Bad combustion. Whats wrong with my stove?

Hello there!
I just build a stove for a friend. Because of the room design, I came up with the idea to place the stove outside and 9 meter of duct - covered with concrete slaps - on the floor inside. It works great! The room is nice and clean and the bench stores the heat well in 2,5 ton of concrete. (Bench, wall and floor)
The stove outside has a good draft, no problems with starting the fire or with smokeback. Heatloss is messured; the top surface is 10 degrees warmer than outside temperature.
The exhaust is app. 30 degrees celcius.

But there is lots of smoke ;-(
Which means loss of heat, wood and work!
As you can see on the video



this is outspoken when new wood is added. But the perfect burning is seldom!
The wood isnt the best, I must admit: Pallets.

Does somebody see what is wrong here?

There appears to be very little, if any at all, re-burn of smoke/gases inside the heat riser. The typical causes for poor re-burn are usually due to one or more of the following: insufficient height of the heat riser, insufficient insulation of the heat riser, and insufficient insulation of the fire box.

Of what material is the heat riser tube made?

I am sorry to say, but your stove doesn't look like a rocket mass heater. The core vaguely does resemble a rocket stove for cooking but that is about all. At this moment in time, there's the Ianto Evans J-tube RMH and relatively recent the batch box rocket was developed. You need to decide what you want it to be, as it is neither of those now.

Also, both concepts won't run very well or at all with the lack of a proper chimney stack. It appears to run now because the whole of the stove's respiration is outside so the pressure differences of the house won't influence the workings. When built like this inside the house I fully expect it'll flatly refuse to work.

Maybe you could give us some numbers like the size of the riser, firebox etcetera?

The 90 degree flow transition between the riser and mass is horrible.   Perhaps tidy that up more smooth.
Also add a vertical chimney at the mass exit, the final exit should be vertical and above the roof.  Also add an insulated riser, maybe the soft insulative firebrick?   Try those 3 things.  

Secondary air can help with cleaner combustion but that's like 10 steps ahead of what you need to correct first.   You don't even seem to have much of a firebox but these are all things you can add or fix

I will end this on positive note.   That mass bench is beautiful.   How is the 180 degree transition inside that beauty?

Im not ashamed to call this stove a bastard, so no need to feel sorry here, Peter
Offcourse you should have some numbers (cm):
- Burnchamber:
Depth: 35
With:  11
Hight: 22

- Riser:
3 elements length: 25 and ø18 with the horizontal tubing leaving the uppermost
which gives 60 cm effektive rising.

The joint between burnchamber and riser is a square 11x15, before the gases enter the much bigger riser.

How would a chimney make combustion better: Do you think the draft is lousy?

Hello John.
Thank you for the compliment First lady wanted these weak colors; I defnetely would have wanted some more contrast.

So you think too, like Peter, that the draft is too weak? The "horrible" 90 degree
transition on top of the riser adds to this. How would you messure the bad draft?

The riser is made of prefabricated chimney elements of chamotte. They are app. 15 mm thick. The space between the riser and the surrounding box is filled with leca/blähton. I had hoped this would give high enough burntemperature. But you indicate, that a low temp could contribute to bad combustion?

By design the basic L-tube configuration burns dirtier than the J-tube rocket, that is the first limiting factor. Then, the smaller the system (not to mention one where the fire is being cooled by outside combustion air) the greater the requirement for really good insulating techniques, and the use of refractory materials throughout the stove's construction.

Byron Campbell wrote:By design the basic L-tube configuration burns dirtier than the J-tube rocket,

Interesting. I stayed away from J because of the cheap wood, as I have learned, that J requires reeeealy dry hardwood in order to prevent creaping fire.
So you advocate fitting a J-feed?

I think I have understood now, that the temp doesnt get high enough to burn all gasses ;-(

These days my home is heated by a 6" batch-box. Here's some links to information you may find helpful to correct your stove. Keep in mind a proper chimney is required if you decide to build a batch-box:

Adventures in horizontal feed: http://donkey32.proboards.com/thread/511/adventures-horizontal-feed

6" Batch in brick: https://permies.com/t/41202/rocket-stoves/Emergency-quick-small-batchbox-sq

Batch fire box sizing: http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions?page=1&scrollTo=6691

Ole Blente wrote:Hello John.
Thank you for the compliment First lady wanted these weak colors; I defnetely would have wanted some more contrast.

So you think too, like Peter, that the draft is too weak? The "horrible" 90 degree
transition on top of the riser adds to this. How would you messure the bad draft?

The riser is made of prefabricated chimney elements of chamotte. They are app. 15 mm thick. The space between the riser and the surrounding box is filled with leca/blähton. I had hoped this would give high enough burntemperature. But you indicate, that a low temp could contribute to bad combustion?

I never said the draft was bad.  
I've heard bad things about clay risers and clay isn't very insulative either.  The 180 looks good in the bench.   I think the smoke may be multiple reasons.  But it's not a total loss.   The 90 at the top of the riser can be port matched, the riser can be changed, and your final exit can be turned upward.   You might consider air/fuel ratio and the size of the air intake under the burn box, and the length of that air tunnel.   The burn box seems small but you could lower that  metal channel or maybe even build a batch box onto the front.

Good luck

Not only the temperature is important in a combustion process. One of the most wanted effects is called turbulence, needed to mix the combustible gases with fresh, preferably hot air.  Rocket heaters are doing a very good job in that area. I'll second Byron in that your best choice might be a batch box rocket. It isn't really complicated but your heater core need to be rebuild for that. As a bonus, the firebox will be larger and is able to contain more fuel. For a comprehensive resource see http://batchrocket.eu/en/. The riser which is situated behind the firebox is acting as an afterburner in this rocket heater implementation. Be warned though: the temperatures inside one of these cores can be close to 1200º C, the top limit for an atmospheric aspirated combustion unit burning wood. And oh yes, this batch box core is running excellent on soft woods like conifers.

I come here, and have nearly nothing to say!

Ole, your building tech isn't that far from mine

http://donkey32.proboards.com/thread/1817/starting-build-220mm-rocket-double

The leca balls aren't that good at insulating, i would say.

Since you have nearly all the parts to make a batch, i think you should go this way.


Take all appart, put the top tube which has been cut upside down, so the cut is at the bottom. put two firebricks in front to create the port. Like i did here

https://permies.com/t/35569/Range-retrofit#344806

Picture 534.

Put your two other tubes above, the lip the right way around so they fit. You will need to buy more, as a heat riser for a 18cm batch is about 1.2m top to bottom.

Then make your batch box out of the brick you have there. And may be some more.  I would pull away the whole setup further from the wall. And use the pozolan chimney elements, to make a downdraft bell, back to the pipe you have through the wall. Insulate it, of course, so you don't loose heat.

Use perlite or vermiculite to insulate the heat riser.

Thank you John for lots of good ideas for optimizing. I suppose you are right about the insulationvalues of clay; also Peter mentions, that even bare steel can perform better than building bricks! So there definitly is room for improvements.
You too mention the batch, like Peter, Byron and Satamax.

I observed your very interesting experiments with the batch, Peter, and on request from the owner I will take a look at that again.
I didnt remember the 1200 degree; thats amazing! If I get 900, I will be happy
From your calculations I can see, that with an effektive heatriser mesuring 60cm, the diameter should be reduced to 9cm. Did I get that right?

I also enjoyed your retrofit last year, Satamax, and have tried something semilar, cutting consumption to less than half. Im glad to hear, that its still upp and running.
Yes, I see what you mean. Your workshop devil burns like hell
Why would you create a downdraft bell between the stove and the wall?

The downdraft bell, or it could be called plenum is just to send the gases back down, from the top of a tall heat riser, to the tube you already have. I don't like pipe transitions. Too much friction opposing the flow of gases.

Satamax Antone wrote:The downdraft bell, or it could be called plenum is just to send the gases back down, from the top of a tall heat riser, to the tube you already have. I don't like pipe transitions. Too much friction opposing the flow of gases.

Got it

Ole Blente wrote:From your calculations I can see, that with an effektive heatriser mesuring 60cm, the diameter should be reduced to 9cm. Did I get that right?

I am in the dark what you mean by reducing the diameter to 9 cm, it is unknown to me. Where do you read that?

My actual heatriser - between firebox and lower point of the tubing leading into the house - is app. 600 mm.

On your chart "Rocket stove calculations" there are options for heatriser lengths at 540 and 720. Mine is in between with 600 mm. Corresponding diameters are 75 and 100 mm. 90 mm is in between.

You are taking the existing riser height as fixed, and reducing the system diameter to go with that. What people have been advising is to raise the riser height to fit the system, which means a masonry bell that comes above the bottom of the window. This may not be highly desirable, but is necessary to get good results. Making the system diameter smaller will reduce the amount of heat it can generate by approximately the square of the difference.

You have an 18cm diameter riser, effectively about 13cm diameter firebox, possibly 15cm duct inside the house. It may be that the long duct would not allow an 18cm batch box system to burn properly, so even though you can't shrink the riser diameter, you might be best making a batch box for a 15cm system, with all dimensions aside from the riser diameter corresponding to that.

Saying you have the makings of an effectively 15cm system, then going to 9cm would give much less than half the power.

Glenn:
Yes, you are right, the hight is pretty fixed. Moving the stove and creating a bell between the wall and stove isnt really an option. Nor is a chimney.
As I had to come up with the answer my self, I had to ponder on the question, and I presume, I have found out: The heatriser length starts at the buttom of the firebox. That gives me at least 20 more cm ;- )

«you might be best making a batch box for a 15cm system, with all dimensions aside from the riser diameter corresponding to that."
Do you suggest to make it 150mm and leave the riser as high as possible?

One of the reasons for the recommended riser height for a batch box is to allow the flames to stay in the super-hot zone long enough to finish burning. Your 18cm diameter riser is 1/3 bigger than the 15cm system, so if it is 3/4 the height of a 15cm riser, it would still have the same dwell time. A larger diameter riser would have less turbulence, which is not helpful, but it may be alright.

If you replace your little L-tube firebox with a batch box (15cm system size), you will deliver heat to the mass faster and have to feed the fire much less often. It would be better to move the riser say 10cm or so away from the wall to allow the hot gases to flow better, but you may get away with leaving it as is and just adding the batch box to the front. You want the batch box as tight to the riser as you can get, removing a few facing bricks to make it nestle in there. The nominal port thickness (back of firebox to inside of riser) is 50mm, but Peter has said that this is not critical. I would try to keep it less than 100mm.

To be specific, for a 15cm system, the batch box interior would be 216mm wide x 324mm high x 432mm deep. The port would be 54mm wide x 238mm high, or a slightly different shape of the same cross section. I presume these are the numbers you are working with.

Glenn Herbert wrote:To be specific, for a 15cm system, the batch box interior would be 216mm wide x 324mm high x 432mm deep. The port would be 54mm wide x 238mm high, or a slightly different shape of the same cross section. I presume these are the numbers you are working with.

Well, it's easier to make the box 440 long which is 2 firebricks end to end.

330 high which is three bricks. Makes life easier.

Thank you Glenn.
This really helped me sort out things. A narrower riser will keep the speed, but reduse burntime. Obvious.
Accordingly I could reduce the box size slightly to compensate for the too short riser.

Satamax: I love you practical aproach always

For dimensions, I just reposted the original calculated table information. Most of these dimensions are flexible enough that adjusting for available brick sizes is fine. (I didn't know what the standard brick sizes were in Europe.)

I don't think there is a need to reduce the firebox size below the 15cm system size.  You will need some chimney height for a batch box, the more the better, but even a few feet may help the draft.

Ok guys.
Building the batchbox outside, which many of you recommended, would give off too much heat. So I reconstructed the stove with a J-feed, better core and insulation.

Buildsize:
6x6

Riser:
1/2" thick refractory walls with 2" vermaculite around.
Hight: 70cm

Feed:
CSA 6x6
Deapth: 11cm

This works great. Draft is exelent, very little smoke the first 6 minutes.
I bet I have created a successful rocket :- )

Then catastrophe strikes.
Not only does it smoke back.
Fire creaps up until the draft in the stove is serverely redused; futher burning is meaningless.

Ok, I know I would be better off with hardwood. But this pallet wood is for free and is what I have!

Are there any rescuing advises to be heard out there, before this oven gets executed once more?

No Chimney? Most RMH installations require a proper chimney. Straight out the wall exhaust works only in some special situations.

Let me guess: the fire creeps up at the back of the sticks, close against the back wall of the feed. The original design of the J-tube shows a feed which is open all around so that air is allowed to come in from all sides. You enclosed the whole thing so the back wall is heating up and the fire creeps up. I mentioned about a month ago there was no vertical chimney so draft would be very weak. What happens in reality, the riser pushes the gases through the bench pipes initially. Further down the bench the moist in the gases are condensing in the horizontal pipes, cools down the gases more and stalls the heater. That could be helped with a proper chimney which would provide the intitial draft and takes over when the vertical stack warms up. Forming of a thick coal bed is a sure sign of weak draft, by the way.

In short: you need a vertical chimney like Byron mentioned plus the feed shouldn't be built in.

You mention you converted to a batch box and say it delivers too much heat? I would think heat is what you want, and when it's too much just make the fuel batch smaller. But then, it's your project, please do whatever you feel like.

Peter: Its so obvious with the backwall! Im ashamed I didnt see it myself :- )
I was so happy to make the passage veeery short. (And ofcourse it makes sense to have the feed build in, in order to avoid rain and snow filling it up all the time.)
Thank you so much, this will give the oven some few more days to live!

Byron: Im not so proud about the horizontal chimney any more ;-(
But as long as there is a room for improvement, I will make the steps, to see what is possible.

Peter wrote: "You mention you converted to a batch box and say it delivers too much heat? I would think heat is what you want, and when it's too much just make the fuel batch smaller."

The room, where the massbench is, is tiny, so the burnunit is outside. I concidered the heat loss from a batchoven bigger and the effort to insulate it bigger too - not to forget the door - and therefore chose the smaller and simpler solution, all though the batchovens are very attractive!

Well, my joy was too quick. Ofcouse the sticks must be cold and aired all way round, but they burn happily without the hot backwall.
Must be the soft wood!

Plus the absence of a chimney!

Draft:
- is not reduced when the riser gets hot.
- does not disappear when the fire extinquishes. Slightest touch of heat rises and pushes the gasses in the tubings.
Highest point is the outlet from the heatriser and all of the horizontal tubing. Which means, that the only resistance is air against the walls.

Creaping fire:
I added 10 cm distance to the backwall (1" vermaculite) but it made no difference. Fire krept up and made a campfire in front of the heatriser.
I took away the vermiculite and gave it a layer of bricks. Feed 6cm from the wall. Instead of the 2,5cm hight over the horizontal canal, this gave 11cm and did the job!

It wasn't the added height in my view. For the first hour or so the feed is cooler due to the added mass so the fuel won't reach self combustion temperature. After a certain stretch of time, when the feed is heated up, you've got your problem back. Just my interpretation of what happened.

Well, after all rebuilds and your polite thoughts, it seems there is really only one simple answer: Draft. Im stubborn, but now Im slowly gain the knowledge too. With a simple bathroom ventilator, drawing 90 m3/hour everything is fine. And I suspect all my builds might have worked just as fine this this kind of draft.
So, actually I could tear down the whole construction; I suppose the riser has no function in front of the ventilator?
And I could be back with the beautiful batchbox...