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My rocket mass heater doesn't work (not enough draft)  RSS feed

 
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I built a rocket stove last summer. That was the RMH minus the 40 ft. of pipe, but with a vent stove pipe. It has worked like a charm for the past year.

I built an RMH this fall. It is the exact same dimensions and materials of the stove that I built last summer, plus 40ft. of pipe. Only difference was the height of the firebox (feed). That was much deeper than the one on the working rocket stove. But, this RMH doesn't work.

I checked for draft at every stage. It had draft. In fact, it still has draft, just not enough.

We started by digging a ditch partially into the ground, partially elevating the level for the pipe. There was a slight incline where I wanted to put my tipi, so we first decided where floor level would be, then adjusted the level of the ground to determine the height of the 8” pipe. The stove was built into the ground. I started with a cement board, then mortared firebricks to make a base, then built the firebox, the burn tunnel, the heat riser, added the water heater around the heat riser, filled with perlite, put on 55 gallon barrel, fit into 40ft. of pipe, covered pipe with rocks, covered all with dirt to floor level.

First trial: Built a fire in the fire box. Drafted through burn tunnel, but not with force, for a few minutes, then fire snuffed out like it had been deprived of oxygen. Multiple attempts, same result.

First solution attempt: Knocked off half the fire bricks from the firebox (feed.) Made it the same height as the working rocket stove. Then the fire burned fine, without being snuffed out for lack of oxygen. But, the draft through the burn tunnel worked more like an ordinary stove would – a draft, but not with any force. The result of this is that the fire would burn UP the sticks, and once the flames were higher than the burn tunnel, the fire drafted in two directions. I could look to the bottom and see that the flames lower than the top of the burn tunnel were drafting through the stove, but the flames higher than the burn tunnel were drafting out the fire box – therefore filling the tipi with smoke.

Then I found online the idea that the heat in the heat riser needed to be way higher than the heat in the barrel, after it went over the top and started down. Well, we had buried the barrel half way up, so that the top of the firebox was at floor level. We figured maybe the differential in the temps was not great enough. So, I unburied the barrel, made an air space all the way around the barrel. Same result: fire goes through the pipes fine until the height of the fire is higher than the top of the burn tunnel, then those flames draft into the tipi, straight up.

Now, here’s what I ‘m dealing with. I cannot rebuild from scratch. I had a massive amount of help collecting rocks, hauling dirt, cutting the barrel and water heater. It can’t be done over. If the problem cannot be fixed at the stove, I’m going to have to call it a huge money waste and dream killer.

To restate the problem: the fire burns and drafts through the burn tunnel, but not with the “rocket” force sufficient to prevent the fire from burning up the sticks and therefore taking the easier exit out the top.

I am wondering if there is any reason not to simply add on an “oven” style firebox in front of or as part of the firebox (feed.) It would open from the side, act like a wood stove, draft out the burn tunnel, go through the system of pipes, exit out the outside. Thoughts?
 
gardener
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How wet is it?
 
Lua Sage
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I am in Colrado. It is dry.
 
Satamax Antone
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Lua Sage wrote:I am in Colrado. It is dry.

I am not talking about your weather, but about your mass? How long has it been drying?
 
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Pictures / drawings would be a huge assistance in trying to troubleshoot....
 
Lua Sage
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I don't know how to add pics. Will try it tomorrow. It is not wet at all. There is no cob. The fire bricks were mortared with a kiln mix and the pipe was simply covered with rocks and dirt. No reason for cob when it is under the floor.
 
Satamax Antone
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It would help to see your transition area, to know your system size, i guess 8".

Yep pics would definately helps.

What is your top and side gap? Do you have any mean to insulate your vent pipe, inside the teepee? Like old batt insulation top wrap around?
 
pollinator
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Lua Sage wrote:I built a rocket stove last summer. That was the RMH minus the 40 ft. of pipe, but with a vent stove pipe. It has worked like a charm for the past year.

I built an RMH this fall. It is the exact same dimensions and materials of the stove that I built last summer, plus 40ft. of pipe. Only difference was the height of the firebox (feed). That was much deeper than the one on the working rocket stove. But, this RMH doesn't work.

I checked for draft at every stage. It had draft. In fact, it still has draft, just not enough.

We started by digging a ditch partially into the ground, partially elevating the level for the pipe. There was a slight incline where I wanted to put my tipi, so we first decided where floor level would be, then adjusted the level of the ground to determine the height of the 8” pipe. The stove was built into the ground. I started with a cement board, then mortared firebricks to make a base, then built the firebox, the burn tunnel, the heat riser, added the water heater around the heat riser, filled with perlite, put on 55 gallon barrel, fit into 40ft. of pipe, covered pipe with rocks, covered all with dirt to floor level.

First trial: Built a fire in the fire box. Drafted through burn tunnel, but not with force, for a few minutes, then fire snuffed out like it had been deprived of oxygen. Multiple attempts, same result.

First solution attempt: Knocked off half the fire bricks from the firebox (feed.) Made it the same height as the working rocket stove. Then the fire burned fine, without being snuffed out for lack of oxygen. But, the draft through the burn tunnel worked more like an ordinary stove would – a draft, but not with any force. The result of this is that the fire would burn UP the sticks, and once the flames were higher than the burn tunnel, the fire drafted in two directions. I could look to the bottom and see that the flames lower than the top of the burn tunnel were drafting through the stove, but the flames higher than the burn tunnel were drafting out the fire box – therefore filling the tipi with smoke.

Then I found online the idea that the heat in the heat riser needed to be way higher than the heat in the barrel, after it went over the top and started down. Well, we had buried the barrel half way up, so that the top of the firebox was at floor level. We figured maybe the differential in the temps was not great enough. So, I unburied the barrel, made an air space all the way around the barrel. Same result: fire goes through the pipes fine until the height of the fire is higher than the top of the burn tunnel, then those flames draft into the tipi, straight up.

Now, here’s what I ‘m dealing with. I cannot rebuild from scratch. I had a massive amount of help collecting rocks, hauling dirt, cutting the barrel and water heater. It can’t be done over. If the problem cannot be fixed at the stove, I’m going to have to call it a huge money waste and dream killer.

To restate the problem: the fire burns and drafts through the burn tunnel, but not with the “rocket” force sufficient to prevent the fire from burning up the sticks and therefore taking the easier exit out the top.

I am wondering if there is any reason not to simply add on an “oven” style firebox in front of or as part of the firebox (feed.) It would open from the side, act like a wood stove, draft out the burn tunnel, go through the system of pipes, exit out the outside. Thoughts?




Lua Sage : Forgive me if I have pounced onto the wrong thing, You have failed to mention the final vertical chimney -that must go up as
soon as you put your barrel over your Heat Riser !

This could be your problem, or something you forgot to mention! Ether way as a Further warning to Others . . . . .

When the Barrel goes on- the Vertical chimney must go up! Whether you are running a horizontal chimney or not makes no difference !

How did you calculate your 40 feet, have you subtracted 5 feet off of its length for every elbow …?

The correct ratio of the Feed Tubes Depth- BurnTunnels length - Heat Risers Height is 1X - 2X - 3X , when you made the total height of the
feed tube deeper you should have increased the height of the Heat Riser ! 1" at the feed tube, 3'' at the heat riser !

Pictures ARE worth a thousand words ! For the craft ! Big AL !
 
gardener
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I don't see a description of any vertical chimney after the 40' of horizontal run. What exactly did you do there? A good chimney, preferably insulated in some way, will help the draw after things get warm. It could even be entirely outside as long as it is sized and located properly.
What is the exit temperature of the flue gases at the end of the run? If it is too low a chimney can't help except perhaps in very cold weather. What weather have you been trying it in?
 
Glenn Herbert
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By the way, an oven style firebox, especially if it is above the burn tunnel, is not going to work any better or even as well as what you have.

Al's comment about elbows is critical. A properly built 8" system should be able to handle about 50' of horizontal run, *counting each elbow as 5 feet*. What effective length do you get when you do that calculation?
 
Lua Sage
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Bunch of things learned here too late. I thought I understood from what I had read before, but am clearly seriously deficient. I still do not understand the 1"2"3" ratio. I did not know elbows equaled 5 ft. This is an absolute disaster and I don't know what to do now. Total straight horizontal length = 38 ft. Five elbows = 25 ft. Vertical pipe = 5 ft. WAY over the limit = 68 ft.

I can put the oven stove below or at least equal to the burn tunnel, if that's the only solution.

Before I try to figure out posting pictures, I would like you to know my limitations. I am 66. I walk with crutches and live in pain. I had a tremendous amount of help with carrying and lifting, but I need to figure out what I'm going to do now on my own, so my capabilities are pretty limited. My income is also seriously low. I tried to get some help way back in the beginning of exploring RMH's, by asking a couple of questions of the experts, as offered on a website, but the consultation fee requested in return equaled a quarter of my monthly income. At that point, I felt pretty bad about asking anybody anything and just started reading and trying to figure this out on my own. I felt pretty awesome when I built the one that works so well, and just barreled into this one without learning enough, obviously.

I will study how to post pics now.

 
Lua Sage
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Ok, I'm working out posting pics. I hope this works. Here's the first one of the cut barrels, the pipes starting to be laid out, the fire bricks pile.



Well, that didn't work. So, how do you get the actual picture to post in the thread? I hit the Img button, put in the link, and this is what I got.




ETA: Fixed the picture link for you. Bill Erickson
 
Lua Sage
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One person has an idea of putting a small electric fan in the end of the exit pipe to help pull the draft through. I have the idea that we're going to have to dig up the end of the pipe after the third run of straight pipe and send the pipe out through the wall of the tipi straight from that point, instead of making the last turn. That would eliminate two elbows and eleven feet of straight pipe. The vertical chimney is 5 ft. tall, uninsulated. I am thinking about going with the fan idea for the winter and then digging up the pipe next summer.
 
Lua Sage
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Ok, so I figured out how to do pics. This one is the stove put together prior to mortaring it.



During the mortaring:



Covering with rocks:

 
Lua Sage
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Heat Riser spacing:



The top of that heat riser is 1.5 inches from the top of the 55 gallon drum.

The outside chimney: 5 feet high.



The fire burning merrily after I knocked off two layers of firebrick in the firebox and before I added one layer back on again to the two sides.



How it looks right now:




 
Satamax Antone
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So Lua, insulate that "chimney" outside. At least, if there is some heat left it will tend to go up. And if you can afford it, do another ten feet of vertical. Insulated vertical chimney counteracts somewhat the drag of the horizontal pipe.

That's called the stack effect. http://en.wikipedia.org/wiki/Stack_effect

Then, did you insulate that heat riser? What i see is not the actual barrel, i think. So this might have been filled with perlite or something?

Increase your top gap, 1.5" is too small for an 8 incher.

Your transition area if the heat riser was centered in the barrel would be too small, but since you're off center, you could be all right.

Hth.

Max.
 
Lua Sage
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Yes, the heat riser is insulated with perlite.

Difficulty with another 5 ft on the vertical pipe is wind - it gets fierce around here. I can insulate it.

I can't increase that 1.5". That's a solid piece of ceramic. That goes into the category of tearing it all apart and starting over. But, I think next summer, I can get help to reduce the feet of horizontal pipe. How about the electric fan idea to increase the draft in the meantime?
 
allen lumley
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Lua Sage : As clean burning as the RMH is and the very low chance that you will have too much heat means you could get away with a very simple fan !

Find out what a similar product would be in the exhaust smoke pipe for a Fossil fuel fired Forced air system and then you will be safe at only moving 2/3rds
as many cubic feet per minute And use some kind of ground fault interrupter !

Wait(!), what do you have onsite for electricity !

You may have to go with more pipe and use guy wires in # places ! Big AL

 
Satamax Antone
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Lua, if you don't mind, just to experiment, and it's simple, get another barrel, and remove your lid, clamp the other barrel in place of the lid. With some aluminium tape to ensure it's airtight.

Raise your chimney, insulate it, and hold it with cables.

And, put something like this on top.

http://www.artiscaps.com/products/chimney-caps/

If you realy need it, i've gathered my fan from a fridge, to start thr draft when cold.
 
Lua Sage
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Ok, I got a fan installed in the outdoor exit pipe and the whole thing is drawing very well now. It is clear that the mistake was not knowing that elbows need to be counted as five feet, and having way too long a pipe. So, next summer, I'll back up to much less pipe and re-channel the exit pipe out the west side of the tipi, reducing the length considerably. I could just leave it with the electric fan, but do not want any dependence on high technology to stay warm in the winter. I get the electricity now from solar panels. A few years ago, our solar system took a direct hit from lightning (yes, we have all the grounding recommended) and had between two systems - my son's and mine - $16,000 in damage. Without insurance and an existing grid, we would have simply and instantly lost all electricity. Since I fully expect total collapse of civilization, I never count on any high tech stuff continuing. I will enjoy it while I have it, but learn to live without when I don't.

Thank you very much, all of you, for your input and education. I wish I had presented a diagram off the plan before doing it. Oh well, live and learn.
Lua
 
Glenn Herbert
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Came across this old thread and noticed something that I should have brought up when it was fresh.

The pictures of the feed tube and barrel indicate that the heat riser here cannot be any more than two times the height of the feed tube, and about equal to the burn tunnel length, when for best results it should be three or four times the height of the feed tube. This may have been helped by shortening the feed tube, but increasing the riser height with the double barrel that Max advised would greatly improve the strength of the system. That combined with a taller chimney might even compensate adequately for the extra-long horizontal run. I expect the OP has already made some modifications, but any later readers might keep this in mind.
 
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Fist thing that caught my eye was the fact I did not see an insulated Core. I use 6" cast iron pipe 2" cob and the outer sheath of my core is aluminum 10" duct pipe. The insulated Core allows the retention of heat that helps the secondary burn of all the wood gas within the riser core, which creates massive amounts of heat.

The second thing that I noticed is wow that is a really long duct. I would imagine it will take a good long time to heat(charge the system) before it will start to draw the way it is supposed to. You may even need to set up some kind of forced air flow(bellows) to get the air started through that much ducting. Possibly a longer riser might help also.
 
Glenn Herbert
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Lua Sage posted Oct 03, 2014 · 03:01 PM:

"Yes, the heat riser is insulated with perlite."

There is just not a photo of that.
 
pollinator
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Glenn Herbert wrote:We haven't heard from the OP in two years.



Edited:

I'm going to throw an idea into the hat.
Perhaps one could add a  heater at the base of the chimney,just big enough to create draft,in effect priming the pump.
Some folks apparently use something as small as a candle to do this.
 
Glenn Herbert
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We haven't heard from the OP in two years. It would be very interesting to know how her system has progressed in that time.
 
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