On Peterberg's site, there is a calculator for bell size. This is just the bell, without the inner surface area of the cob bench, correct? See the related answer above. -What is the minimal/ideal length of the chimney? As a rule of thumb: 5 meters from where the exhaust of the heater is entering the chimney. Perhaps it could be shorter, provided there isn't any tree, hill or building near by. -The brick size that I've found here seems to be slightly different from what Peterberg uses. I could easily stack bricks so that the inner diameter of the riser is 190. I guess this gives me 2 options: take 10mm off and work with a 180 size; or keep the 190 and use a chimney of 200? You could use the 180 mm chimney and 190 mm system: it's csa is 89% of the size required for a 190 mm system. Within reasonable tolerances I'd say, provided there aren't any retrictions in the gas path and a straight insulated chimney. Give lots of room, at least 150% of system csa at every 90º change of gas direction and 200% of csa for every 180º change. These are minimum values, wider is better. And oh yes, the bell around the riser need to be at least (again!) five times wider than system csa. For example: in my own redbell it's 23 times.
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Post by philippeelskens on Oct 31, 2019 9:23:10 GMT -8
Many thanks for the replies!! Just a couple more questions: -How much ceramic blanket should I wrap around the riser? Is 4cm overkill compared to 1, 2 or 3 cm thickness? Also, how do I attach this stuff so it doesn't just fall down? -Not sure if I understand these answers (see below): The chimney has to be 5 vertical meters after it exits the cob bench, correct? I'll first have 2 horizontal meters to get out of the tipi. So it's not so that I can have 2 horizontal + 3 vertical meters? I need 2 + 5? -So you're saying that for the 90 degree turn, I should have connector pieces that are 150-200% the normal chimney diameter?? Also, why would I use a 180 diameter for a 190 system instead of a 200? Does it not make sense to not have the chimney as a limiting factor. -Lastly, when you say the bell needs to be 5 times wider than CSA: in my case that would be the cross-section of the barrel (0.2564 m2), correct? CSA is 0.028652 m2, so that's 8.94 times. -What is the minimal/ideal length of the chimney? As a rule of thumb: 5 meters from where the exhaust of the heater is entering the chimney. Perhaps it could be shorter, provided there isn't any tree, hill or building near by. -The brick size that I've found here seems to be slightly different from what Peterberg uses. I could easily stack bricks so that the inner diameter of the riser is 190. I guess this gives me 2 options: take 10mm off and work with a 180 size; or keep the 190 and use a chimney of 200? You could use the 180 mm chimney and 190 mm system: it's csa is 89% of the size required for a 190 mm system. Within reasonable tolerances I'd say, provided there aren't any retrictions in the gas path and a straight insulated chimney. Give lots of room, at least 150% of system csa at every 90º change of gas direction and 200% of csa for every 180º change. These are minimum values, wider is better. And oh yes, the bell around the riser need to be at least (again!) five times wider than system csa. For example: in my own redbell it's 23 times.
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Post by martyn on Nov 1, 2019 0:16:29 GMT -8
Ceramic blanket is best formed inside the riser, lots of folk use 25mm personally I have found 50mm to work well and hold its shape inside the metal tube.
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Post by peterberg on Nov 1, 2019 5:13:58 GMT -8
-How much ceramic blanket should I wrap around the riser? Is 4cm overkill compared to 1, 2 or 3 cm thickness? Also, how do I attach this stuff so it doesn't just fall down? Anything above 5 cm I'd consider as overkill. It can be held together with chickenwire. -Not sure if I understand these answers (see below): The chimney has to be 5 vertical meters after it exits the cob bench, correct? I'll first have 2 horizontal meters to get out of the tipi. So it's not so that I can have 2 horizontal + 3 vertical meters? I need 2 + 5? You need 5 m vertical in my opinion. You stand a good chance to get away with 4 m provided the rest of the system has very little friction in the gas path. -So you're saying that for the 90 degree turn, I should have connector pieces that are 150-200% the normal chimney diameter?? Also, why would I use a 180 diameter for a 190 system instead of a 200? Does it not make sense to not have the chimney as a limiting factor. That 2 meter of pipe to get out of the tipi need to be wider than system size in my opinion. And it need to be packed around with insulation material, perlite of vermiculite otherwise the surrounding ground would act as a giant heat sink. Using a 180 mm chimney for a 190 mm system is about cost: the 200 mm is more expensive. But you need to avoid friction altogether in order to make this possible. That's what I was talking about. -Lastly, when you say the bell needs to be 5 times wider than CSA: in my case that would be the cross-section of the barrel (0.2564 m2), correct? CSA is 0.028652 m2, so that's 8.94 times. Your understanding of this isn't correct, sorry. In order to act as a bell the free space inside, in effect around the riser, should be 5 times system size. When it's smaller it's just a downdraft channel employing no stratifying at all. I said it would be best to create lots of room. Building a large system with a bench that isn't overly long and a capable chimney could compensate for friction spots. If that's what you want, forget what I've said and go ahead.
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Post by philippeelskens on Nov 2, 2019 11:54:39 GMT -8
Thanks for the answers! I feel I'm starting to understand this stuff relatively well :-) Ceramic blanket is best formed inside the riser, lots of folk use 25mm personally I have found 50mm to work well and hold its shape inside the metal tube. Which metal tube? I'm stacking fire bricks. So if I put the ceramic blanket inside the riser, held together by chicken wire (as Peterberg suggests), the chicken wire will just melt, no?
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Post by martyn on Nov 2, 2019 13:23:08 GMT -8
Well there are numerous ways to build a heat riser
You can use fire brick or better still insulated high temperatures bricks if you like and there may be some benefit from a high mass riser in certain circumstances, however ...a simple metal tube lined with ceramic blanket is much easier to build and in most circumstances much more effective .
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Post by drooster on Nov 3, 2019 9:45:59 GMT -8
Thanks for the answers! I feel I'm starting to understand this stuff relatively well :-) Ceramic blanket is best formed inside the riser, lots of folk use 25mm personally I have found 50mm to work well and hold its shape inside the metal tube. Which metal tube? I'm stacking fire bricks. So if I put the ceramic blanket inside the riser, held together by chicken wire (as Peterberg suggests), the chicken wire will just melt, no? The chicken-wire was suggested to hold ceramic blanket wrapped around an efficient riser, just as insulation. The outside of the ceramic blanket does not get hot enough to melt chicken-wire. Inside : different story. I hope this helps clear things up.
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