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Post by patamos on Aug 17, 2016 8:05:29 GMT -8
Well, i'm guessing this has been asked before… but i could spend ages trying to find the thread. So: A friend wants to know if i can build a little Rocket heater in their 160sq.ft tiny home. I'd place it up on a slab of metal on rubber bushings to deal with vibration and offer air circulation underneath. Also have a hot plate above the heat riser. Probably rectangular shape bell made of brick shiners. Maybe insulated firebrick on the back wall face... But i wonder whether to go with a 4" batch or 4 or 5" J-feed. I wonder how well does a 4" batch box fare with half loads? All thoughts welcome
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Post by peterberg on Aug 17, 2016 10:26:04 GMT -8
Last year at the Innovators gathering at Paul Wheaton's I built two tiny house heaters. One 4" Dragon Heater's J-tube in a 55 gal open head barrel with ordinairy red bricks dry stacked as shiners around the perimeter inside. Exhaust by means of a stove pipe through the lid, reaching down as a plunger tube to 6" above the bottom of the barrel.
The top of the barrel got hot enough to cook on. The bricks were extracting so much heat that I was forced to make a vertical slit in the stove pipe just under the lid in order to maintain a high enough temperature in the stove pipe. We tested the thing outside, so I don't really know how long it could store its heat. I understand the thing is now inside the red cabin, a tiny house at the premises. Last winter it has been used to warm the place.
The second one was a 4" batch box rocket made entirely out of vermiculite board inside a much larger barrel. This size is known as a salvage or overpack drum, always open head type. If I remember correctly, it was a 96 gal item. Those are not easy to come by and also quite expensive. We used red bricks on flat all around the perimeter so there happened to be quite some weight in there. Stove pipe configuration the same way through the lid, ending about 6" from the bottom again.
Both heaters were ran without bricks initially, both burned without any hesitation and got quite warm all around, less at the bottom. The exhaust pipe was in both instances touching the steel side of the drum, the bricks were stacked against the pipe left and right. The bricks extracted more heat than the bare steel drums, much to my surprise. That was the point that I made the slit in the pipe to provide a small permanent bypass.
The batchrocket core is probably the simplest to mount in a drum, although the door would make it complicated. When I wanted to build one of these cores I would do it like this: making a box out of high-rated vermiculite board, assembled with stainless steel screws, lined with firebrick splits. Or a steel support frame around the board box when I wanted to make it very reliable. This way the mass of the core is still relatively high but it would be much more durable than the bare vermiculite board. The one I built did fare quite well with half loads, just like its bigger brothers as long as the double vortex is maintained.
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Post by Deleted on Aug 17, 2016 13:15:03 GMT -8
The bricks extracted more heat than the bare steel drums In accordance with the laws of physic. Very awkward.
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Post by patamos on Aug 17, 2016 18:58:06 GMT -8
Thanks Peter and Karl, So the bricks extracting more heat… would have something to do with warm ambient air temperatures? Peter, how does this affect your thoughts about bare metal ISA and denser mass ISA? Hearing that the batch box burned fine with half a load is good news. For cooking hot plate purposes and low tending maintenance i think it is preferable. But then it will still benefit form a larger ISA tha a J feed. Thinking of our mild climate (rarely below 0c thru winter) and the very small 160 sq.ft. moderately insulated living space, i now wonder what the optimal ISA would be for either. I'm guessing those shiners inside the 55gal drum amount to about 20sq.ft. ISA. Plus 2 or 3ft. of barrel metal top. Peter, when you say the exhaust pipe was touching the sides of the rum, do you mean at the back of the drum, opposite the feed tube/door? many thanks
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Post by Deleted on Aug 17, 2016 23:58:03 GMT -8
Bare steel drums have virtually no storage capacity and thus can only take as much heat as they can immediately emit. Bricks can buffer heat and have due to the thicknes a larger outer surface. If square the girth is extended by 8 times the thicknes, if round still by about 6.3 times. For a big free standing brick stove the outer surface can be two or more square meters larger. There are a more effects.
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Post by peterberg on Aug 18, 2016 5:56:53 GMT -8
So the bricks extracting more heat… would have something to do with warm ambient air temperatures? No, not at all as far as I can see. Peter, how does this affect your thoughts about bare metal ISA and denser mass ISA? There are two effects that have its influence on the ISA and the temps in the exhaust pipe in this case. The bricks were stacked loosely, so the surface area exposed to the hot gases was much larger than you expect, due to all the gaps and crevices. It would be different when the brick wall was completely closed. Second effect was the exhaust pipe, in the bare barrel it was near completely exposed to the hot gases so it would warm up quite a bit. With bricks added, only the front half of the pipe was exposed so it didn't warmed up as much. All together, I would say the metal or brick inner surface doesn't make much difference when the area is the same. This also showed to be the case when I compared my shop heater with the red bell batchrocket. Both are very close to each other ISA-wise, the difference is small in my view. The exhaust temperature of the red bell is slightly lower overall so that could be the difference in heat extraction. Peter, when you say the exhaust pipe was touching the sides of the drum, do you mean at the back of the drum, opposite the feed tube/door? In both cases the exhaust pipe wasn't situated opposite the feed exactly. It was at the back against the inner surface of the drum, just a bit off to the left or right. As long there's space enough, the pipe could be anywhere at the perimeter though, because it's a bell construction. I'd think this barrel construction sports the smallest footprint and the safest on foot temperature. Keeping it from the floor is recommended though.
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Post by patamos on Aug 18, 2016 8:55:25 GMT -8
Thank you Peter
This information is very helpful.
Yes to keeping everything off the ground.
So my idea is now tending towards a 4.5" (due to size of firebrick splits) J-feed with metal barrel bell and bricks spaced inside as you have described. Heat riser below top right of barrel. Flue exiting top left. By 'loosely stacked' do you mean the bricks were almost or sometimes touching? Or was there more of an intentional space between them?
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