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Post by mrdman on Feb 16, 2015 8:49:18 GMT -8
Building an underground off the grid house this summer and im working out dimensions for a RMH. The heater will be a 4" j tube design built into a large cob tree in the center of the house from the floor of the main level up into the ceiling of the second floor. The exhaust will feed a small bench then run up thru the tree to the roof.
The air inlet will be piped under the floor from a 50 ft soil pipe. The air will enter the feed tube thru a grill built into the side of the feed tube and the top of the feed tube will be closed off with a plate on a hinge.
My question is about the 1:2:4 equation, since my air inlet is on the side of the feed tube near the bottom does the length of the feed tube really matter? With a long (2ft+) feed tube the idea is that i can load long pieces of wood in and close the lid for extended burns where the stove is most efficent.
Any thoughts?
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Post by Donkey on Feb 16, 2015 10:46:39 GMT -8
I think you will find (as I did) that the side air doesn't work as well as one would hope. What tends to happen is that it will blow smoke out the feed. The lid for the feed will get quite hot, wood will begin to pyrolyse up in the feed itself and every time you open the lid to add wood, you get smoke and sometimes flame streaming into your face. So far, it has been best to flow air over the top, down through the wood and into the fire. This can still be done with an external air source to good effect.. Also, it seems (so far) that if you want to be able to feed in more wood, the Batch-Box is a better option. Murphy's law has a particular thing for long pieces of firewood and tall wood-feeds.. They tend to hang up and fail to slide down. I don't use wood longer than about 3 fingers more than the depth of the firebox, I've had burning wood roll out and onto the floor too many times, after hanging up, burning away the base and toppling out..
On the other hand, If you find a good solution for the above issues, please tell the rest of us! Your experiences experimenting with these ideas will be valuable whatever the final outcome!! On that note, to ACTUALLY answer your question: with air coming in the side, the feed height won't matter, no.
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Post by mrdman on Feb 16, 2015 14:39:13 GMT -8
Ok, so keep the air inlet near the top of the feed and make sure the feed tube is smooth to help prevent hangups. The feed tube will be sealed by the lid so there is no chance of burning wood falling on the floor. I just like the look of the j tubes, going to put a window in the front of the feed tube soyou can see whats going on in there as the wood burns down.
Thanks for the info.
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Post by ronyon on Feb 16, 2015 16:46:41 GMT -8
I am surprised no one has advised you go bigger, especially with a J tube. 4" rockets are often said to be harder to do right, and underpowered in heating applications. Of course you will be likely to be well insulated and have plenty of mass, so the required heat output would be minimal. Still, I would favor a 6" or bigger, add an oven, and a bypass, or two to avoid heating the mass when you just want to have a fire, or heat the oven.
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Post by mrdman on Feb 17, 2015 18:16:22 GMT -8
6" pipe isnt very common and I was already planning on laying down 4" soil pipe for house ventilation thats why I thought a 4" RMH would be the simple way to go. The house is small, both floors only equal 1080 sqft, as well as being mostly underground and will be well insulated. At the same time it would suck to have to rip at all apart to make it bigger later on. I like the idea of a bypass, would be easier to light with the exhaust going straight to the chimney then switch to the mass once warm.
What were the common issues with the 4" heaters?
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Post by erikweaver on Feb 18, 2015 9:41:26 GMT -8
There are all kinds of issues with 4-inch systems. Everywhere you turn, from what people say. The reason is simple, there are air drag, resistance, from the walls. That adds up. There are critical dimensions, and 1/8-inch change is a much larger percentage change on a 4-inch system than a 6-inch system, so all your dimensions and turns and cuts become more critical. You have a lot less room to be sloppy and not have a negative result due to sloppiness. Thus, for new builders, a 6-inch offers a lot of advantages in terms of being able to concentrate on the broader issues, and being less concerned with whether there is an 1/8-inch too much or too little here or there, or wondering why the air is jamming up in the tight spots.
Then there is the area of a circle. Going from 4- to 6- to 8-inch circles is each a very big change, much more so than suggested by the simple 4, 6, 8 progression. Just run the CSA calculations for each and you'll see this immediately. So, with each step up you get a lot more air movement, and capacity. I read that as options. If the thing is too big, you can always burn it less often (assuming you have thermal mass). But if it is too small, you can only burn constantly, and after that, there is nothing else to do but tear it out and make it bigger.
Thirdly, a 6-inch ought to do quite well for that size of space. Provided you are insulating yourself from the earth. A common misconception is that the earth makes a good insulator, and that's why building underground is so beneficial.
It is beneficial, but that's not quite the reason.
Due to depth, the volume of earth does moderate changes in temperature. If deep enough there can even be a flywheel kind of effect. And that is great! Far better to heat and cool a living space if the outside walls are always looking at something like 55-60 F.
But the earth also sucks! Big time! It will suck, and suck, and suck the heat out of the living space. So it is really very important to insulate the living structurally from the planet. If you do that, then you get both the benefit of temperature change moderation and living in a well insulated space, which you should be able to heat and cool to good effect, with minimal energy input.
And do read "The Book" by Evans if you haven't already. And watch all you can by Ernie and Erica Wisner. I even recommend picking up one of their plans, but I like seeing building details. For some that might not be as important.
And give careful thought to your materials. Refractory materials in the firebox. Insulate the firebox from the surrounding structures appropriately. Have a good chimney system. Maybe if you're going to also put a sealed liner over your underground house you can get away with the side-ways style Evans chimney, but otherwise, most folks report they need a standard chimney (I am one of those cases, sideways out the wall did NOT work for me). Really important details.
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Post by erikweaver on Feb 18, 2015 9:45:10 GMT -8
I wanted to emphasise this point. I am concerned when I hear you say 6-inch pipe is not common, but 4-inch pipe is, and will be used elsewhere. Don't make your heater out of pipe. Make it out of refractory material. The easiest refractory material to use for most folks is fire brick. But you can do castable is that seems more natural to you. If you don't understand why pipe is a poor choice, and why refractory is so widely recommended, I would suggest you spend more time researching your proposed build. I suspect you will end up making some modifications to your current plan. Better and cheaper to make changes on paper heheh
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Post by mrdman on Feb 20, 2015 18:49:30 GMT -8
The stove will be made of steel with the burn chamber and riser lined with firebrick. The whole unit will be wrapped in rocksul (2150 F) and encased in COB. Ive seen the pictures on here of the stoves that melted down from the heat, not very safe.
I picked up some fire brick and layed out a 4" stove and yeah it does look too small for a house, more like a ice shack heater. 6" it is.
Question: It is commonly mentioned on here that the CSA thru the stove should remain constant, but as soon as you put wood in the feed tube doesn't that reduce the CSA considerably? Shouldn't we then build the feed tube larger than the rest of the stove or am I missing something?
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Post by shilo on Feb 21, 2015 10:25:27 GMT -8
" Shouldn't we then build the feed tube larger than the rest of the stove " no!
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Post by erikweaver on Feb 21, 2015 12:21:03 GMT -8
I suspect you'll be far more pleased with the results (and construction) of a 6-inch system. Good choice.
As pointed out, the short answer to increasing the area of the feed tube is "no."
There are several reasons. It tends to increase smoke back into the house. I made mine a little longer (along the long axis of the firebox) and ended up putting a fire brick inside to take up some of that extra room. If it were a permanent install, I'd cut brick [to make a smaller, permanent feed tube opening], but this is only a test stove and will be removed after this winter.
Also, with a standard J-style feed tube you need to monitor it rather closely, and make sure the wood falls, and snug the wood together every now and then (several times an hour, or poke and prodding the wood about every 5 to 20 minutes, depending). Part of this process is keeping the covering fire bricks snugged up against the wood to reduce the feed tube opening. Doing these things keeps the draft going and helps keep the smoke from getting back into the room.
Extra space in the feed tube acts like an "eddy" for air, and draws smoke into it. What we really want is the smoke to be drawn into the burn chamber and burned up. So extra space in the feed tube works against us in terms of air flow.
Do investigate the Peter Channel or the P-channel, which Peter VDB designed. It is widely reported to help J-style rocket heaters burn better, as is making a trip wire.
The trip wire adds air turbulence and the P-channel provides a thin stream of fresh air to flow down the feed tube right above the opening to the burn chamber. Experimentation has determined this may be the most effective air intake design for a standard J-style rocket heater.
The wood is snugged up tight, and an air gap is provided between the wood pieces (and the P-channel if one is being used). But the optimal percentage of air flow is much less than the feed tube opening. Something like 20% of the system CSA, as I recall; others can provide the correct figure, but I think that's in the right ball park (an American expression meaning, suitable range or reasonable estimate).
What is critical is that the burn chamber and the 90-degree upward turn into the fire riser have the smallest CSA of the entire system. Different areas of the system will work best with various dimensions thereafter, but the CSA of the burn chamber must be the smallest dimension; I personally include the point where the burn chamber transitions into the fire riser in this statement, but I have not tested that, so maybe that can already get a little larger than the CSA of the burn chamber. My guts say this is true, but also that one will do best keeping that as close to the CSA of the burn chamber as is reasonable.
EDIT: Corrected typographical mistake & [added remarks] for clarity with regard to fitting a cut brick to the feed tube opening, to reduce CSA.
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Post by mrdman on Feb 26, 2015 19:02:48 GMT -8
Thanks for all the great info!
Another question, is it better to have the wood at the bottom of the fed tube sit closer to the burn chamber opening, the center of the feed tube, or away from the burn chamber. I am incorporating an ash drawer under the feed tube and believe I will be able to influence somewhat where the wood sits based on how I construct the grate and angle it. The grate will be a series of small diameter round bar welded beside each other with small gaps between each to alow the ash to fall thru but not the wood.
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Post by erikweaver on Mar 1, 2015 16:49:56 GMT -8
I've not read of a lot of success with metal grates (they don't seem to survive the high temps well). So be sure to post back after a season of burning and let us know how it works for you.
The answer to your question is the wood burns best when up against the burn chamber opening. It smokes less too, or more accurately, the smoke is more easily pulled into the burn chamber; same applies for the dimensions of the feed tube, smaller volume encourages the smoke to draw as desired (such has been my experience at any rate).
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stoker
Junior Member
Posts: 61
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Post by stoker on Mar 15, 2015 14:18:34 GMT -8
If I were building any kind of new house I would be looking at Passivhaus and similar standards, and at the techniques, technologies and materials that other people have used to achieve those standards. en.wikipedia.org/wiki/Passive_houseen.wikipedia.org/wiki/MinergieGoing off-grid makes it tricky, but a lot of the ideas can still be used, and I presume you'll be generating some electricity at the house (e.g. solar plus batteries). One of the big improvements over most houses is to seal up all the air-leaks in the house and then ventilate using forced-flow (ducts and fans) with a heat exchanger in which outgoing stale air warms (and is cooled by) incoming fresh air. This way you get good ventilation with minimal heat-loss.
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Post by mrdman on Apr 11, 2015 18:46:28 GMT -8
Rocket stove is built! Now I just need to build the house to put it in. Cold air intake preheated within the Cob, Lid to close off feed tube, Ash grate on the bottom of the feed tube and a drawer not shown, Front viewing ceramic glass window, firebrick lined burn tunnel and riser, room in the top of the barrel to later install a copper coil to heat water. Attachments:
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Post by satamax on Apr 11, 2015 22:54:03 GMT -8
Nobody told you? Metal is doomed!
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