Another smokey Burn question !!!

Hi, New here and my first post.

I have gone ahead and built a small rocket stove in a little flat I have. The design is a little unconventional so I will describe the details for you;
-I have built the thermal mass out of stainless stove pipe and concrete. It is about 7ft long and uses concrete homemade bends for inspection.
-The Burner is made from a Pumice flue system. I went 6" throughout. The burn chamber is only about 3 inchs taller than the horizontal burn tunnel and extends down below the burn tunnel about 5inch's. I have enclosed a loose fitting stainless flue section inside the burn tunnel to help keep the flames off of the surface. The burn chamber is topped by a 7inch tall stainless steal pan which is partially enclosed within a pumice concrete slab.
- The heat riser is made out of sections of flue system which have approximately 3inch walls. I added a 3inch section of fireclay pumice top to get it nearer to the pot lid. The total height of the heat riser is 26inch's.
- The heat transfer pot is made out of a 28 inch gas bottle with a section of cast iron welded to the top.
- The plenum is made out of plastered block and the plenum to pot seal is achieved with stove rope.

The mock up went reasonably well and I went ahead and did a mini workshop with friends to build it out in the space. First test burn didn't work at all with a fight to get it to draw at all. This was with the thermal mass flue simply exiting horizontally through the wall. I added a section of 6ft of vertical pipe outside and this made the draw consistently positive which was a great improvement. I used heat resistant tiles to choke off the burn chamber to about half its original size and this helped as well.

First two burn made almost no impact on the thermal mass - but most of the energy went into drying it out so I expect that to improve greatly once fully dry. This hopefully should help a little with the draw.

However the burn proceeds in a rather strange fashion, for which I think I understand why. After lighting a simply blow down the feeder pot gets the draw going in the right direction. Using thin wood and pine the burn initiates well and is clean as it should be. It proceeds to burn reasonably well for about an hour or so, but then over time the draw gets progressively more lazy and the feed pot starts to smoke back into the room. I added a lid at this point and choked off the inlet air supply to a small area which killed the smoke back. However the longer the burn goes on the more the tendency is the for smoke to return. At this point the feed tube and burn chamber are starting to get really hot (to hot to touch) and there is lots of smoke swirling around in the feed pot.

What I believe is happening is that the feed tunnel is heating up and generating its own counter draft which is fighting the heat riser draft. Adding the lid is temporarily curing it by increasing the air velocity into the burn chamber - but is also helping to trap more heat into the burn chamber and feed pot. This becomes a positive feedback loop which increasingly fights the main draft. So a simple lid is not the solution to this problem. The insulated slab which is around burn chamber and part of the the feed pot is probably amplifying the problem (and is a noted issue within the book).

I want this burner to not smoke, and not need a lid, to not smoke since it will be used by people with minimal experience.

I suspect I have three solutions;

- remove part of the insulated slab around the feed pot. this is not appealing and I suspect it would only present a partial solution.
- add a forced air fan. Again not ideal since it will be difficult for inexperienced users to know when to switch it on and off.
- add another 10inch flue section to the heat riser. I will then have to add some crane type arrangement to allow single handed man handling of the heat exchanger pot.

I suspect I am almost there and am fairly happy with the level of performance I am getting with minimal wood input. I nearly boiled water on the top last night - but it didn't quite make it. I suspect the best option for me is to buy another flue section and raise the heat riser by another 10inch's. I strongly suspect that this will make the thing run perfectly. What do you guys feel.

Stephen

I just did a little addition to my feed pot based on the "Peter Channel", which made almost no difference. However when reading Peters original research he discussed a factor which i had not adequately considered in the form of the Lamina effect. It seem to me that the designs which discuss using 24" as the minimum heat riser length are optimised for an 8" system. The lamina effect in a 6" system might be reducing my draw by as much as a 1/2 or a third. That means that a minimum heater riser length of 36-48" is suggested with a 6" system. By this simple logic i will not get this working well without increasing my heat riser by a minimum of 10" which sort of confirms my original thinking.

Stephen

Steven Brightwood : Welcome to Permies, and a Big Welcome to The Rocket Mass Heaters RMH and Wood Stove Forum threads, With ~25,000~ Fellow Members
you should be able to come here 24 / 7 and talk to someone who wants to talk about what you want to talk about. You obviously have a very creative mind and
are not afraid to think outside the box! free dialog with your Fellow Members Will Stretch your mind as much as it Stretches theirs !

I think that I am following the major segments of your build, it is difficult to follow as you have created Alternate Labels for the various sections of your Build, and
also appear to have both changed the Dimensions of the Feed Tube, Burn Tunnel and Heat Riser, and have Different Measuring points !

It is a Truism that at the temperatures a well burning Rocket Mass Heater RMH will get to, All steel+ and especially Concrete will fail, as concrete is hydroscopic, steam
will be trapped within the Concrete and cause it to spall,and also the Lime in the cement will under go Chemical change destroying its mechanical bonds ! Both of those
materials have secondary rolls they can play but there are better materials, and you need to insulate around your Entire Combustion zone !

If the outer most wall of the Heat Riser is not well insulated from the inner wall of the Heat Riser there will soon be a nearly zero Temperature difference between the
Ascending Gas Stream and the later Descending Gas stream, with a more conventional Barrel over the Heat Riser replaced with Plastered Block at a certain point, you
can not get rid of enough heat Energy thru the Block Plenum to maintain the difference between the Ascending and Descending Gas Streams, Depending on build this
could happen as soon as 30 minutes into the Burn, or after a few hours !

If this is your major problem you can test to see if your Choking off the top of the Feed tube makes a difference in how long it takes to Start smoke -back in any case
cooling any part of the system prior to the Descending Gas stream on the outside of the Heat Riser is never the answer ! This would require two tests starting out from
cold both times and using similar amounts of wood !

Pictures and a sketch-up might help but going back to 'The Book' to make sure we are using common measurements is pretty much a must if you want us to Critique
your Build ! For the Good of the Crafts !

Think like Fire, Flow Like a Gas, Don't be the Marshmallow! As always your comments and Questions are Solicited and are welcome ! Big AL

+ Do a Google search for High Temperature Hydrogen Attack on steels ! A.L.

Much has been misunderstood from my description, which I can only blame myself for.

To clarify. The burner elements are built out of a Pumice flue system. All internal dimensions are the same at 6" diameter. The pumice flue liner was chosen because it is inherently strong and INSULATING. That means that the burn tunnel and the heat riser are indeed insulating and serving their intended purpose of creating a temperature differential between the inside and outside surfaces. The heat riser is clean on the inside after a burn and black on the outside which clearly indicates that the heat exchanger pot(equivalent to the barrel) is doing its job and dumping the heat to the outside. The gas bottle heat pot (barrel) is considerably hotter at the top than at the bottom which shows that the gas is indeed condensing, contraction and falling is taking place as designed. This means that the components of the burner are working as intended - just not well enough yet.

All other components which are not in contact with the flame and super hot flue gases are made of concrete, and the temperature of gas they have to deal with is well within their capability (ie below 400 degrees centigrade). The main heat mass has no contact between the gases and the mass since there is a stainless steel flue separating them. Until the thermal mass dries out it is not possible to say how it will behave in a normal burn situation since driving off moisture is keeping its temperature down.

The final exit gases at the top of the vertical flue outside the building is steam once the burner gets to operating temperature, and the internal surfaces of the heat mass are not blacking up to any significant degree. This again shows that smoke combustion is almost complete and the burner is working as intended.

Stephen

Just to clarify some dimensions;
-the bun chamber has a diameter of about 4". It rises 2" above the outlet to the burn tunnel and extends down 5" below the outlet.
-the burn tunnel is 6" diameter and about 7" long.
-the chamber at the bottom of the heat riser is about 8" diameter and extends below the burn tunnel by about 2". This should act as a swirl chamber
-the heat riser constricts from the 8" diameter swirl chamber down to 6" diameter flue. It is 26" tall and has a wall thickness of 3" of insulating pumice.
-the gas bottle heat exchanger pot has a domed top and is 28" tall giving a gap between the heat riser and the pot of 2"
-the gap between the gas bottle and the heat riser is about 2" on all sides, meaning there is no constriction of the flow between the heat riser and the gas bottle.
- the only point where there is a constriction is where the gas collecting plenum that the gas bottle rests on goes to a 6" horizontal flue outlet - but this is not smaller than the internal burner dimensions

Hope that all helps.

Stephen

I was reading over at another site where Peter of the "peter Channel" fame made the observation that the tighter the gap between the top of the heat riser and the barrel the more resistance offered to the heat riser draft. It seems that the recommendation of keeping the gap to 1.5 -2" is to allow for hot enough temps on the top plate to boil water/cook on and has little to do with setting the draft. He says that the draft will increase with a bigger the gap - with no theoretical limit to the amount this applies. This seems to make logical sense to me as there is less of a tendency for the gases to be compressed at the exit of the heat riser and hence develop the dreaded back pressure. So I am going to do a little experiment where I will raise the gas bottle(barrel) by six inches and see if that improves the draw any.

Stephen

make temp notes while your at it. top, top side, middle, and bottom of barrel. that should tell a story all its own there. i would think raising it would start to be disadvantageous at some point? interesting experiment!

I read the description but would really need a picture to help me understand the design. Sounds like a pretty cool project.

I will try to take some photo's tomorrow.

What did I learn today - that raising the barrel by just four inches improves the burn by a noticeable amount. Still not there yet as the same progression develops after about 2 hours but its slower and the smoking is less.
The top plate is cooler and is now unable to boil water so this seems to confirm that the burn improves with a bigger gap but that the 2" specification is for a cooking hotplate. The thermal mass is still driving off its moisture content so its not doing quite as well as it should overall - but I fear any improvement in this regard will be insufficient in off itself.

So what I am thinking is that I will get a new section of heat riser (10") and then raise the barrel by 14". This is a manageable modification and I feel it will cure the issues. If not I will remove the feed pot and that should remove the offending counter chimney.

Stephen

As a little experiment I added a 4inch computer fan driven from a 9V battery to the stack to see what effect it would have. This is a very lazy fan but it massively improves the burn.

I am almost tempted to cut my losses and just go with a serious 12V fan.

Stephen

Some pictures

And the thermal mass and outside flue

Things to note from the photo's.
The tram lines across the thermal mass show clearly where the burner is still driving off moisture.
Generally when I take the burner apart the internals to the top of the burn tube are white, this time they are part blackened which indicates a drop in burn efficiency.
The bottom assembly within the manifold shows two cracks. I am hoping that the cracks will stabilize and help alleviate thermal stress. The actual surface of the material is standing up to the punishment extremely well. I have considered adding a piece of lose fitting stainless pipe to the inside to take some of the abuse and it can be replaceable.
It all looks very messy at the moment as the final details have not been set and it will receive a final unifying coat of plaster and paint eventually.

I have come to the definite conclusion that 26" of heat riser from top of burn tunnel to top of riser is inadequate and I will be adding a further 10" section. If anyone has any final opinion on whether they consider this to be enough I am all ears. Will be going for a 4-6" gap between the gas bottle and the heat riser.

Stephen

Your idea of the additional heat riser height will give you a stronger draw. My heat riser is about 32" tall. One of the problems I encountered was my house vents were creating pressure drops equal to or greater than the RMH pressure drop. I increased my chimney height to equal to that of the HVAC, plumbing vents, water heater vent, etc. and I have very strong draw now and never a smoke back issue. You may be fight the stack effect of your house plumbing when trying to run your RMH.

I have a 4" gap for my system but it is an 8" system (cross sectional equivalent of 8" diameter).

When you included the electric fan I suspect your problems are competing pressure problem or pressure drop problem in the RMH. It doesn't look like you exceeded the maximum length for a 6" system (~ 45 feet of straight run pipe). Subtract 5 feet from the max length for every corner you turn. The lower section of the RMH under the barrel looks really rough, if it is not too much work smooth this area out to prevent pressure drop. Can you increase the height of the external chimney to match potential house vents?

Thanks for photos.

The plenum/manifold below the pot is very large compared to most and is plastered on all surfaces so it is smoother than it looks. i do not believe it is the issue here.
I can add another few feet to the outside flue - but there are no vertical vents for it to compete against so again I think it wouldn't make much of a difference.
I believe you are spot on in saying the additional heat riser should improve the overall draw. All i now have to do is work out how to raise the lip of the manifold adequately to take it.

Stephen

Brett raises some very valuable points, Stack Effect, and Whole House Stack Effect ! Just simply running a clothes Drier simultaneously with your Rocket Mass Heater
Can cause this Smoke Back ! A search within Permies or the internet can clarify many of the important points of Stack Effect !

Other possible villains are simultaneous running of A Fossil Fuel fired Furnace, A non-electric water heater, even the exhaust fans in the Bathroom and the 'Range
-hood' over your cooking stove !

Also important is having your Outside vertical chimney Ending 4' -5' above the peak of your roof on the Downwind side whenever Possible ! For the Good of the
Crafts !

Think like fire, flow like a Gas, Don't be the Marshmallow! As always your comments and questions are solicited and are welcome ! BigAL

Did some modifications which helped, but left it for a week whilst I got on with some other things. Went back to it today and it was intermittently smoking. I partly attribute this to the heat mass been cold - but not all of the problem can be attributed to that.
After raising the heat riser by 10" and adding a bit of an extension to the external flu I am still getting smoke back from the feed pot when things start cooking.
I have come to the conclusion that the feed pot is at least a significant part of the problem. The burn tube and feed pot are insulated so they act as a chimney which creates a counter draft.
Tomorrow i am going to decisively chop the feed pot down to about 2" from its present 7"

http://donkey32.proboards.com/post/8770/thread




Stephen, two other things, your barrel to flue transition, imho is too small. And your outside chimney is too short.

Sooty burn could also indicate a lack of insulation on the whole J tube.

One trick i think is intresting, is to insulate the outside chimney.

Off to work then!

The barrel to heat riser spacing has been tried at a few heights so its not that. The barrel to flu transition is the same size as the flu so if it is the bottle neck then the flu is also a bottle neck - at what point would the flu not be a bottle neck in this case ? The collection manifold is simply enormous compared to most designs.
I can accept that the outside flu could be higher and a i added a few more feet last night. When I get a bit more flu i will add it on.
The J-tube is made up of pumice which is highly insulating. The whole J-Tube assembly is better insulated than almost all of the designs I have seen using firebricks. This is not the issue. In fact the probable issue is that the fire chamber is to well insulated causing a very hot burn and strong convection currents above the fire.

However neither of these two issues really explains why its starts smoking back as it gets to its hottest, for that a counter flu explanation seems best. I am going to go ahead and reduce the height of my feed pot today as I feel certain that this is the primary problem.

Stephen

simplify the stove problem. what you are doing is upside down syphon . with the primary feed high, your exit outside chimney flue will need to exceed it by far.

Stephen Brightwood wrote:The barrel to heat riser spacing has been tried at a few heights so its not that. The barrel to flu transition is the same size as the flu so if it is the bottle neck then the flu is also a bottle neck - at what point would the flu not be a bottle neck in this case ? The collection manifold is simply enormous compared to most designs.
I can accept that the outside flu could be higher and a i added a few more feet last night. When I get a bit more flu i will add it on.
The J-tube is made up of pumice which is highly insulating. The whole J-Tube assembly is better insulated than almost all of the designs I have seen using firebricks. This is not the issue. In fact the probable issue is that the fire chamber is to well insulated causing a very hot burn and strong convection currents above the fire.

However neither of these two issues really explains why its starts smoking back as it gets to its hottest, for that a counter flu explanation seems best. I am going to go ahead and reduce the height of my feed pot today as I feel certain that this is the primary problem.

Stephen

Well Stephen, you seem very sure of yourself, so why should i continue trying to help?

Anyway, i'll do it.

On this pic



The flue transition is the hole that we see on the right? If yes it is too small. As the majority of the flow of gases comes from the top, they can't enter a tiny hole with squared edge easily enough. The bottom part of the hole is nearly inactive, since the flow comes from the other way. Plus, it forms an elbow, and a badly shaped one, in an elbow, the gases are near stalling on the inside of the elbow, and the boundary that it forms slows the rest of the gases. It's not for no reason that most of the people who have had successfull builds; say that the transition area should be two to three times bigger than the system size.

Plus, looking the hole of your heat riser, it seems bigger than the flue hole.

I can't seem to find R values for pumice on its own, but pumice concrete mixture is at R1.5 per inch. the minimum value advised for a rocket is R4, so a good 2.5 inch is needed.

Usualy the guideline is 1 inch perlite, or two inches of vermiculite, or rockwool. 4 inch cob. To me, it's just the soot which seems to indicate an incomplete combustion.

Don't rush reducing your feed, there might be another solution.

You see here, the metal feed tube on one of Dragon heaters builds based onto Peter van den Berg's work.

http://donkey32.proboards.com/attachment/download/425 i can't insert it here.

It has a P or Peter channel. Which as well of adding some air, cools down the feed tube. You could implement something similar which would drop down into your feed tube and cool it.

Konstantin did something along thoses lines there

https://permies.com/t/32099/rocket-stoves/doubling-power-RMH-cooling-feed


Then, please try what i was saying, find a skip with rockwool or glass wool and insulate your outside chimney, it sheds it's heat to the outside air, while it would be better to keep this heat to power the draft than being lost.

Is this the stuff you used?

http://www.schiedel.co.uk/products/pumice/pumice-range

The feed tube has a Peter channel on it already and it helps a bit - not on the original photo. As can be seen the manifold is much bigger than on most builds which generally constrict considerably at the barrel support, where as the manifold in mine is open all around the base of the barrel. offering minimal resistance and plenty of thermal mass to dump its heat and sink for exit to the horizontal flue.
The internal dimension of the heat riser is 6" and the flue out is 6" - its an optical illusion that suggests they are different as the feed out hole is low down. The heat riser is made of pumice and is around 2.5" inches across so it meets the spec for insulation. The internal surface of the heat riser is white after burning, and it is only the outer surface which has any blacking. From my reading few people get away without a bit of soot in the cool areas so I am not really concerned about inadequate burn temperatures. The pumice in the J-tube heats up quicker and stays hotter than firebricks and the sacrificial stainless flue liner in the burn tunnel glows red hot after about 20mins.


I decided to take your advice regarding trying a flue extension to the outside section - so I added another six foot. The outside flue is now 12'. I did another test burn and had a three hour burn with no significant smoke back. I had it burning with a lid on the feed tube for most of the time - and without it there was still a bit of smoking - with it on it burnt for extended periods without smoking. This is significant as previously with a lid on after a short time the feed pot got so hot that it smoked and the reverse flue action took over. I assume that two things have changed - the flue extension has tipped it over to been adequate draw and the thermal mass is behaving better as it has been burnt for two days running. Previously I was getting steam out of the top of the external flue, now there is no visible steam which means it is condensing before it gets to the flue exit. Effectively its a 100% clean burn at this stage.

So thanks for the perseverance and I can now proceed to apply the final plaster coat and get a proper lid for the feed pot.

Stephen

Stephen Brightwood : The U.S. Federal Government endlessly repeats the mantra about dealing with 'the terrorist threat' and how they have to be 'Right' 100% of the
time, and the Terrorists needing to be right only once !

Standard practice for an Exterior chimney is a minimum of 4-5 feet above the Peak of the roof AND over any other nearby objects, It is also recommended that the
exterior chimney be on the 'lee' side of the house, insulated and have a positive Storm Cap.

With 'storm of the Century occurring at ever more frequent intervals it is difficult to plan for a multi-day Storm front that allows strong winds to come from a different
direction than "Normal '' making a marginal chimney unusable, 12 ft is not close to tall enough under those conditions, and can be totally useless If Stack Effect and
Whole House Stack Effect are disregarded ! Big AL

It seems many people are getting away with much shorter stacks and it certainly isn't laboured that strongly in the "literature". However 12' takes it over the ridge since it is a flat so i hopefully will be OK for the future.

Stephen

Stephen Brightwood wrote:It seems many people are getting away with much shorter stacks and it certainly isn't laboured that strongly in the "literature". However 12' takes it over the ridge since it is a flat so i hopefully will be OK for the future.

Stephen

Stephen, you said it, "it seems" There's plenty of people who want to tell stories of sucess, not failure. That's why some will never say that they have a smokey burn, nor even admit it. It's bad to the poind that some people might even fool themselves, and never learn.



There's been a discussion lately about the fact that people tend to read half the manual, and modify whatever they read about, because they think they've understood, and believe they are cleverer than engineers. And there's also the case of, if you don't want help; don't ask questions! Very frequent over here. They do ask, but don't want to listen.

Stephen Brightwood wrote:It seems many people are getting away with much shorter stacks and it certainly isn't laboured that strongly in the "literature". However 12' takes it over the ridge since it is a flat so i hopefully will be OK for the future.

Stephen

at work we had a barrel stove, fine 3 out of 4 days. wind coming from wrong way, put pressure and did not draft, smoked us out.