Modified RMH?

Hello all,
I’m going to build an A-Frame cabin on sandstone bedrock which is only one-foot below grade. The cabin will be built using post and beam construction. For reason of not wanting to build a massive support system under the wooden floor joists to support the thousands of pounds of cob used in a rocket mass heater, I need an alternative method. Besides, I have no readily available material in which to make cob. But I like the idea of the J-tube and heating a mass of some kind, albeit one somewhat lighter than 3,000 pounds of cob. So armed with the limited knowledge I have and the little bit of research I’ve done, I’ve come here in hopes of getting some insight into the design I’ve come up with that would accommodate my needs and the limitations I am faced with. I am in no way trying to improve upon the rocket mass heater design, I just don’t like the idea of the 55-gallon drum, or a ton of cob. I’m simply trying to find a viable way around them.

I've attached a PDF file for a visual reference. I hope it shows up.

1. I have a basic understanding of the J-tube and its function. But by removing the metal bell, I’ve changed the parameters. I feel the heat riser would no longer have to be insulated because the heated gasses would no longer have an effect on the cooling of those gasses as they descend in the down-draft tube (which takes the place of the bell). Instead of the cooling gasses descending down the inside of the bell, they would now pass over a wall between the heat riser and down-draft tube. The top of the wall would have a, say, quarter-inch steel plate (which can be removed for inspecting and cleaning of both tubes). That steel plate would not only cool the gasses, but heat up fairly quickly and give off immediate heat into the room. Not to mention make some morning coffee.

2. The gasses inside the down-draft tube would also heat up the brick of that structure and radiate that heat into the room. When the fire went out and the system closed down, the two tubes would cool down at the same rate, and being that the heat riser tube started out hotter, it would always to be hotter than the down-draft tube continuing the upward flow of heat and into the down-draft tube. The system should continue to operate until the entire system cooled down.

3. When the cooling gasses reach the bottom of the down-draft tube, they turn 90° and enter the mass. A brick bench. There will be plenty of air space between the bricks to allow the gasses to travel within the bench, but not a direct path to the 8” stove pipe located at the other end of the bench. This should allow the gasses to heat up the bricks and warm the bench, and, hopefully, store enough heat to dissipate it slowly throughout the day.

4. The stovepipe exiting the brick bench will be supported off the floor by stand-offs increasing in height as the pipe makes its way to the outside wall some ten-feet away. The stovepipe will have 2-90° bends. One at the bench and one at the wall in the middle of the cabin (then makes its way to the outside wall). Because I don’t have the mass that cob would provide to soak up a lot of the heat from the system, I figure that the heat in the stovepipe would still be fairly hot and give up a lot of that heat as it travels along the wall inside the cabin. This would also be an immediate source of heat. And, because I’ve been told that condensation can accumulate inside of the stovepipe, I would put a small hole in the lowest end of the pipe to use as a drain, and might also be used as a propane torch hole to prime the pipe for a cold start.
So that’s it in a nutshell. Please show me where and why this system won’t work, or if it has a chance to work with some modifications, what those modifications might be. Thanks for any and all insight you might have.

Hi William,   I have not had any personal experience with your modified proposal so I really can't give you any advice on that however, what I can strongly suggest is to start off with a tried and true rocket mass heater that has had years of tweaking and innovations to get it to where its at before making modifications. The Rocket Mass Heaters builders guide is a great place to start. It will answer a lot of your questions.
What is also suggested, is to get yourself some bricks and make some mock up stoves outside to get a feel of it. Move the bricks around and get a feel of what changes occur to the burn each time you move them. The experience is well worth the time invested before building one in your house and possibly getting to the point of saying "Holy Crap!... What have I gotten myself into!!"
Look forward to hearing from your adventures!

If you have bedrock one foot below grade, you have excellent support for a massive RMH with little additional structure. A horizontal bench of cob or masonry of whatever kind can be easily supported by a reasonable timber frame anyway. It has been said that if a floor can support a king size waterbed, it can support an RMH.

The idea of eliminating mass means that you would need to keep the fire burning all the time when it is cold outside, and either overheat in mild weather or damp down the fire and create a smoky, inefficient, creosote-laden exhaust stream. An RMH duct configuration MUST be burned fast and hot whenever it is running in order to be safe. There is no substitute for thermal mass in an RMH, though there are many ways of achieving that mass. If you are short on clay for cob, rock mortared together with a small amount of clay would work excellently. If you build a masonry containment structure, you can fill it with any kind of dirt around the duct. Cob is just the best kind of dirt

You could eliminate the barrel and have a downdraft channel, but the heat riser would still need to be insulated, as that is a large part of what creates the complete combustion. This arrangement has not been tested that I know of, and I would advise making a full-scale working mockup outside before experimenting in your house.

Finally, you would be much better off regarding both draft and heating effectiveness if the exhaust goes up through the house and exits near the peak, rather than having a completely external chimney which will get cold and may be hard to start.

A similar core layout to your proposal has actually been published, the maker sells plans but it is easy to figure out if you are decent with masonry construction:
https://permies.com/t/59672/advice-RMH-heat-sq-ft#507403

This one is pretty small, but it could easily be scaled up, especially making the cross sections more square than rectangular for better flow. (I would do this by stretching the plan lengthwise so the riser is more like 6" x 9" than 4 1/2" x 9".)

Thank you all for the replies, you gave me a lot to think about. I'll be back!

Careful, downdraft tubes don't work very well in rockets. Most of the time.

Satamax Antone wrote:Careful, downdraft tubes don't work very well in rockets. Most of the time.

So, all downdrafts are not created equal? Please explain the differences between the downdraft in a bell design and the downdraft in a tube design. They both have them.

In my minds eye the main function of the heat riser is to thoroughly burn the gasses locked inside the wood. By the time the gasses reach the top of the riser, that function has already taken place. The bell or downdraft tube is simply a means by which the gasses would exit the heat riser, cool, then sink. By having the downdraft tube isolated from the heat riser, the downdraft tube would naturally be cooler since it's not absorbing any heat from the riser. It should function better than a bell design.

That's the way I see it anyway. Please correct me if I'm wrong and point out any flaws in my thinking.

William LeMieux wrote:So, all downdrafts are not created equal? Please explain the differences between the downdraft in a bell design and the downdraft in a tube design. They both have them.

In my minds eye the main function of the heat riser is to thoroughly burn the gasses locked inside the wood. By the time the gasses reach the top of the riser, that function has already taken place. The bell or downdraft tube is simply a means by which the gasses would exit the heat riser, cool, then sink. By having the downdraft tube isolated from the heat riser, the downdraft tube would naturally be cooler since it's not absorbing any heat from the riser. It should function better than a bell design.

That's the way I see it anyway. Please correct me if I'm wrong and point out any flaws in my thinking.

Satamax is right, not all downdrafts are created equal. I think you mean a kind of U tube, placed upside down. One leg being the riser, the other one being the downdraft channel. The difference with a bell is that all the hot gases need to pass through that U while a bell is spreading the heat load. Moreover, hot gases tend to stay in the highest region so the heat stress will be enormous in that spot. I've seen some examples build out of firebrick, within weeks the top of the construction cracked badly. I can't say anything about it being working correctly or not, although I'm inclined to think one would run the risk of equalizing the temperature in both legs so the draw would be hampered badly.

In other words: where heat stress in a barrel or bell is spread out, this isn't the case in a U-turn. So heat stress will be concentrated in the bend of the U.
But if you want to see for yourself, please build it and post the results.

Reading your first post over I come across the idea to place a steel plate of a quarter inch on top of the U-turn. Such a plate will inevitably warp spectacularly because of the temperature difference.

Here's a construction like what you are planning. I've met the builder during the Masonry Heater Association annual gathering in 2015. He told me this construction did crack all around the top three layers, leaking like mad.
http://endeavourcentre.org/2014/12/rocket-mass-heaters-with-andrew-brunning/

Hot gases tend to rise so in the downdraft leg you would be pushing hot gases down. Not sure the riser can push this.  It already strugles with a barrel without chimney.

Peter van den Berg wrote:
...Moreover, hot gases tend to stay in the highest region so the heat stress will be enormous in that spot.
In other words: where heat stress in a barrel or bell is spread out, this isn't the case in a U-turn. So heat stress will be concentrated in the bend of the U-turn.
But if you want to see for yourself, please build it and post the results.

I'm not sure that I follow you sir. I could see that happening in a stagnant system, but not one with moving air. A closed barrel left out in the sun will accumulate heat at the top of the barrel. The air at the bottom of the barrel will be cooler. But move air through the barrel and that is no longer the case, at least not to the same degree. Both systems have a high spot and what's good for the goose is good for the gander I would think. If heat stress is concentrated in the bend of the U-turn, then it is also concentrated at the top of the bell. What's the difference?

As far as the steel plate warping because of the heat differential, I hadn't thought of that. And that is why I posted here. To find the flaws in my plan. I liked the idea of a steel plate for two reasons. 1) for the instant heat it would provide and 2) for serviceability of the two tubes. I've seen a barrel removed and the accumulated ash at the top of the riser. It would be nice to just have to lift off the steel plate to remove that ash periodically. In a closed-off brick configuration, that wouldn't be possible.

So what if the steel plate covered only the heat riser opening at the top? Would it still warp and open a gap between the rope seal and steel plate? I was hoping the weight of the plate alone would keep it sealed. Or if the heat was great enough to warp the plate, perhaps it could be extended above the turn in the U-turn a foot or so? But I'm not sure that would make much of a difference. It might also provide the heat trap you mentioned.

What are your thought on these two points?

Satamax Antone wrote:Hot gases tend to rise so in the downdraft leg you would be pushing hot gases down. Not sure the riser can push this.  It already strugles with a barrel without chimney.

But isn't the same thing happening with a bell system?

Thinking more on the steel plate. What if there were two steep plates? One at the top of the heat riser, angled to deflect the hot gasses towards the downdraft tube. Then an airspace of, say, 4", then another steel plate, laying horizontal, to make up the top of the riser. There could also be a small airspace between the two plates for when the lower plate warps. That would allow the gasses between the top and bottom plates to be diverted to the downdraft tube. I'm thinking the airspace between the two plates would remain cool enough to not warp the upper plate. Sound reasonable? I'm not against building a prototype out back, but I'd like to work out the bugs on paper first. What think?

The bell or barrel  will have much more surface area to absorb heat around the top, and be able to cool more. The downdraft tube will after a while get very hot all the way down, and eliminate the cooling downdraft effect. If the downdraft tube becomes very large in cross section, it would assume more of the function of a bell. I still think it is beneficial to have the riser inside the bell of whatever sort, as the temperature differential between riser and bell versus riser and room is much less, so the same insulation will keep the riser distinctly hotter, which helps combustion. You can always spend more for insulation if you are determined to keep the riser out of the bell.

A deflector plate above the riser to absorb the main heat shock might help keep the top sealing plate from warping as much, as it would no longer have a centered blast of heat but uniform heat. I think it would still warp, but how much can only be known by experiment.

William LeMieux wrote:

Satamax Antone wrote:Hot gases tend to rise so in the downdraft leg you would be pushing hot gases down. Not sure the riser can push this.  It already strugles with a barrel without chimney.

But isn't the same thing happening with a bell system?

Nope

Hot gases tend to stagnate up top, and as these are cooling, they go down, and exit through the chimney. But because the volume of the bell is far bigger than heat riser and flue, the bell acts as a faster cooler than a downdraft channel, Reducing the hot plug effect.  A updraft/downdraft channel acts as a siphon, more or less, where the bell having more surface doesn't do this effect as much.  Gases, in the updraft/downdraft channel, have far less contact surface to cool down, hence stay hotter for a longer time, forming that hot plug, which doesn't happen in a bell. Theoretically this could happen in a bell, when all gases, and surrounding mass reach a steady state, being all at the same temp. But this can't happen as easily, as the bell's temperature exchange surface is bigger, and it's outer radiating surface is even bigger.

Ok, with all that said, let me clarify one thing (since I didn't include it in my drawing). I did plan on the system having an insulated heat riser. Firebrick making up the inside of the riser, insulating the riser with whatever type of insulation on the outside, then wrapping the entire riser with red clay brick. The downdraft tube would be the same configuration but without the insulation. Perhaps even bricks laid out so they stick out of the main body of the tube to act as a heat sink to cool the tube. The heat sink bricks could even act like miniature shelves to place little trinkets on. Does any of that matter in the grand scheme of things? And if it's surface area in the downdraft tube I'm after, perhaps the I.D. could be made larger? Is there a ratio of air volume between the riser and downdraft tube one should adhere to?

Well, i don't want to wreck your dreams. But to act vaguely as a bell, your downdraft tube should be 5 times the CSA of the heat riser. And that's an absolute minimum.

So, please, build it, and report.

Have you ever built a working Rocket before?

Satamax Antone wrote:Well, i don't want to wreck your dreams. But to act vaguely as a bell, your downdraft tube should be 5 times the CSA of the heat riser. And that's an absolute minimum.

So, please, build it, and report.

Have you ever built a working Rocket before?

So if I calculate the volume of air inside the riser tube, and multiply that by five, that's the air volume I should have in the downdraft tube?

And, I have never so much as even mortared two bricks together in my entire lifetime. I'm well below the rookie level here. But I have two years to figure it all out. How hard can it be!? (I'm figuring at this point y'all are wishing me a lot of luck.) :)

6.6"x6.5"x48" (the I.D. of the heat riser) = 2,028 somethings. Multiply that by 5, and 10, then divided by 2 = 15,210 somethings. That would be 7.5 times the volume of air in the downdraft tube as opposed to the heat riser. 18"x18"x48"= 15,552 somethings. The I.D of the downdraft tube. Do I have that right? I could make that, although I don't know how I would span the top of 18" with 9" brick. Maybe use the two-plate method as I did with the riser?

18" x 18" would be easy to cover with a corbeled brick arrangement, similar to the one in the link I gave. Or you could corbel in the top of the chamber by a couple of steps to say 9" x 9" and cover that with a plate, for easy inspection and maintenance.

Thanks Glenn. Speaking of the link you gave and to the guy's build, his arrangement and description of his system appears to have the heat riser and downdraft tube of similar dimensions. If one is larger than the other, it's certainly not by 5 times. Is Satamax pulling my leg here?

Not taking the mick.



5 times CSA of a downdraft channel, i haven't spoken about ISA nor volume.

We don't know how well that system really works, or if it depends on assistance from the down channel at all. Being all one brick thick, that system would shed heat fairly rapidly from all surfaces, and might get some down channel cooling, at the cost of less heat retention in the riser and less complete combustion.

Satamax- I'm not sure what the animation is suppose to represent sir. Nor do I know what CSA or ISA means. If you would be so kind...

William LeMieux wrote:Satamax- I'm not sure what the animation is suppose to represent sir. Nor do I know what CSA or ISA means. If you would be so kind...

The animation is the movement of gases in a bell.

CSA, cross sectional area of the heat riser.

ISA internal surface area of a bell. Or pipes in the bench. Which is heat extraction surface.

Satamax,
No matter how long I study or stare at the animation, I can’t find the connection between that design and mine (if I can call it that).

See if I have this correct. The main goal with this type of heating design is to store the heat generated by the fire. A secondary function of generating instant heat can also be incorporated into the design if we so choose, but at a loss of total stored heat. An acceptable trade off in most instances, and usually preferred.

A metal 55-gallon drum is certainly the way to go if one wants to pump a lot of heat into the living space in a hurry. There’s a lot of surface area there to give off that heat. But what if it starts to get too hot in that living space but there isn’t enough heat yet stored in a mass to last throughout the day and night? (if that’s even possible. Some say it is.) I don’t have nearly enough information yet to know how often the fire needs to be lit and attended to so all I have to go on is knowing what I want to achieve and a life’s worth of common sense to try to figure it out. And what I think this system is able to achieve, if designed correctly for any given set of circumstances, is to give off a measured amount of heat immediately, just to take the chill out of the air, while at the same time store the majority of the heat produced to maintain a comfortable atmosphere throughout the day. That, at least, is the goal I have in mind. I’m thinking that the metal drum is simply going to give off too much heat. Heat that I would prefer to save for later. I don’t want to have to open a window to keep from overheating.

But from a maintenance standpoint, the drum has an advantage over a fully brick-encased heat riser. I would think it would be easier to replace the drum, or just remove it, tend to any issues inside of it, then replace it. Unless the drum were partially encased in cob. Then it would be a draw. With a brick encased heat riser the entire assembly might have to be demolished and rebuilt should any issues arise. These are things I consider as I move forward. But the brick riser has an advantage over the drum for the fact it won’t readily give of its heat but rather would transfer it to the mass for storage.

I’m not sure if a heat riser is dependent on a bell or not. That is, that the heat on the outside of the riser created by the heat inside the bell, is necessary for complete combustion of the gasses created by the wood. But I would think not. The system would burn dirty until the heat penetrated the heat riser’s insulation and firebrick. So with that train of thought, with a stand-alone heat riser, more heat is available for storage, and less heat is lost into the living space.

Then there’s the stand-alone downdraft tube. Another disadvantage if there were issues on the inside. But there’s a lot less heat involved so I think less likely to have issues. Heat related issues anyway. There’s two ways I look at this design since posting this thread and getting feedback. It can be a simple transfer tube, or be used as a heat sink. If the volume of air inside this tube were the same as the heat riser, it would be a transfer tube. If the volume of air were greater than the heat riser, it would become a heat sink. I liken it to a solid rod being pushed through a similar sized hollow tube. The rod would displace about 99.9% of the air in the tube. Make the rod diameter smaller and pass it through that same tube, the rod will displace less air, and in incrementally smaller amounts as you downsize the rod incrementally (The same being true in increasing the volume of the downdraft tube). If all of the heat in the downdraft tube is not continually and completely replaced, the tube will become hotter faster because hot gasses will linger in the tube (instead of getting displaced) heating it up, especially at the top of the tube since hot air rises. There would now be less heat available to heat up the mass in the next part of the system. I think if one does want to get some relatively instant heat from the system, I think the downdraft tube is the place to get it from. It’s inside diameter being the determining factor. Please correct me if I don’t have the science correct on this one. It’s a grey area for me. As is most of the system.

TLDR

The riser inside a bell or barrel does not depend at all on heat coming in from the bell; it should be insulated and preferably lightweight so that the immediate heat of combustion warms the inner surface very quickly, ideally coming up to full operating temperature within 5 to 10 minutes. Heat will slowly pass through the riser wall, and will warm the bell some but hopefully not much beyond the hot gases passing down around it.

If the downdraft tube is similar to the riser, hot gases will move quickly through it. It will all get very hot, since the gases will have moved on before transferring much of their heat to the tube. There will be more heat left for the following duct or channel.

A bell, being very large relative to the riser, will let the gases move slowly down through it, giving up heat to the much larger internal surface, and leaving less for the following channels. This is the same for a barrel or a masonry bell, the difference being that the barrel lets the heat pass quickly on to the room while the bell passes the heat through layers of masonry on its way.  Gases do not move as a plug leaving the perimeter undisturbed; they generally move as a whole, with some degree of turbulent mixing depending on the design.

I would never build a masonry bell without including an access panel for inspection, cleaning, and if necessary, riser repair or maintenance. This panel if sized and located appropriately would serve some instant heating function.

I'd like to thank you all for the input you've given me in this thread. It was valuable indeed! I'm going to start gathering materials to build the thing and see what happens.
My local Big-box-store has 4 1/2x9x1.15" Pelletized Vermiculite Firebrick that is rated at 2,300*F. They weigh in at 2.7 pounds each. Is this material suitable to use for the heat riser, etc.? I also found 1" Ceramic Fiber Blanket that is also rated at 2,300*F, has an 8-pound density, an R-value of 5, and is made from Alumina-Silica. Is this insulation good for the outside of the heat riser?
Keep in mind that this design is with the U-tube configuration, not a traditional bell design.

I'd like to incorporate into the design of my RMH, an outside air supply for the system. It would be a simple matter of running a pipe through the floor of the cabin near the burn tunnel. I was thinking about locating the pipe at the bottom of the wood chamber pointing it directly at the burn tunnel. The air would flow across the bottom of the burning wood and continue on into the burn tunnel. I would have a damper in the pipe that would close off the air supply when I shut down the system for the day. Is there anything wrong with this idea?

Not wrong as such, but you do need a certain amount of air exchange inside your space for health and comfort, so I would route the air supply (say) past or through the hot core enclosure, letting the heat warm up the incoming air, and letting stale room air be what goes to feed the fire.

You do not want to add an air supply at the base of a J-tube, as that will reduce the draft through the feed and increase the likelihood of smokeback.

I think your firebricks would be suitable for the feed tube and burn tunnel. I would use the ceramic fiber blanket as the inner part of your heat riser, as it will come up to full operating temperature instantly. Look around for "5 minute riser" and see that this is the latest design that innovators are using.

I agree with that, though outdoor air will come in somehow, and deliberately running it through a warm mass would be good.

Thank you gentlemen for your insight into my query. It gave me a lot to ponder. And Graham- that was an interesting article, I thank you for that.

Now about the Ceramic blanket (which is also available in board form) on the inside of the heat riser, etc. I immediately saw how that would be useful. Being exposed to direct flame, does the blanket, or board, ever need replacing? Does it wear out over time?

Also, if the insulation was on the inside of the firebrick, I would think the firebrick would expand and contract less and therefore less likely to crack at the mortar joints? Is this a correct assumption? I'm asking because of the video above where the guy said his brick bell eventually cracked and leaked real bad. If the ceramic blanket were below the top layer of brick (at the top of the heat riser), perhaps the cracking, and therefore leaking, might have been prevented?

William LeMieux wrote:Thank you gentlemen for your insight into my query. It gave me a lot to ponder. And Graham- that was an interesting article, I thank you for that.

Now about the Ceramic blanket (which is also available in board form) on the inside of the heat riser, etc. I immediately saw how that would be useful. Being exposed to direct flame, does the blanket, or board, ever need replacing? Does it wear out over time?

Also, if the insulation was on the inside of the firebrick, I would think the firebrick would expand and contract less and therefore less likely to crack at the mortar joints? Is this a correct assumption? I'm asking because of the video above where the guy said his brick bell eventually cracked and leaked real bad. If the ceramic blanket were below the top layer of brick (at the top of the heat riser), perhaps the cracking, and therefore leaking, might have been prevented?

http://donkey32.proboards.com/thread/3378/minute-riser-owners

The ceramic blanket as heat riser would eventually need replacing, but it would probably last several years, maybe decades. It is mechanical abrasion that kills that material, which is why I would not use it inside the feed or burn tunnel. The riser has nothing but hot gas, which is what the blanket is designed to handle.

Using some ceramic blanket or board where the riser exit hits the top of the enclosure would protect it, or just using firebrick could be enough, as that is designed to stand up to those conditions.