Looking at external air source.

I have looked at the information on RMH and the single biggest flaw I see is that living in a passive solar house the house is incredibly tight so if I am going to go RMH the combustion air needs to be externally sourced. Nearly all the plans are for internally sourced air.

This leads me to the second thing I am not understanding and that is talking about adding turbulence to get good air mix. Turbulence equals lost energy so that goal should be reducing air flow. Which if you have to much already may be okay but if you are adding the restriction of piping to draw external air it is possibly a problem. Here shouldn't the goal be maximum air mix while maintaining laminar flow to reduce losses? This thought leads me to think the guys who are trying to get a vortex in lower part of the riser tube are on the right track. The trouble is they are all restricting the flow in some form rather than maintaining constant cross sectional area. Success seems to be mixed there. But what if we did it and maintained the constant cross sectional area rule?

Now another thing in my thinking is I understand mass heat storage having lived with as the passive solar heat storage for basically 30 years. I know that to recharge the walls of the home I want to apply slow heat over a longer time. To me that means a small fire running steadily for hours.(Yes I realize I can heat one mass which will shed its heat to the other masses over time.) So that of course leaves me asking the question that I have seen asked probably a hundred times here about how can I put more fuel in the stove but burn it slowly and still stay clean. Does using outside air change how this might work?

Having been around numerous wood stoves growing up I know that I hate cleaning ashes. We had 2 stoves that stood out in this regard in a good way. The first one was a wood and coal cook stove that because of how the ash was channeled into its pan was nearly totally mess free to pull the pan to take it out. The other was a stove with a huge ash pan. It was poorly design so the ashes fell around outside the pan so taking it out was messy but it was large so you only had to do it once or twice a month. RMH with their lower fuel use should improve on this but they don't have a good ash collection system in any I have seen video or online pictures of. External combustion air ones the most common thing seems to be to let the air come in the bottom of the firebox and the ash fall through into the pan. But from the videos it is obvious this leads to at least some unburned material. So lets relocate the ash collection point in this process to have maximum time in the flames for the ash but collect it as best possible.

Finally I have followed the arguments about wanting the coldest air possible going into the stove because that brings more oxygen in because of greater air density. They then use the engine argument to prove the point. But the people arguing for this seem to be getting mixed or bad results. To my thinking the problem with this is that unlike an engine the wood stove has to vaporize its own fuel first and hot air should help with that if we can source enough of it.

So all together is leads me to the attached drawing. Lets start with the feed tube. The rules limit its height to prevent back burns. But what if if it was built of smooth steel and such that it opened up as it went down and added an air tight door at the top. For added protection build the door so it fell shut under gravity and a fusible link support like is used on solvent tanks so if it ever back burned it would fall closed on its own. Since the feed tube is so long we will want a lower access so add a fire box level door also. Set it back a short distance from the firebox to keep embers from falling out. Add airflow in along its sides to flush ash back towards the fire and source. If the doors are air tight there should be no smoking back and no back burn under normal operation?

Now from the firebox lets taper the tube up and down but narrow it to maintain constant cross sectional area clear to where it enters the riser. The taper up should help it draw while lighting the fire and the taper down should help the ash flow in the breeze to the collection point at the bottom of the riser. By narrowing this now the it is possible to feed the air into the riser to create a vortex. Notice the spiral is tighter at the bottom than at the top. The bottom should be the coldest air and it will be fed up the middle of the vortex by the tighter spiral. The fact that it is heavier should cause it to mix in a laminar fashion as everything spins. The hottest air should be near the top of the feed with most of the ash having fallen out of it.( For now skipping it but an air injection possible here)

Beyond this though I am looking at 2 other thoughts First to prewarm the air incoming as much as possible for the cleanest burn. What about running the air intake line right down the middle of the chimney out. It will mean increasing the diameter of the chimney out enough to maintain cross sectional area.

The second thing here is has anyone tried adding a vortex cleaner to get the ashes out after the barrel so the chimney doesn't need to be cleaned out as often?

So where am I being stupid or missing the point? Other suggestions? The shape would be complex to build but is well within what I could construct from cardboard and paper mache and then burn out later.

A couple of thoughts - turbulence in the burn tunnel and heat riser is, I believe, desirable because it gives a good mix of the Fuel and the Air. Complete combustion is dependent upon this mixing.

So, lamellar flow of the air coming into the feed box and then into the burn tube is a fine thing - and if you're drawing outside air in, you want minimal drag on that, so turbulence not wanted in that area. But in the actual combustion zones, you want turbulence to get the mixing for complete combustion.

As to cold air versus pre-warmed - I would agree that pre-warmed is a good idea, and there seem to be quite a number of designs that have been developed to provide for it.

With regard to ash coming out of the barrel - that is where most designs include some sort of ash pit and a clean out access, with much of the ash that gets through the heat riser ending up in this ash pit.

I think trying to bring outside air back in through the chimney gets very complicated. You can't bring it back through the heat riser, so there needs to be a point where the incoming air diverges from the chimney to bypass the heat riser. Seems to me it's just easier to have a second run of ducting, that might run along the chimney for some distance to pick up some pre-heating, but not trying to run inside the chimney. Also some issue of cooling the gases in the chimney too much and inhibiting the draw.

I am afraid I am not following your various internal shaping ideas clearly, so I don't have any comment on them

C. Letellier : Your thoughts were laid out clearly and in an organized manner, and it will be a pleasure to follow them and give you my best answer !
Unfortunately, I could not get any thing from your pictures, I probably am blinded by what I think I should see not what is there !

I understand your concerns about lost energy due to turbulence , if We were pumping Tar sands oil or flowing gas from a series of fracked Nat Gas
wells, I too could get excited, but in this case the Turbulence along with Exposure Time, and Temperature (our trinity ) give us our Clean burn, and
the energy lost due to to turbulence shows up as Noise the rocket in our rocket stove, and heat, imagine that a heater that makes heat as a by-product !
This may be judged to be simply' 'but, we've always done it this way!", I call it 'if it works don't fix it !'

Again you stated your clear view of how Heat Energy storage in a thermal mass is usually accomplished, But because Within 3-5 minutes or less of
starting our fire we are at or beyond 1000*F heading towards 2000* Fahrenheit and combustion efficiencies over 90% and near the theoretical limits
of burning a hydrocarbon fuel with Room air we have a lot of heat energy that we HAVE to dump some where now, and we have evolved in our craft
enough to use the 20', 30', 40' plus! of horizontal flow to capture that energy leaving a pittance ~140* F~ for our final temps in our Vertical Chimney !

Oh, what you said, There is so much crap in U-Tube land that It is nearly impossible to pick out the good ones! As soon as one idiot shows up with an
other Franken-stove of flaming death, We can be sure that 4 months later there will be another crop of Franken-clones. Near the end of this I will send
you to a video where you can get the back-story and be satisfied that we do indeed deal with our fly ash better than any stove you have seen before !

Any one who has done any winter out door camping can tell you the colder the air that we oxygenate the fire with, the more wood we burn, houses
actually help us conserve our wood fuel as long as we do not try to warm our houses with warm air to warm ourselves !

Because we have moved to lightweight Fire brick and surround the combustion chamber bricks further with ceramic refractory materials 'Steel is Dead'!

I have found the clearest information on what happens to steel and why in the High Temperature Hydrogen attack articles, the one in wikipedia is Very
good !

When it comes to vortexes , I have always believed that the hottest gasses were trapped in the center of the vortex and basically all they added was more
turbulence, It was also my understanding we were trying to break laminar flow as at the materials surface where we wanted heat exchange to take place
we had to deal with a layer of static air and Laminar no-flow, I hope I have this right, because of the difficulty of relearning it correctly if I am wrong !

I love vortex inline cleaners, but have always assumed that they had to have suction to work, now I have to refigure on that Definitely !

Running the 'make-up air' past our hot exhaust gases would require some more refiguring and rebuilding as the Exhaust gases would have to be flowed into
the chimney/heat exchanger to be sure that they did not cool to the point that they would stall !

Find the Permies toolbox near the top of the page, and do a search for Jesse Biggs here in Permies who is attempting to make a mold for a Rocket burner as
we speak !

If you find the permies>>forums>>rocket stoves at the top of this page and click on the rocket stoves you will find the total list of all rocket stove
threads, 'latest visited' at the top, you are looking for video: great rocket mass heaters in upstate N Y and Quebec

I know the lady in the first video and can tell you she does not know there are any RMH agendas ! watch and enjoy Big AL !

I will share a little background information first off.

I have spent nearly 30 years keeping the coal stoker that heats the shop running. It is being pushed far harder than it was ever designed for. So I know about environments that eat steel like cotton candy in water. Part of it runs so hot that best fire brick you can commonly buy is eaten in 4 or 5 years. To replace metal parts near the combustion front takes monels because inconels and stainless steel are not good enough. The original cast iron parts have long since failed in the worst of that environment. So I don't think anything a wood fire can do damage wise will more than mildly surprise me.

On other fronts I have done pottery so I have some familiarity with molding clay to complex shapes. With normal pottery you would need to be able to remove the internal mold before the clay dried to keep shrink from cracking the clay. But having watched a bunch of the videos it looks like they are getting away with immediately burning internal molds out while the clay is still wet. Since I did a bunch of scale models in cardboard growing up I know I can build a cardboard form and cover it with paper mache to get nice smooth curves and shapes in the shape I envision. This shouldn't take much more to burn out than the wood forms others are using for the core molds. I assume they get micro cracking in the inner liner layer but if the layers are fiber reinforced it doesn't look like a problem. At least everyone seems to be getting away with it. I can source fire clay in 50# bags through the same outfit in Helena that I have been getting pottery clay through for a reasonable price and the fiber glass is just a matter of visiting the insulation blowing machine at the lumber yard to have it already fluffed and shredded. The pearlite is the most difficult part and it sounds like that isn't bad. So the casting part doesn't sound that bad.(there are always lessons to learn doing something new of course)

Now you say it has always been done that way and is working. Thing is there seems to be very little good information on external air sourced rockets. So is it working for them? The catch is that a foot of pipe is a foot of pipe where the stoves is concerned. So if a 6 inch rocket could support 30 linear feet of 6 inch pipe if I spend 15 linear feet of pipe to bring my air in that is 15 ft of pipe I can't have on the outlet end. Now suddenly that drag and turbulence is far more important. And that is why I am concerned about it. Most of the videos of people running external source air seemed to be having a problem with cold air quenching the burn at least some of the time. Run on inside air the stove rocketed but on outside air even with a short inlet pipe suddenly it doesn't rocket or doesn't rocket reliably. My guess is the cold air is quenching the burn to much. If I am going to preheat the incoming air and do it in a short distance then I need a really hot heat source or I need a cooler heat source and a longer/larger heat transfer area. The trouble is the only really hot heat source is the fire box, burn tunnel and riser tube. And I don't want to tap any of those for energy because if I do am destroying the very thing that makes a rocket stove work and valuable. That leaves tapping room temperature, tapping the thermal mass bench, tapping the exhaust pipe or tapping heat in the barrel as possible. Others have done tapping the mass. It doesn't sound like it warms the incoming air enough in cold weather and it will be really slow to start up when everything is cold. Tapping the room heat is going to demand a fairly large heat exchanger plus it is mildly defeating the purpose. That leaves the barrel and the exhaust stream as possibles. Because of limited size I question whether the barrel by itself is the answer.(it might be as I can see 2 possible ways to use it.) I don't have the answers I am simply asking questions.

I agree much of the You tube videos are garbage. They violate primary design criterion and violate good science in many cases. But some of them have things to teach too. For example one of the ones to come out of those was the fact that the inlet and outlet needed to be on the same side of the building to keep pressure equalized in windy conditions to help prevent back draft conditions in an external air source system. That one makes total sense. I know that from the fact that under the right conditions the exhaust fan at the shop runs at nearly full speed unplugged because of those sorts of pressure differentials on the sides of the building. Part of the purpose of discussing it here is to help filter the junk that has crept into my thinking out.

As for stopping the air flow through the stove as long as the heat riser and barrel are doing their jobs and the pipe run doesn't get to long, it shouldn't matter how much the outlet air is cooled should it? since they push the stuff out the stack from there. The other thing is that air heated going into the burner should be heat that is almost without loss. The fire burns hotter if it starts with hot air so most of that heat will simply be recycled repeatedly.

As for why on the vortex in the burn tube. If we can complete the burn low in the riser that means we have more energy to lift the rest of the height of the riser adding velocity to the air vs a burn that actually finishes on the bottom of the barrel. Added to that say the vortex is wrapping around the tube at a 45 degree angle. That means for ever 1 ft vertical rise, that it has travelled 1.4 ft and it has been scrubbing the wall of the tub hard from centripetal acceleration. There is the concentrated heat we want for a clean burn There is the increased time in the hottest zone we want for the clean burn. So the question is can we get the air mixed with it well enough to achieve the clean burn without major turbulence? That one is one I don't know. And I will admit the vortex consumes some energy too in friction so it may not be the answer. But my guess is that it uses less overall energy than turning the corner with turbulence. By having the burn chamber get tall and skinny as it goes to the riser tube I am mildly violating one of the principles of the design. That is square or round cross sections for minimum surface area for a given cross sectional area. So my heat loss into the refractory will be higher because there is greater surface area approaching the riser.. But if it is well insulated this should mostly be reduced to so small it doesn't matter.

As for vortex cleaners they do not need vacuum to work. They need flow velocity and that can be either pressure or vacuum to cause it. Since centripetal acceleration = (V^2)/r the higher the velocity is the better(exponentially better) and the smaller r is the better. They are simple a circle for the air to race around and a larger area open hole in the to for it lift out the center of the top while the debris falls downward in the cone. We had then on grain grinders growing up that only had the fan pushing them on an old belt drive hammer mill. The ground grain fell out the bottom and the really find dust and air went out the top.

I will try doing the pictues a different way later but lets try a verbal description of my thinking. Lets start out at the air inlet. Right at the right angle bend going up outside lets put a bigger boxy shaped elbow in. Lets put a bunch of small cross tubes like a scaled up version of the heat recovery units normally put in stove pipes in to try and tap the 140 degree outlet exhaust. Lets say we are at 0 so maybe this will get us 70 degrees in the first step. It should also produce steady condensation on those cross tubes and that moisture should act as a minor ash filter for stuff going by. From there drop into an air inlet pipe running right down the middle of the horizontal chimney pipe run. For a 6 inch stove this will probably be an 8 inch pipe to mimimize friction losses and improve heat transfer. Around the outside outside of this will be the chimney pipe which will be 10 inches. The difference between the 2 gives the 28 square inch cross section of our 6 inch pipe equivalent for the constant cross sectional area on the exhaust stream. This run should maybe have the inlet air up approaching 200 degrees by the time it is time to skip out at the barrel. Now here comes one of the questions in this. Can we rob heat for the loop in the barrel too? If the riser is 6 inches and its outside is 10 inches then an 11 inch pipe placed around this and a 13 inch pipe is placed around that and capped at both ends we would have a heat exchanger to fit inside the barrel that would allow flow down both the inside and outside. Maybe we can get the air another 100 degree hotter bringing it to maybe 300 before it goes to the firebox air feed. Since we are not feeding the fire box from the top instead lets feed it from the sides. Use curved half pipes to route the air up and over the edges of the fire box and back down in. Carry these half pipes clear out towards the outer door at firebox level so there is air coming from up stream of the fire box. This should do the same thing as the down draft in a interior air sourced stove. Lets say the burn tunnel starts out as a rectangle 4 inches high by 7 inches wide. By the time it is up to go in the riser tube it is a vertically oriented rectangle with round ends top and bottom. There lets say it 2 1/2 inches wide by 9.2 inches high with a 2 1/2 inch half circle on each end. Box 9.2x 2.5 + 1.25^2 x pi = 28 again. Dump this tall narrow stream into the riser tube offset to one side so that it will create a vortex in the tube.( a quick aside if you are having trouble picturing this take a toilet paper tube. On one end squish it to a square or a rectangle and on the other squish it to a narrow vertically oriented oval with flat sides.) If it works the vortex should complete the burn low in the riser tube so we have maximun heat energy to drive the lift part of the cycle up the riser tube. Now out into the barrel we should have not only the barrel cooling increasing draw but the heat exchanger for the inlet air cooling the air. These 2 things should cause the stove to pull air harder if I understand RMH design. Now comes the question of can we add a cyclonic filter of some sort. A single large one would probably not filter good enough so can we create 4 exits from the bottom of the barrel to sort of put one on each corner of the barrel. The exhaust stream from these will come out straight up and now it is time to go horizontal for the horizontal exhaust pipe run in the area between the 10 inch out pipe and the 8 inch inner pipe. Now we will lose more heat to the air in pipe than we would have under the old system. But remember the fire should be hotter because it didn't have to heat that air as much to make the burn. So much of this heat should just keep running around in a circle so the total heat to go into the mass should only be reduced slightly. Now there are some expected problems with this. The first is if we run a cold pipe down the middle of a hot steamy pipe we can expect condensation. Also because that pipe is damp we can also expect it to collect a certain amount of ash from the exhaust stream coating the pipe and reducing its heat transfer capability. The dripping condensation will need to be collected to minimize corrosion instead of just leaving it in the system. From there the exhaust will steadily be cooled by the inlet air and by the mass. At the outlet it will hit that whole cluster of cross tubes as it turns the corner to go up the stack and be cooled even more.

Very interesting insightful thinking though somewhat verbose:) I love intelligent discourse.
Not sure what I can add to this right now. It's a lot to take in. I think I'll read again later and ponder a worthy point of input.

C Letellier : W.O.W., An old farmer would say that ''that cud had been well chewed!!'' Again you raised your points clearly, laid them out well and your points
all seemed to flow one from another! You should have your own blog! Seriously I think I would check in even if it were about button hook collecting !

I too am going to need time to digest your flow of ideas! Only a few times in my life have the arrival of ideas arrived so fast that I felt as though I was trying
to drink from the stream of a large diameter fire hose !

Since you are not shy about sharing ideas, would you consider going to the permits toolbox near the top of the page clickon> the My Profile Button, and then
on the next page follow the prompts and add your general location and climate zone number ?

Because we a designing our final exhaust temps at our (usually internal) vertical chimney to be at 140 * F, we are already seeing the recapture of the 'latent
heat of evaporation' before our vertical chimney but only rarely do we ever see moisture problems in our Cob Thermal Mass After it has finally dried out !

Do you think that the Lady in the video I sent you to who is so anal retentive she saved 5 months of Fly Ash would tolerate a leaking heater ? also as we are
producing fly ash only, there is no lye being formed to corrode the system ! Win -Win !

My form of dyslexia, which predates my T.B.I. means that when I finally get any idea lodged firmly in my brain, it is then part of my structured universe and
I actually like thinking safely and comfortably ''with-in' the box'', however here at Permies we try to appreciate people like yourself who can think outside the
boxFrom people like you our future, our continued existence flows. I am very glad you are on our team !

I am thinking on how to add a cyclone fly ash collector, partially as an possible entertainment factor!

For now I leave you with the final thought that Over and Above everything else, the number one thing our Rocket Family needs is a big picture window to
allow us to see the pretty flames and that is washed by the high temperature flames so as to be self cleaning but still refracts most of the heat back into the
flames, A different form of Aerogel, or maybe some Transparent Aluminum ? For the Good of the Craft ! Big AL

So that of course leaves me asking the question that I have seen asked probably a hundred times here about how can I put more fuel in the stove but burn it slowly and still stay clean. Does using outside air change how this might work?

It is not possible to burn slowly and cleanly at the same time. Contrary to the laws of physics.

Outside air does not change how this might work. The rockety sound from burning wood comes to some extent from the expanding gases released when the wood is heated. Without turbulence, some parts of the gases would not be mixed in properly and consumed by the heat of the fire.

We have tried adding some coal to our rocket heaters and it doesn't work the same as wood. We can add a little for some additional BTU's, but we can't have a fire of just coal. Our theory as to the reason is that the gas expansion which occurs with wood does not occur with coal.

Peter van den Berg has a background in masonry heaters. He perfected a design for the burn tunnel; you can read about his testing, etc. on the Donkey board. Dragon Heaters is the licensee for his design; I am a partner in Dragon Heaters.

His burn tunnel has features which preheat the secondary air and induce turbulence.

Cindy Mathieu wrote:

So that of course leaves me asking the question that I have seen asked probably a hundred times here about how can I put more fuel in the stove but burn it slowly and still stay clean. Does using outside air change how this might work?

It is not possible to burn slowly and cleanly at the same time. Contrary to the laws of physics.

Actually if I understand correctly it is possible to do just that because it is something an RMH already does. Because the sticks only burn very slowly up wind and into the in rushing air. Since they are being burned off as fast as this slow progression into the wind happens you don't end up with the whole stick on fire at once. I will bet that if you can get long straight stuff feeding into any RHM that the fire would never progress up it as long as it didn't get jammed. If it falls smoothly and steadily into the fire box as it burns off it should be possible to burn sticks that reach the ceiling. In a normal rocket stove it would be dangerous because if anything jams the fire could progress outside the in rushing air and then you could have the whole stick on fire at once very rapidly. But if you can enclose the upper part of the sticks in an air tight chamber they shouldn't be able to burn very fast upward because their own smoke rising and lack of oxygen should snuff them out in the upper regions. Now you have a slow burn of a larger quantity of fuel.(Or am I missing something??) An RMH burns clean because of its high heat internally and because it limits the burn rate by limiting the amount of fuel on fire at one time. This is also why though you can't increase the size of the firebox. Because then you have to much fuel on fire at once and it can no longer burn cleanly.

Covers over the Feed Tube and long Feed Tubes : A really quick scenario, Jim Smith has started the evening fire an hour before he is going down to the lodge to
meet the boys, he knows lighting the Rocket Mass Heater RMH is not his wife's favorite task, she will be home shortly and does not mind tending the fire after it
is going !

Opps, an hour later, the time has gotten away from Jim, and his wife is not home! What to do! The RMH is up to full operating temps, with a dull red glow
when you look deep inside the Burn Tunnel, that last chunk seemed to spontaneously burst into flames !

Jim knows as the last guy there he could get stuck for the drinks, and -well a happy Wife makes a happy life !

In desperation, sure that his wife will show up any minute, Jim seals off the top off the feed tube with one large smooth paver, and leaves !

The rockets stove deprived of air flow and oxygen still continues to pyrolyze the fuel, filling the entire combustion chamber with flammable gases, searching for
oxygen!

Jane Smith, comes home sees the capped Feed tube is puzzled, she was sure that her hubby would have started the fire for her before he took off for his 'meeting',
but gathers up tinder and kindling before starting to slide the cap stone away -

INSTANT Flashover, as the oxygen meets the superheated fuels the flame front moves at the speed of an explosion as the tinder and kindling in her hands burst into
flames, so does her hair ! Poor Jane, poor Jim, unhappy wife, unhappy life !

This scenario brought to you to reinforce the idea that forming seals at the mouth of the Feed tube has the potential to create more problems than it solves.

We say fast burning you say slow burning, is it possible that the reports of how little wood the RMH, 'uses' has convinced you this is a slow process? We limit the size
of the individual pieces we burn, but burn many of them at a time this is part of the reason for its reputation for stingy-ness, the rest comes from the number of hours
we don't run the RMH but it is still heating the house! Again, you say slow we say fast, just as long as no-one gets called 'half fast' Big AL !

In a properly designed rocket burner, the tips of the pieces of wood which are burning are burning fast. The heated wood pops and crackles because its releasing gases; many of these gases are combustible. If the fire is not hot enough and the oxygen (20% of fresh air) not sufficient, these volatile gases will not burn. When the volatiles do not burn, they go up as smoke. This is what I mean by burning cleanly.

So, maybe you could arrange a holder for your long sticks to that they would gradually fall into the feed tube. We have found that, in general, nature is not cooperative in this area. Most sticks have bends and knots in them, keeping them from falling neatly into the fire.

C. Letellier wrote:

Cindy Mathieu wrote:

So that of course leaves me asking the question that I have seen asked probably a hundred times here about how can I put more fuel in the stove but burn it slowly and still stay clean. Does using outside air change how this might work?

It is not possible to burn slowly and cleanly at the same time. Contrary to the laws of physics.

Actually if I understand correctly it is possible to do just that because it is something an RMH already does. Because the sticks only burn very slowly up wind and into the in rushing air. Since they are being burned off as fast as this slow progression into the wind happens you don't end up with the whole stick on fire at once. I will bet that if you can get long straight stuff feeding into any RHM that the fire would never progress up it as long as it didn't get jammed. If it falls smoothly and steadily into the fire box as it burns off it should be possible to burn sticks that reach the ceiling. In a normal rocket stove it would be dangerous because if anything jams the fire could progress outside the in rushing air and then you could have the whole stick on fire at once very rapidly. But if you can enclose the upper part of the sticks in an air tight chamber they shouldn't be able to burn very fast upward because their own smoke rising and lack of oxygen should snuff them out in the upper regions. Now you have a slow burn of a larger quantity of fuel.(Or am I missing something??) An RMH burns clean because of its high heat internally and because it limits the burn rate by limiting the amount of fuel on fire at one time. This is also why though you can't increase the size of the firebox. Because then you have to much fuel on fire at once and it can no longer burn cleanly.

I think you are misunderstanding. The rocket burns cleanly because it burns hot, which also means fast. It is a question of three ingredients, fuel, energy and oxygen. The feed tube is typically small not because you cannot make it work at a larger size, but because if it were much larger and still getting the needed oxygen levels then the fire would be too much energy for the purposes RMHs are used for.

If we took a cubic foot of wood in one inch square sticks, put one into a rocket mass heater and one in a conventional fireplace, I am confident the RMH would burn through the fuel much faster. The RMH has the benefit of being a forced air fire, while not needing energy to drive a blower. THe air flow through the burn tunnel kicks the fire to a different level than in more conventional systems.

On the matter of heat in the chimney, you need it there to keep the flow moving. You cannot rely on "pressure" from the barrel to push the air out the chimney. You need the air coming out of the chimney to rise because it is lighter than the surrounding air.

I do understand the RMH is a fast burn. That much was obvious by the batch lengths in the stories on it. When you can light and then burn the batch in 20 minutes or so and then compare that to lighting and burning the wood stoves I grew up with that were just barely producing heat in 20 minutes let alone heating the house.

The reasons I was looking for longer burns without tending is that being in a passive solar house heating and cooling need to happen slowly because changing the temperature on the mass happens slowly. So my thinking was that the goal was the smallest RMH it was possible to build with longer run times to minimize the amount of heat the house took in at one time. It may be the mass of the RMH alone would accomplish that. Done right probably even with a string of overcast days the house would float for days on a single burning of the RMH.

As for the comment on creating a back draft condition in the feed tube if opened at the wrong time point taken. That was the reason for wanting a self closing door at the top of the feed tube. Some things that might possibly improve on that. If one side of the feed tube was glass so you could see how much fuel was in it. Then you would know you didn't open it unless there no fuel in the tube or the fire was out. A low cross tube sliding door that you closed while feeding the feed tube would also provide protection. If fuel was blocking sliding it in then you didn't open it till the fuel burned down or the fire was out. Another possible would be to run the air in through the whole length of the feed tube. But I see bridging problems here with the possibility of fire up in the feed tube. So how about if you fed the air in low normally but added ducting so you could move a damper temporarily for 15 or 20 seconds before you opened the feed tube door to flush the combustible gases collected in the feed tube back to the fire box. Some combination of those will surely let the feed tube exist safely. As for sealing the feed tube, it is necessary if it is going to run on outside air. The whole stove needs to be sealed from the house to prevent air interchange.

I do understand about bent, knotty and so on sticks not feeding. But if the feed tube tapers larger as it gets closer to the fire box and if it is completely smooth inside it should feed. All the pictures I have seen for longer feed tubes people are using pipe or square tubing which will jam because their cross section is constant instead of increasing as the wood falls. Most of those will also have a rough internal weld seam to make life even worse. I was also thinking of a longer fairly straight fuel source that would be fairly easy to harvest that no one I know is using as a fuel source. There are hundreds of willow patches in this area. 4 to 6 feet long with stems about 1 inch at the base. I know from experience that if the roots and crowns aren't harmed a patch will regrow in just a few years so it is highly renewable. That would mean no splitting also.

The whole purpose of this thread is to shoot holes in my thinking and see if I can plug them or add to them. Thank you everyone.

C. Letellier wrote:I have looked at the information on RMH and the single biggest flaw I see is that living in a passive solar house the house is incredibly tight so if I am going to go RMH the combustion air needs to be externally sourced.  Nearly all the plans are for internally sourced air.

I tried outside air into the firebox but it gets down to 40 below here and the cold air hurts the secondary combustion so I decided to build a heat exchanger.

This is for a 4 inch stovepipe
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Our of the stove with a 90 facing up
6x4 reducer and out of the house

On top of the 6x4 reducer, put a 6 inch T with the leg pointing towards the wall.
4 inch pipe out the wall.

On top of the 6 inch t put 2 feet of 6 inch stovepipe with 3  2 inch bolts sticking inwards on the top to rest against the 4 inch pipe to center it

Incoming air enters the T and goes up around the 6 inch pipe.

I get 100 degree incoming air within 1 minute of lighting the stove and get hot air before the stove even gets hot.  Incoming air is preheated and heats the house.

Robert Bodell wrote:

C. Letellier wrote:I have looked at the information on RMH and the single biggest flaw I see is that living in a passive solar house the house is incredibly tight so if I am going to go RMH the combustion air needs to be externally sourced.  Nearly all the plans are for internally sourced air.

I tried outside air into the firebox but it gets down to 40 below here and the cold air hurts the secondary combustion so I decided to build a heat exchanger.

This is for a 4 inch stovepipe
-------------------------------------------------------------
Our of the stove with a 90 facing up
SNIP.

Here is a picture

My two cents on why I like fresh air from outside for my lungs, and the stale inside air full of farts, halitosis, BO and whatever is in the compost bucket to get sucked outside: