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Rocket mass heater question of ratios - 1:2:4 vs 1:1.5:3

 
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Hi, I’m looking to build an RMH for my home but where I’m constrained to putting it is very limited for vertical space (under a stairway right next to where my chimney is). I’m looking to build an 8” system but with what little space there is it would be impossible to fit in a 64” riser using the 1:2:4 ratio. I just stumbled upon a few older posts mentioning that a 1:1.5:3 is also very common. Given that my maximum height for this system would be a little over 48” I think I might just be able to squeeze it in if I place everything perfectly. However my question now becomes would going to the 1:1.5:3 ratio mean efficiency loses? If so are the loses minimal or quite noticeable over the 1:2:4 system? Thanks for any input!
 
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Hi Justin,   Let me explain a little bit of what I know about the ratios you mentioned so you can get a better understanding of why they are that way and plan accordingly. Here are some of the basics:
The 1 represents the depth of the feed tube which should be around the length of the wood your planning on burning. Theoretically, the wood should be not sticking out of the feed tube when burning. It can of course go many times higher if your standing there watching it but it does pose a risk of fire creeping up the wood and then smoking or even flames coming into the room. Also as its burning, the wood could topple over rather than fall straight down into the coals at the bottom - not a good thing either. You also don't want to make it any taller than necessary as then it makes for needing a much proportionally  taller heat riser to compensate so there is not the competing chimney effect.

The 1.5 or 2 is the burn tunnel length which is where the wood gasses are produced and begin to burn. Too short and there is not enough dwell time for this to happen, too long and there is excess travel time and friction from the boundary layer effect which can slow things down. I would say that as long as it falls somewhere in between these two you should be fine.

The 3 or 4 is the heat riser height. If you've ever burned a J tube rocket stove with no barrel on top, you've probably seen flames shooting out the top of the heat riser. Again, theoretically speaking, you would want all the gases to be burned off within the heat riser before they exit the top but its not a perfect world and its also not practical to make a 6 foot riser. So taller is usually better for maximum efficiency but compromises sometimes have to be made to accommodate height restrictions, cooking on top of the barrel, etc..
Of course, all this only applies to a J tube core, not a riserless core, sidewinder, double shoebox, batch box or other stove.

Hope this helps.
 
Justin Hadden
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Hi Gerry, thanks for the reply! Another question for you, I see the most common 1:1.5:3 measuremts for an 8” J tube system seems to be 16”, 24”, 48”. I’ve read that you want your feed tube height to be 1.5-2 times the diameter of your system. So would it be possible to build a system measuring 12”, 18”, 36”? Or even using the 1:2:3 measuring 12”, 24”, 36”? Or are those systems too impractical? The reason I ask is because of my vertical space limitations, but obviously I don’t want to lose too much efficiency otherwise I may as well buy a conventional wood stove.
 
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As Gerry mentioned you could look at alternative designs that dont require a tall riser.
 
Justin Hadden
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Hi Fox, 36 inches would actually be a good height riser, I just need to know if either of the two measurements I gave would give me a decently operating system or if I would lose too much efficiency.
 
Fox James
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Hi Justin, lots of thing may be possible but we dont know enough about your proposed heater, how much mass you want to heat etc....
 
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Justin, it might be a good idea to include a description or pics of the area and the intended rmh along with the heat riser question. It is a tuned system, and the better the draft of the system, the shorter the heat riser can be. Looking at the heat riser by itself may not tell you much, because either will likely burn just fine in open air without the rest of the system. The materials used also may play a big role in how short the riser can be, and the chimney.

I'm concerned about what you say about trying to squeeze it in under this staircase. Is the staircase flamable? Is there going to be room to build the system around the heat riser once it is in? What about future maintenance?
 
Justin Hadden
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Hi jordan. The stairs are already fire proof, and it’s already in the area of my pre existing pellet furnace. Money is tight and I can’t afford to build a cob and clay system. However I have several old 250 gallon oil tanks. So I’m thinking of filling them with water and running about 25 feet of pipe through them for my flue gases to perform a a heat exchange. From there’s it’s probably about 20 feet of chimney to the top. Also the reason for wanting a shorter riser is because of the design of the tank I can only raise the tank up so much, as I only have an 8 foot ceiling. A 36” riser would meet the bottom of the tank well if I put the tank on stilts. Any higher of a riser and the first flue pipe section would end up in the middle of the water tank.
 
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Can you at least fill the 250 gallon 'containers with dirt/sand/mud after you run the flhe piping thru it.

I know that gas powered water tank leak. I know that oil tank leaks. So I am concerned about a leak, and humidity or worse boom squish.
 
Justin Hadden
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No need to worry about boom squish, the tank will be vented to atmosphere. I have many certified welder friends who can do water tight welding for me. Also the tank will go directly over my sump drain so even if it leaks it will not flood the house. Where I’m located there is little to no clay in the ground that is easily accessible. And regular dirt will not give me the good thermal mass qualities I would like.
 
Justin Hadden
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I should also mention I have access to existing ducting for a fresh air intake for the stove, so back smoke isn’t a concern. So with that in mind how much efficiency would I lose with a 36” riser compared to a 48” on an 8” J tube? Could I have a 16” feed tube, 24” burn tunnel and 36” riser? Or givin the 36” riser would I need to shorten my feed tube to 12”? I know some may say to just build a batch box but I think that’s a little advanced for me as I’ve only built a handful of prototype J tubes. Also I’ve had lots of trouble finding information on how exactly a batch box system works, and I don’t ever build things that I don’t feel I have a decent understanding of first.
 
Jordan Holland
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Justin Hadden wrote:No need to worry about boom squish, the tank will be vented to atmosphere. I have many certified welder friends who can do water tight welding for me. Also the tank will go directly over my sump drain so even if it leaks it will not flood the house. Where I’m located there is little to no clay in the ground that is easily accessible. And regular dirt will not give me the good thermal mass qualities I would like.



It sounds like you may be on your own on this one. I haven't noticed anyone successfully making such a system so far. If your friends already have experience welding together such a RMH, and it has run successfully for years without cracking, I would follow their design exactly. I don't know that anyone here can really help if they haven't had any experience with such a system, especially without being able to see the design.
 
Jordan Holland
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Justin Hadden wrote:I should also mention I have access to existing ducting for a fresh air intake for the stove, so back smoke isn’t a concern. So with that in mind how much efficiency would I lose with a 36” riser compared to a 48” on an 8” J tube? Could I have a 16” feed tube, 24” burn tunnel and 36” riser? Or givin the 36” riser would I need to shorten my feed tube to 12”? I know some may say to just build a batch box but I think that’s a little advanced for me as I’ve only built a handful of prototype J tubes. Also I’ve had lots of trouble finding information on how exactly a batch box system works, and I don’t ever build things that I don’t feel I have a decent understanding of first.



The only way anyone could tell you about any efficiency comparison, is if they have built a very similar system and tried both sizes of risers. Even if their system was identical, and their house was identical, just because it worked in their geographic location doesn't guarantee it will work in yours. We don't know anywhere near enough about your design to know if anyone has attempted anything even remotely similar. The only thing we can really say is that a taller riser is more likely to work than a shorter one. That is the reason they updated the riser ratio to the 1:2:4. Several people were having problems with the shorter risers.
 
Fox James
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Hi Justin, if it were at all possible to draw a diagram of what you plan to build and perhaps a photo or two it might help us understand a lot more.
I am sure that  if your plans are feasible, we can help but personally  I am a little lost and confused about its potential placement and function.

A stand alone 8” rocket with no restrictions will work with a shorter riser but without knowing more details about your own personal design requirements it is impossible to say if it will work well enough  for you?
 
Gerry Parent
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Justin Hadden wrote: Also I’ve had lots of trouble finding information on how exactly a batch box system works, and I don’t ever build things that I don’t feel I have a decent understanding of first.



Justin,  Check out Peter van den Bergs website batchrocket.eu which will give you all kinds of information about batch box rockets .
 
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