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

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!

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.

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.

As Gerry mentioned you could look at alternative designs that dont require a tall riser.

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.

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....

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?

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.

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.

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.

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.

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.

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.

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?

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 .

I see all these posts about risers and those ratios but my biggest question is how tall it too tall for a chimney/exhaust stack?

Id like to put one of these in the middle of my basement but I'd either have to send the exhaust all the way up through the first floor and then the attic, which would be about 20' total... or send it out the side of my house almost horizontal. I don't see too much in Ernie and Erica's book about this. Seems like the assumption is that the RMH is built along an outside wall.

Thoughts?

Higher is usually better with the exit stack. The main rules that apply to chimneys are applicable with an RMH as well:

Exit point should be at or above the elevation of the highest roof point.

Vertical runs are best.

If you have strong prevailing winds from a single direction, be aware that a zone of high atmospheric pressure could set up on the leeward side of the building. But right next to it will be a zone of low pressure (rolling turbulence). The effects of this feature are mitigated by chimney height.

Insulated flue pipe on exterior runs helps keep heat in the exhaust gases and increases the stack effect.

Decreasing the diameter progressively as you approach the end increases the velocity of the gases and helps system draw.

Erica did one of her marvelous detail-rich posts on How Chimneys Work a while back and it is superb.

A recent article about this stated that the ratio sequence was dependent on the cross sectional area (CSA) of the material in a “J” tube configuration.

It assumed that the CSA was the 1.  The height of the magazine (vertical section) was 2 x CSA; the burn chamber was 4x CSA and the heat riser was 6 x CSA. Given those figures, your heat riser would be 48”. However, I’ve not seen a combustion chamber 32” long.

In the original research by Larry Winiarski at the University in Wisconsin, the design was an “L” shape where the combustion chamber was 1 unit and the heat riser was 1.5-2 units (not necessarily based on the CSA). So, a 12” combustion chamber got a 18-24” heat riser. I’ve built several “L”shaped rocket stoves based on this figure and have had excellent results.

The efficiency of the insulation around the heat riser was of significant importance but without insulation, the ratio remained unchanged.

There has been significant research on this since then. Peter Vandenberg and the Wisners have contributed greatly as well as “Donkey”.  “J”tubes and modified “J”tube styles have come along since then and the “P” channel have improved the performance  greatly.  

I share the concerns regarding spacing of your project from ANY flammable surface. On the rocket stove I bought (Liberator), at the top of the “bell” can easily reach 600°F and I have photos to prove that. The one I built comes close to that. I can keep the temperature of the bell between 300 to 400°F in a large Quonset building with no insulation on the walls. The minimum distance from anything that hot should be greater than 30”(more iyam).

Hello ratios
Really useful to have simple instructions, like ratios, to configure a build.
Our build nearly required logarithms and calculators for the calculations.
We followed the Builder’s guide instructions, did the tests with the bricks outside etc.
In the event, the dragon was a smoking beast, thank god for the Godin stove on the other side of the house.
This year, height has been added to the chimney in the new taller barrel.

What I really wanted to add to the burn question and indeed ratios, how would a Peter panel affect efficiency?
I get sporadic indestion from bulemic reading of opinions and advice but what the heck, hey ho.

Looking forward to responses with whipped cream and chocolate flakes, please.
Thank you and blessings
M-H