optimizing rmh in weird situation

So, asking for a friend...the rocket mass heater has only a twelve foot cob bench,, because of code reasons, and a 6" duct through that.  

It has an 8" j tube with an extra high barrel and extra large--65 gall, plus 8" off a second barrel, and that's in addition to the manifold.

But wait, there's more!  Act now and get half off your next bell.

There's a second bell for a stratification chamber (to compensate for the short bench) and this has been mostly covered with cob on the sides, about 2" .  (My friend is running out of clay but can get more in a few weeks).  The top has a tub of water in a big aluminum cauldron with a lid, maybe 20 gallons, it's squat and almost fills the whole barrel top with its girth.  It reaches temperatures of maybe 130 F after a burn.  

There's no cob hat on the first barrel but there could be...but it does have a metal "bonnet" of sheet metal curved over to reflect some heat back toward where it's needed instead of just heating the ceiling there.  (The ceiling has not been complaining about the cold).  

The bench gets only slightly warm.  The draft may still be too strong.  

The room goes back down to about 55 degrees rather quickly (ie overnight).  

The insulation situation between the first floor there and the. basement is still kind of sketchy--my friend has insulation but fitting it in around ducts and figuring out of how to remove ceiling panels has not yet occurred.

There is a crude baffle in the bottom of the exit chimney.  It could be more baffled.  (I, on the other hand, am almost fully baffled already.)

8" J tube with 6" duct shouldn't be a problem because of Charles's Law, the temperature has gone down from over 1000F to 300 by that point, and also because Uncle Mud said so.

I like to be able to cook on the top of the main barrel, including a tall pot of stew, but I'm wiling to let that go if I must, I could put a cob hat where there never was a hat.  (If you get that reference that makes me happy.)

What would you prioritize?

Do you agree that a baffle is helpful?

What else am I missing?

Thanks, team.

Bump. AI is recommending that I put an extreme baffle in, is that to be trusted?  

Another factor that I should mention is that the rocket mass heater is downstairs a two-story house, so radiant heat maybe heating up here that is gradually conveyed upstairs overnight, such that the drop-in temperature in the downstairs room is more because of that, then a lack of performance. I can try putting the thermometer upstairs for a while to see how we’re doing up there, but I have been hearing some complaints of”it’s too cold up here”

Two more wrinkles –

– The riser is more like 40 inches instead of 36. The burn tunnel was made too long, by mistake, so the builder compensated.

And then the barrel is more than 2 inches above the top of the riser, whereas Erica and Ernie’s book recommends 2 inches. That would seem to mean more turbulence and less heat transfer from the top of that barrel, so potentially more Heat would go into the stratification chamber? And then more out the chimney than we want?  The uninsulated part of the exit chimney feels like maybe 200°. The meat thermometer gives a reading up 130 Fahrenheit(but that’s not necessarily super accurate.)

Some information we need to assess the situation would be the size and character of the house, its age, insulation, weathertightness, a description of the chimney...

What exactly do you mean by a baffle? How close to the floor of the final bell barrel is the exhaust to the chimney? How tightly do you cover the J-tube feed between fires? Do you have natural draft when the heater is cold? How strong is the draft when it is operating?

As long as the main floor is not leaky (letting cold air percolate directly up), I think the underfloor insulation is not the main issue (but good to get it fully insulated). A 6" duct in the mass would possibly be too quick of a reduction from 8", though the extra large barrel(s) above the riser may shed enough heat to make that not a problem. My experience with a 6" stovepipe and chimney from the bell of an 8" J-tube is good, but that has all the heat extraction in the zero-friction bell.

Some information we need to assess the situation would be the size and character of the house, its age, insulation, weathertightness, a description of the chimney...

What exactly do you mean by a baffle?


—I put a piece of aluminum food container, the disposable kind, up the chimney so that it blocked maybe half of the airflow. It didn’t seem to do much of anything to the draw when I operated the rocket mass heater after that.

How close to the floor of the final bell barrel is the exhaust to the chimney?

—The second bell is between the normal rocket mass barrel and the Mass. The exit of the stratification chamber is about 3 inches from the floor, I would guess, whatever height the pipe that goes through the mass starts at (as it gradually rises through the mass)

How tightly do you cover the J-tube feed between fires?


—I use two jumbo bricks, it seals pretty well, there are some gaps between bricks that I have filled in with a bit of Cob. it is not as good as seal as the metal prefab thing from the liberator though.


Do you have natural draft when the heater is cold?

—If it’s cold, as in cold-start cold, I don’t know, I would say maybe slightly. If it’s as in “cold in the morning,” there’s definitely a draw, I noticrd the difference when I was away for a few days and had to do a cold start, it was really hard to get it going. in fact, I removed my rudimentary baffle at that point, because I was worried that there was something seriously wrong, and I haven’t put that baffle back in again.


How strong is the draft when it is operating?

—I would say it is very strong, I don’t have much point of comparison, but it’s at least the same as the liberator was. Strong enough that it’s pretty easy to start the rocket mass heater. that aspect of this is way easier to use than liberator was.

As long as the main floor is not leaky (letting cold air percolate directly up), I think the underfloor insulation is not the main issue (but good to get it fully insulated).

—I’m doing the best I can. It’s a funky situation down there, lots of pipes and things to get in your way.  They’re definitely isn’t any leak, but it’s a tile floor, so it was kind of taking upon itself to radiate heat down into the basement, and I decided to put tinfoil under it as well as the insulation stuff.


A 6" duct in the mass would possibly be too quick of a reduction from 8", though the extra large barrel(s) above the riser may shed enough heat to make that not a problem. My experience with a 6" stovepipe and chimney from the bell of an 8" J-tube is good, but that has all the heat extraction in the zero-friction bell.

—Thanks for that information. So I think you’re saying that it should be OK, as long as a lot of heat gets shed before it goes into the Mass, right? And now I hope it is clear that there are two opportunities for Heat to be shed, the initial barrel, which is 1 1/2 barrels, and the second barrel that is a stratification chamber.  

My question is still if I should put the baffle back in and make it even more baffling. Claude the AI said to put 5 1-inch diameter holes in a circle of metal, which would be a pretty extreme baffle. Was Claude hallucinating?


A few other points, I have almost finished, coughing around the stratification chamber, but not completely.

I have put a “Cob hat“ on top of the first barrel, by which I really mean a gigantic aluminum animal feed tub filled with water. I haven’t measured its temperature, but it definitely has not gotten up to boiling. Which leads me to think that the top of the riser is not close enough to the barrel, to really shed the kind of heat you want to shed in this situation, and that would be a better optimization. I also have a metal tub on top of the second stratification chamber, and that one is about the right temperature for fermenting animal feed, which is nice, but I also wish that it got too hot, that would be a nice problem to have, which I could solve by putting another layer of bricks between that stratification chamber and the feed fermenting tub. These tubs can hold maybe 30 gallons of water.

Thanks so much for your help, I really appreciate it!

Thanks for your reply, Glenn, I wrote back and I messed up the thing to indicate that I was quoting your post. I put my answers with a DASH (—) before it.

Joshua Myrvaagnes wrote:Some information we need to assess the situation would be the size and character of the house, its age, insulation, weathertightness, a description of the chimney...

What exactly do you mean by a baffle?


—I put a piece of aluminum food container, the disposable kind, up the chimney so that it blocked maybe half of the airflow. It didn’t seem to do much of anything to the draw when I operated the rocket mass heater after that.

How close to the floor of the final bell barrel is the exhaust to the chimney?

—The second bell is between the normal rocket mass barrel and the Mass. The exit of the stratification chamber is about 3 inches from the floor, I would guess, whatever height the pipe that goes through the mass starts at (as it gradually rises through the mass)

How tightly do you cover the J-tube feed between fires?


—I use two jumbo bricks, it seals pretty well, there are some gaps between bricks that I have filled in with a bit of Cob. it is not as good as seal as the metal prefab thing from the liberator though.


Do you have natural draft when the heater is cold?

—If it’s cold, as in cold-start cold, I don’t know, I would say maybe slightly. If it’s as in “cold in the morning,” there’s definitely a draw, I noticrd the difference when I was away for a few days and had to do a cold start, it was really hard to get it going. in fact, I removed my rudimentary baffle at that point, because I was worried that there was something seriously wrong, and I haven’t put that baffle back in again.


How strong is the draft when it is operating?

—I would say it is very strong, I don’t have much point of comparison, but it’s at least the same as the liberator was. Strong enough that it’s pretty easy to start the rocket mass heater. that aspect of this is way easier to use than liberator was.

As long as the main floor is not leaky (letting cold air percolate directly up), I think the underfloor insulation is not the main issue (but good to get it fully insulated).

—I’m doing the best I can. It’s a funky situation down there, lots of pipes and things to get in your way.  They’re definitely isn’t any leak, but it’s a tile floor, so it was kind of taking upon itself to radiate heat down into the basement, and I decided to put tinfoil under it as well as the insulation stuff.


A 6" duct in the mass would possibly be too quick of a reduction from 8", though the extra large barrel(s) above the riser may shed enough heat to make that not a problem. My experience with a 6" stovepipe and chimney from the bell of an 8" J-tube is good, but that has all the heat extraction in the zero-friction bell.

—Thanks for that information. So I think you’re saying that it should be OK, as long as a lot of heat gets shed before it goes into the Mass, right? And now I hope it is clear that there are two opportunities for Heat to be shed, the initial barrel, which is 1 1/2 barrels, and the second barrel that is a stratification chamber.  

My question is still if I should put the baffle back in and make it even more baffling. Claude the AI said to put 5 1-inch diameter holes in a circle of metal, which would be a pretty extreme baffle. Was Claude hallucinating?


A few other points, I have almost finished, coughing around the stratification chamber, but not completely.

I have put a “Cob hat“ on top of the first barrel, by which I really mean a gigantic aluminum animal feed tub filled with water. I haven’t measured its temperature, but it definitely has not gotten up to boiling. Which leads me to think that the top of the riser is not close enough to the barrel, to really shed the kind of heat you want to shed in this situation, and that would be a better optimization. I also have a metal tub on top of the second stratification chamber, and that one is about the right temperature for fermenting animal feed, which is nice, but I also wish that it got too hot, that would be a nice problem to have, which I could solve by putting another layer of bricks between that stratification chamber and the feed fermenting tub. These tubs can hold maybe 30 gallons of water.

Thanks so much for your help, I really appreciate it!

Another factor I just realized I should mention, this is the sunroom, it gets really good Solar gain in the daytime, but has terrible shades at night. Some of them are r2 beehive, shades, and some are just a thin rough-weave cloth, which hopefully traps a little bit of air between the glass and the curtain.

Yesterday, when it was about zero Fahrenheit outside, which is not the coldest it ever gets here, it felt like the cold was reaching fingers into the room. I was sitting right next to the rocket mask heater after the fire had gone out, and I was wearing my coat. I made another fire and then I felt better.

At sometimes when I have measured the temperature of the chimney as it’s exhausting, it has been about 160°. I’m wondering if all the Heat is getting radiated into the room from the barrel, and it would actually be good to put Cob up higher around the bottom of the barrel?

Please forgive the weird capitalization of words, I am using dictate to type this. Thanks so much for your help.

From your description, you did start with mostly instant radiation heating, only storing heat from the largely cooled gases. Therefore, you will not have much stored heat compared to instant room heating, so when you make the room comfortable, you don't have much for overnight. Cobbing the second barrel with say 4" would get you a much better balance between instant and delayed heating. It may be good to cob a good part of the first barrel too.

I would not have less than a 2" gap between riser and barrel; 3" would allow easier flow, and the only heating difference would be less concentration on the middle top and more on the sides and downstream. The fact that you are not getting a lot of heat in your first water tub is puzzling, assuming you get complete combustion and end up with only fly ash and no charcoal after a burn.

Josh;
You mentioned that the burn tunnel was made longer than it was supposed to be.
How long is it?   Just the roof, from the trailing edge of the feed tube to the leading edge of the riser.
Stock specs call for this to be 10" up to 12." I built one at 15"... BUT, only one time.
I had to do a mid-winter teardown to shorten that build. It seemed to burn OK, but was filling my pipes with ash.

Those two barrels share plenty of heat while burning, but they also shed plenty of heat when not burning.

3" is now considered to be a good gap for riser to barrel clearance.

Thanks, this gives me some good information.

The bucket of water on top is propped up on bricks around its circumference, mimicking the Cob hat, i’m not sure if I had mentioned that.

I’m going to thicken the Cob that surrounds the second barrel for sure, cob higher on the first barrel, and also ask the person who helped build it how much space was between the riser and the barrel. I think it may have been more than 3 inches, and I think it would actually be worth it to concentrate the heat more in the center of that barrel, so that it would be really directed at the tub of water to be heated rather than radiating into the room.

Another issue is that the Cob hat is way up in the air—not where people are. I put a metal bonnet on top of that to reflect heat down again, but it’s not very ideal.  It doesn’t feel like it’s radiating a lot of heat when the fire is out, but I suppose that that’s not so easy to feel unless you’re right up near it.

The burn tunnel is 33 inches long, I believe, one extra length of brick. I’m understanding you’re saying that that is a serious problem, yes? Do you know why that creates more ash instead of Heat?  

One obstacle to changing that is that the size of the barrel is 65 gallon rather than 55, and there’s not very much space between the firebox edge, and the edge of that barrel currently. Almost none. Certainly not 9 inches. And I thought we had solved the problem of the extra burn tunnel by making the riser taller…?

Another thing I’ve discovered is that I can cook or reheat a pot on top of the firebox, as long as I don’t let it boil over, the firebox radiates a lot of heat upward. Even though the draw down is very powerful.  My pot got up to a boil, reheating some starch.  This is helpful because I have sacrificed to the top of the barrel to just heat that big tub of water.  My friend still has a way to make a pot of buckwheat for dinner without needing to use electricity.

My 8" J-tube is 24" long from front of feed to back of riser mounting. The burn tunnel ceiling is about 8-9" long. It works excellently, leaves nothing but fine gray ash in the floor, and in two cleanings of the bell in 8 years, has had maybe a few gallons total of fly ash settled on the bell floor.

If you can remove the barrel over the riser, your best bet is shortening the burn tunnel from that end, moving the riser closer to the front of the barrel.

Propping the water tank up away from the barrel top will greatly reduce the amount of heat that goes into the water. I would set the tank directly on the barrel top. The amount of water you describe will not boil dry quickly, but will heat much more quickly. Is the tank covered? Evaporation will release a lot of heat from the open top.

Yes Josh;
A 33" burn tunnel is much to long.
That makes the roof of your burn tunnel 17" long.
Glenn has 8=9", I prefer 10", but have found 12" to work, but no more than that.

Your buddy is asking that slowed down 8" J to push through 6" pipes, again, not to specifications.
A recipe for an ash-plugged system.
I used a 2.5" gap on my 8" for seven years.
I also had a 48" riser. I recorded a 1100°F reading on an 8" glowing red circle on my barrel top.
The riser can be within an inch of the barrel on one side.
What kind of transition area is the barrel dumping into?

You asked why a long burn tunnel is bad.
To create the temperatures at which J-Tubes are recorded.
You must allow the gases free easy movement, although it looks like your J-Tube is burning properly, I assure you it is not.
The core portion of any RMH must be built to specifications if you expect it to work like we say it does.

In your example, the gases are slowed by their horizontal travel. When they reach the riser, they cannot get the 1800°F temperature that it is touted to reach.
This allows Ash to reach the transition area and enter the pipe system.
Increasing the riser height to compensate does not solve the problem.
After the riser, he is asking for 8" of hot gas to travel through a 6" pipe.
It fills with Ash, slowing it down to the point it will not light properly anymore.
I know this to be a fact, as my 15" burn tunnel roof, plugged with ash by mid-winter...
Prior to that, I thought I had a Bad Ass Rip Roaring J-Tube...  I did not.

I believe it needs a rebuild to "factory" specifications, if he wants factory high-performance results.

Hey Josh.
I forgot about the "Baffle", leave that out, and please don't put any restrictions in the chimney system of an RMH.
Next, regarding AI, Liz has found it extremely useful for her business. However, she is asking for keywords or descriptions for artistic work.
In this case, you are asking it to make mechanical suggestions on a subject with significant conflicting information, due to ongoing innovation.
Ask at Permies, Donkeys pro boards, Walker stoves, Peter Berg himself, even Dragontech will help answer questions.
Call me old-fashioned, but I don't believe AI is as qualified as the folks who developed these stoves.

Thanks for this post, I’m reading it again and I think finally understanding it better.  I’m not sure I’m understanding how you would most easily change the length of the burn tunnel by moving the riser within the barrel, however, since that means taking everything apart, but I I’m assuming that I can leave the manifold intact, have the riser be closer to one side of the barrel, lopsided in other words. No need to rebuild the feed tube at least. If I misunderstanding, please let me know?

My friend had had an inexperienced builder, who did really good work in a lot of ways, but missed a key element here and made the one mistake that really undermine the workings of everything it seems. Live and learn.

thomas rubino wrote:Yes Josh;
A 33" burn tunnel is much to long.
That makes the roof of your burn tunnel 17" long.
Glenn has 8=9", I prefer 10", but have found 12" to work, but no more than that.

Your buddy is asking that slowed down 8" J to push through 6" pipes, again, not to specifications.
A recipe for an ash-plugged system.
I used a 2.5" gap on my 8" for seven years.
I also had a 48" riser. I recorded a 1100°F reading on an 8" glowing red circle on my barrel top.
The riser can be within an inch of the barrel on one side.
What kind of transition area is the barrel dumping into?

You asked why a long burn tunnel is bad.
To create the temperatures at which J-Tubes are recorded.
You must allow the gases free easy movement, although it looks like your J-Tube is burning properly, I assure you it is not.
The core portion of any RMH must be built to specifications if you expect it to work like we say it does.

In your example, the gases are slowed by their horizontal travel. When they reach the riser, they cannot get the 1800°F temperature that it is touted to reach.
This allows Ash to reach the transition area and enter the pipe system.
Increasing the riser height to compensate does not solve the problem.
After the riser, he is asking for 8" of hot gas to travel through a 6" pipe.
It fills with Ash, slowing it down to the point it will not light properly anymore.
I know this to be a fact, as my 15" burn tunnel roof, plugged with ash by mid-winter...
Prior to that, I thought I had a Bad Ass Rip Roaring J-Tube...  I did not.

I believe it needs a rebuild to "factory" specifications, if he wants factory high-performance results.

Well, I measured the bottom of the burn tunnel, and I got 25.5 inches. Which would mean that the roof should be 10.5 inches, I might understanding, (subtracting 7.5×2.).  Confirmed by awkwardly sticking my hand in there and measuring the roof directly. That is well within the range of what should work.  Shortening it by another one and a half inches doesn’t seem like it’s going to make that much of an improvement, so I will just put that in.

Can you clarify what you meant by “ what sort of transition area is the barrel dumping into“? There’s a pipe, 7” diameter, to the stratification bell.

I think I will focus on putting more Cob on the side of the first barrel, but there’s one more question.

Is it much better to have no Cob on the very top of the barrel, at least in the center of the top, for rapid heat shedding to create a strong draw? And if that’s so then how come a Cob hat doesn’t mess up the draw?  Another words, if the Cob had retains a lot of heat, then there’s less of a rapid cooling affect there, right? And if that’s the case, then how come other designs like the cyclone (thick mass all over the top, very little possibility for shedding Heat) could work so well, too well in fact? ( I would love to have the problem of a cyclone.)

I have put a thin layer of Cob over the top of the barrel, for this purpose: to make a flat surface for the big tub of water, sitting up there, rather than having a gap. I noticed that when I had spilled water in between those two metal areas, there was much better heat transfer into that tub of water, and it heated up way more than without that bit of water. So I’m thinking conductive surface between their works better than just radiative.  (Pouring water in there every time you operate the rocket mass heater doesn’t seem like a safe idea.). Can anyone clarify this matter of the heat shedding?Thanns

Joshua Myrvaagnes wrote:Thanks for this post, I’m reading it again and I think finally understanding it better.  I’m not sure I’m understanding how you would most easily change the length of the burn tunnel by moving the riser within the barrel, however, since that means taking everything apart, but I I’m assuming that I can leave the manifold intact, have the riser be closer to one side of the barrel, lopsided in other words. No need to rebuild the feed tube at least. If I misunderstanding, please let me know?

My friend had had an inexperienced builder, who did really good work in a lot of ways, but missed a key element here and made the one mistake that really undermine the workings of everything it seems. Live and learn.

thomas rubino wrote:Yes Josh;
A 33" burn tunnel is much to long.
That makes the roof of your burn tunnel 17" long.
Glenn has 8=9", I prefer 10", but have found 12" to work, but no more than that.

Your buddy is asking that slowed down 8" J to push through 6" pipes, again, not to specifications.
A recipe for an ash-plugged system.
I used a 2.5" gap on my 8" for seven years.
I also had a 48" riser. I recorded a 1100°F reading on an 8" glowing red circle on my barrel top.
The riser can be within an inch of the barrel on one side.
What kind of transition area is the barrel dumping into?

You asked why a long burn tunnel is bad.
To create the temperatures at which J-Tubes are recorded.
You must allow the gases free easy movement, although it looks like your J-Tube is burning properly, I assure you it is not.
The core portion of any RMH must be built to specifications if you expect it to work like we say it does.

In your example, the gases are slowed by their horizontal travel. When they reach the riser, they cannot get the 1800°F temperature that it is touted to reach.
This allows Ash to reach the transition area and enter the pipe system.
Increasing the riser height to compensate does not solve the problem.
After the riser, he is asking for 8" of hot gas to travel through a 6" pipe.
It fills with Ash, slowing it down to the point it will not light properly anymore.
I know this to be a fact, as my 15" burn tunnel roof, plugged with ash by mid-winter...
Prior to that, I thought I had a Bad Ass Rip Roaring J-Tube...  I did not.

I believe it needs a rebuild to "factory" specifications, if he wants factory high-performance results.

Hi Josh;
Your riser can stay where it is.
Your feed tube does not need to be removed, but the inside of the outer wall needs to come closer to the riser.
In other words, by removing bricks from the roof of the burn tunnel, 17" down to 10" or so.
The feed tube needs to remain 8 x 8 x 16, but it needs to move seven inches closer to the riser.

The current outer profile can remain the same, but the inner wall needs to move.
Your barrel will be close to the riser on the feed tube side and have a larger gap on the riser side.

Well, that changes things.
Don't make any changes there.
The fact is that he has an 8" stove, transitioning through a 7" reducer to a 6" piped system, with numerous bells.
We need pictures.

thomas rubino wrote:Well, that changes things.
Don't make any changes there.
The fact is that he has an 8" stove, transitioning through a 7" reducer to a 6" piped system, with numerous bells.
We need pictures.

Information about the house:

1800 ft.², two stories
Has blown in cellulose insulation in the roof spaces, whatever the state program would put in with their rebate thing
Cellulose in the walls??
The attic space over the kitchen (adjacent and open to the sunroom, where the rocket mass heater sits), weirdly, has zero insulation, it’s just an empty attic. The state rebate program would not put in insulation there unless it was certified to be rodent proof, and the exterminator was going to charge $2000. for a $300 rebate.  So that is still uninsulated.

Zone is 5A, temperatures last winter in the northeast were brutal, for long stretches of time, but still, I doubt it was as cold as a Montana winter.

Another piece of information: the liberator, which this Repair of a rocket ;Mass heater replaced, would make clicking sounds when it got up to temperature, and the new barrel does not make those sounds.

In terms of the amount of ash that this produces, he would get a lot of ash out of the feed tube, several gallons a season. That was true with the liberator, as well as with the Repair.  When cleaning out the pipe in the bench, there was only a quart or two of ash. It was nowhere near enough to block airflow or make starting the rocket mass heater difficult.

So, I think it would be worth making it a little less ashy, by moving the feed tube in 1 inch, and I’m wondering if it’s really necessary to widen the 6 inch channel and have a larger chimney.

The reasoning for the 6 inch bench was that, by some law of physics, the air is actually smaller once it has cooled, to a significant degree.

In some ways, it seems like the problem was too strong a draw.

The baffle experiment seemed to have no effect whatsoever.  (other than baffling me, as I have said, and I promise that’s the last time I’m going to make that stupid joke.)

Regarding using AI, I agree that it’s better to talk to actual people. But sometimes AI can at least throw out some possibilities that I haven’t thought of, if I’m not able to reach a person. Then I can reality test those possibilities. But so far, it has not scored any victories here.

Plugged after use: aluminum tray mashed flat, bricks holding it down. The firebox is smushed up against the lip of cob surrounding the barrel manifold, so a brick won’t fully seal it. A little air can probably sneak past. The mortar has fallen out of the space between the nearest brick and the ones on either side of it.  I should stick some cob in there.