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Rocket stove made out of steel.

                              


Joined: Nov 01, 2010
Posts: 22
I have a friend who is a welder and he's wanting to build a rocket stove out of steel just like a cast iron wood stove. Does anyone have any info on this or know anything that has been tried before.  Thanks
Erica Wisner
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Joined: Feb 10, 2009
Posts: 779
Location: Okanogan Highlands, Washington
    
  96
Dabumb wrote:
I have a friend who is a welder and he's wanting to build a rocket stove out of steel just like a cast iron wood stove. Does anyone have any info on this or know anything that has been tried before.  Thanks


Been tried - Ernie says it worked OK for a little while, but then they started seeing degradation in the gas output = it started smoking.  It works OK 'til you put the barrel over it, basically.  There's not enough friction or temperature differential in the burn area to maintain a clean burn.  Same thing happens using stovepipe elbows instead of brick for the burn tunnel.

Line the burn tunnel with firebrick to get the right turbulence and heat in the flame path, especially the horizontal part.  Or kiln-brick if it's available. 

If he installs it within a masonry casing to store that good heat, and lower surface temps to safer ranges, the expanding steel will try to crawl around and crack the masonry unless he includes a good expansion joint.  So making the housing out of metal can make it more difficult to combine the burn unit with a masonry heat-exchanger.

The main use for a metal one would be a small, portable, clean-burning woodstove.  Without the masonry heat-exchanger, it could work as a radiant heater for a small hunting cabin or apartment like those little vertical woodstoves used to be made for.  Or maybe a cookstove or working furnace (for small forge work).

You could always go bigger to burn more fuel to make up for not storing the heat...

Building a metal one defeats much of the purpose of the mass heater as we understand it: it makes it more expensive, requires more energy-intensive materials, reduces the efficiency, reduces safety by exposing residents to high-temperature metal instead of warm masonry, and limits effective heat storage.  The stoves are not designed to burn overnight, either, so you'd be looking at short heating periods followed by fast cooling periods. Better for cooking than for comfort.

But if what you want is a woodstove, and what you have is a lot of metal scrap and a welder, and you are not in a situation that allows for heavy masonry installation or regular occupancy... experiment away.

Please pass on our recommendation that every prototype be tested in a safe location outdoors before installation in any occupied area.  We can't predict the surface temperatures, and your friend will need to determine generous clearances and/or heat shielding after observing how hot the stove gets with local fuels and a several-hour test fire (with intended chimney adding extra draft...).

Good luck, and please get some pictures or a report on how it goes!

- Erica Wisner
www.ErnieAndErica.info


Play with nature, make nifty stuff:
www.ErnieAndErica.info
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
Dabumb wrote:
I have a friend who is a welder and he's wanting to build a rocket stove out of steel just like a cast iron wood stove. Does anyone have any info on this or know anything that has been tried before.  Thanks


this guy has done it:
http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/

Some things to note.... From everything I have heard here he has done it wrong 
1) 4 inch CSA is too small.
2) riser is insulated... but just barely.
3) exhaust is aluminum.

Some of the things he has done right...
1) no mass, this makes up for a lot of the other funkiness.
2) Riser is insulated some... better than none.
3) feed tube depth (top to bottom) is short. It doesn't look it, but remember only 4 in. CSA.
4) He hasn't made it permanent.
5) I think one of the reasons he has no smoke back is that he only burns for a short while...
      note his comments that the room gets too hot quite quickly... Mass would even that out.
6) the whole bottom of the burn tunnel slides out for cleaning, Barrel can come apart for cleaning.

Anyway, it works as it is. For a portable unit like this, a good mass might be solder. Find some that melts around 350F.... (the stuff you use for plumbing is a little higher I think normally 50/50, you would use the 60/40 stuff) Put another cylinder outside (or inside) the barrel sealed at the bottom... fill the gap with your mass. It should give off a lot of heat as it transitions through phase change. A light layer of insulation on the outside would keep the stove from heating the room too fast when fired and slow down the cooling of the phase change mass after firing.

Wish I had a few hundred pounds of solder to play with   (I may not need that much)

One other comment... his comments about how it works are sparse, which for his purposes is ok. We don't know how clean the burn is really, just that the thing gets hot and doesn't use much fuel to do so.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 779
Location: Okanogan Highlands, Washington
    
  96
Len wrote:
this guy has done it:
http://www.iwilltry.org/b/build-a-rocket-stove-for-home-heating/

Some things to note.... From everything I have heard here he has done it wrong 
1) 4 inch CSA is too small.
2) riser is insulated... but just barely.
3) exhaust is aluminum.

Some of the things he has done right...
1) no mass, this makes up for a lot of the other funkiness.
2) Riser is insulated some... better than none.
3) feed tube depth (top to bottom) is short. It doesn't look it, but remember only 4 in. CSA.
4) He hasn't made it permanent.
5) I think one of the reasons he has no smoke back is that he only burns for a short while...
      note his comments that the room gets too hot quite quickly... Mass would even that out.
6) the whole bottom of the burn tunnel slides out for cleaning, Barrel can come apart for cleaning.

Anyway, it works as it is. For a portable unit like this, a good mass might be solder. Find some that melts around 350F.... (the stuff you use for plumbing is a little higher I think normally 50/50, you would use the 60/40 stuff) Put another cylinder outside (or inside) the barrel sealed at the bottom... fill the gap with your mass. It should give off a lot of heat as it transitions through phase change. A light layer of insulation on the outside would keep the stove from heating the room too fast when fired and slow down the cooling of the phase change mass after firing.

Wish I had a few hundred pounds of solder to play with   (I may not need that much)

One other comment... his comments about how it works are sparse, which for his purposes is ok. We don't know how clean the burn is really, just that the thing gets hot and doesn't use much fuel to do so.



Cool project!  This isn't a Rocket Mass Heater, though - it's a radiant container around a very small Rocket Cookstove (Aprovecho-style).  The barrel never comes in direct contact with the exhaust.  That may be an important part of why his 4" system is working.

I agree about the aluminum exhaust, and the description of a leaky water heater makes me nervous because he's relying on foil tape for the seal around the heat riser/exhaust junction.  I'd want a CO detector near this one for sure.

I think his insulation might be surprisingly good - we used a thin, mineral-felt insulated pipe for demonstrations, and it does maintain a good heat differential, for a good long time.  There must be some heat escaping to heat up that radiant surface, though.

Mass evening out the heat differentials is one of the main benefits of any mass/masonry heater.  Short-term heaters are best for spaces that are occupied only for short intervals, like a church, college classroom, or ski cabin.


Solder is a very interesting idea.  Haven't heard that one yet. 

Makes me think of the liquid metallic sodium on nuclear piles....

Could be very dangerous if you have mercury or flux in your solder, the vapors could be emitted at unpredictable times.  And you'd need the right container, nothing that would let the molten metal escape or pressurize...

But with the right metals for the right temperatures, it could be a very interesting project.

And portable, too, with the solid solder helping to protect the other parts from shifting or warping.

Of course, the RMH originator would probably roll his eyes, and suggest that solder is best used for soldering useful things like cook-pots, circuitry, and water-tanks, and that dirt is a much more economical thing to use for mass.
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
Erica Wisner wrote:

Cool project!  This isn't a Rocket Mass Heater, though - it's a radiant container around a very small Rocket Cookstove (Aprovecho-style).  The barrel never comes in direct contact with the exhaust.  That may be an important part of why his 4" system is working.

Ya, it does, the picture with the exhaust on top of the riser was just for testing without the barrel, you can see when he welds it in he is using a 1.5inch thick piece of wood to space the top of the riser from the barrel. The exhaust is a round flange welded onto the barrel.

I think his insulation might be surprisingly good - we used a thin, mineral-felt insulated pipe for demonstrations, and it does maintain a good heat differential, for a good long time.  There must be some heat escaping to heat up that radiant surface, though.

It is also thicker than I thought as I reread that the riser is actually only 3 inch! Tunnel is 4x4 and exhaust is 4inch.


Solder is a very interesting idea.  Haven't heard that one yet. 

Makes me think of the liquid metallic sodium on nuclear piles....

Could be very dangerous if you have mercury or flux in your solder, the vapors could be emitted at unpredictable times.  And you'd need the right container, nothing that would let the molten metal escape or pressurize...


But with the right metals for the right temperatures, it could be a very interesting project.

And portable, too, with the solid solder helping to protect the other parts from shifting or warping.

Of course, the RMH originator would probably roll his eyes, and suggest that solder is best used for soldering useful things like cook-pots, circuitry, and water-tanks, and that dirt is a much more economical thing to use for mass.

I was thinking for applications where space is limited and the ability to carry mass is also limited. I was in fact thinking of a boat I would like to build I was hoping not to carry much in the way of fuel oil (if any).

Solder is mainly a lead/tin mix. It is cheaper to get it without flux core. While it is solid the expansion coefficient would be close to steel, not so sure after it melts though... probably expands faster. That was why I thought a second cylinder close to the other would work even sealed. There is a large area of steel that could safely be distorted.... though being round might make it too strong. Using a bench would not work because the temp might be too low by then to melt the the medium.

At home here, I will probably use brick... I have some other(tm) ideas I want to try. I have been playing around out back with some bricks... I need to cut up the water heater (not leaky.. the guy replaced it because he felt it was near enough to the end of it's life to change) so I have a barrel to try. From what I have read, it sounds like I will need to wrap my brick riser with insulation. Clay mortar will be my friend... easy to take apart

I want to try the bench with a relatively large space/passage. In the range of 6inches wide (assuming a 6inch riser) and 12 to 14 inches high... or two 6inch pipes in parallel with joins every once in a while. The exhaust from the barrel goes into the top one and comes out from the bottom at the other end to go up the flue. The idea is to slow the gases and give them some chance to stratify. The masonry heater boys call it a bell. (the barrel is actually a kind of bell)

I also want to try making a bell with just a vertical flue that is blocked. The long term use would be to heat a room one floor up.

Anyway, when/if I get that far I will tell all my findings.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 779
Location: Okanogan Highlands, Washington
    
  96
Len wrote:
I want to try the bench with a relatively large space/passage. In the range of 6inches wide (assuming a 6inch riser) and 12 to 14 inches high... or two 6inch pipes in parallel with joins every once in a while. The exhaust from the barrel goes into the top one and comes out from the bottom at the other end to go up the flue. The idea is to slow the gases and give them some chance to stratify. The masonry heater boys call it a bell. (the barrel is actually a kind of bell)

I also want to try making a bell with just a vertical flue that is blocked. The long term use would be to heat a room one floor up.

Anyway, when/if I get that far I will tell all my findings.


Look forward to seeing them.
Sounds like you're on track with most of it... the boat explains a lot BTW, Ernie is also a mariner.  Seems like sailors have a leg up on the fluid dynamics and general hands-on innovation stuff.

Re: the bell: With the right configuration, this may work just fine.  But you may need to reduce the heat-exchange length, and keep the exhaust hotter like the masonry heater guys do.

One reason the RMH's are so fussy about dimensions is that it allows a much longer heat-exchanger, pulling much more of the heat off the exhaust before release.  Our mentor has built systems that actually exhausted at 70f or below, effectively scrubbing all useful heat from the exhaust, and pushing the system on the draft of the internal heat riser alone.

Stepping the exhaust duct down slightly in size along a continuous run maintains good flow rates, whereas stepping it up can create sluggish flow and intermittent draft problems.

So be prepared to swap in other features that increase draft, as you swap out the RMH's dimensional constraints.
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
Erica Wisner wrote:
Look forward to seeing them.
Sounds like you're on track with most of it... the boat explains a lot BTW, Ernie is also a mariner.  Seems like sailors have a leg up on the fluid dynamics and general hands-on innovation stuff.

I've built a few, all small though (2 8 foot and a 16).... The one I want to build is over 50ft.

Re: the bell: With the right configuration, this may work just fine.  But you may need to reduce the heat-exchange length, and keep the exhaust hotter like the masonry heater guys do.

I was thinking a 7 or 8 foot bench... but the total length would then be double... of course the bench can be longer than the plumbing inside. I figure if we loose power for any length of time (unlikely as we are closer to the dam than the rest of town so we generally get power back soonest) we could do the Russian thing and sleep there as a family.

A taller riser may help, I was reading in some of the Aprovecho documentation that seemed to indicate if the riser was too high it would actually blow the flame out. Most seem to be 36inch, but I think my "barrel" would be good to 48 or so. If it was constantly blowing itself out.... the cutting torch is my friend. Our house is split level (open from the bottom level to the top (third) level. The flue and the spot where the old wood burning heater was is in the bottom. The flue was originally 12inches square inside, but at some point (when the wood burner burned through the firebox) they put a gas fireplace in with a 4inch pipe. I am not sure what they stuck down the flue to connect to it, but I will pull it out. I'll see what shape the original lining is in at that time, while I should be able to put a 8inch lining in to replace the 4, the hole through the wall is only 8 inch and I am not sure how close combustibles come to it, 6 inch insulated flue should fit though. Again, I will see when I remove the stuff in there right now. Anyway, the area is surrounded by a brick wall and the floor is tiled so it is the best place to put both the barrel and the flue.

Just as a side note, I had our nat. gas disconnected about three years ago. Our electric bill (all our heating is right now electric) is less these last few years than electric and gas together were before. Some wood heat at least would bring that down some more. It would add 600sqft of living space that we don't do much with in the winter as well. I could probably run an RMH off of just the trimming people leave out at the curb every year... a lot is 1/2 inch but there is a lot up to about 3inch as well.


So be prepared to swap in other features that increase draft, as you swap out the RMH's dimensional constraints.



Innovation is a high percentage failure.... or trial and error. So far the feed/tunnel/riser seems to work... for as long as I have run it, but I need to get the barrel on so I can hook up a flue. Then I can do a long burn and get it all up to temp. Then I can play with the rest... The vertical blocked flue has already been tried (aka clean out), but not to the height I envision. The rest I will see. I think I will try a 4 foot riser just for fun.
Erica Wisner
volunteer

Joined: Feb 10, 2009
Posts: 779
Location: Okanogan Highlands, Washington
    
  96
Len wrote:
A taller riser may help, I was reading in some of the Aprovecho documentation that seemed to indicate if the riser was too high it would actually blow the flame out. Most seem to be 36inch, but I think my "barrel" would be good to 48 or so. If it was constantly blowing itself out.... the cutting torch is my friend.


Sounds like you have your experiments all laid out, I won't interrupt except to say keep us posted!

Just FYI, the Aprovecho folks and the Rocket Mass Heater folks were not on speaking terms for about 15 years.  So they developed different 'rules of thumb' for different types of stove.  Be aware these are two separate labs, and they don't necessarily use each other's research.

In our experience, a heat riser where (without the barrel) the flame path reaches up to near the top of the heat riser, tends to work pretty well for RMH's.  A little flame poking out, or flame almost to the top, is fine.

I don't think I've seen anyone build a too-tall heat riser on an RMH, if only because the barrel and indoor height tend to set effective limits on what's convenient.

Taller = more draft, faster burn. 
(Putting out the fire may be more likely with the Aprovecho L-shaped cookstoves, where the fuel must be hand-fed inward, and air is often drawn over the top of the fuel instead of through.  I know they recommend adding a fuel/air shelf to get more air in underneath and help deal with this. 
The J-type downdraft stoves tend to benefit more from extra draft, burning hotter, and self-feeding the wood faster.  I haven't seen one 'blow out' aside from the very initial kindling stages.)

Shorter riser (or longer burn tunnel) = slower draft, more chance of smoke escaping into the room.
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
Erica Wisner wrote:
Just FYI, the Aprovecho folks and the Rocket Mass Heater folks were not on speaking terms for about 15 years.  So they developed different 'rules of thumb' for different types of stove.  Be aware these are two separate labs, and they don't necessarily use each other's research.

Figured that... I do tend to be a little on the socially inept side, but that much is pretty plain. Aprovecho do tend to deal with a wider variety of setups and their focus seems to be more on cooking in third world countries. Anything they have on the RMH seems to be old... not their "thing".  Even between those who are "working together" there seems to be different rules of thumb. I've worked with enough organizations... and probably been the cause of dissension myself a few times ... to understand. However, I did feel it was right to give my info source.

In our experience, a heat riser where (without the barrel) the flame path reaches up to near the top of the heat riser, tends to work pretty well for RMH's.  A little flame poking out, or flame almost to the top, is fine.

I don't think I've seen anyone build a too-tall heat riser on an RMH, if only because the barrel and indoor height tend to set effective limits on what's convenient.

That is helpful. I will look it up in Canadian standards for iron wood stoves and see how far from the roof I need to be and go from there. (it'll be metric) ... the MHA happens to have that info on their web site.

(Putting out the fire may be more likely with the Aprovecho L-shaped cookstoves, where the fuel must be hand-fed inward, and air is often drawn over the top of the fuel instead of through.  I know they recommend adding a fuel/air shelf to get more air in underneath and help deal with this. 
The J-type downdraft stoves tend to benefit more from extra draft, burning hotter, and self-feeding the wood faster.  I haven't seen one 'blow out' aside from the very initial kindling stages.)


The design I was looking at (out of interest, not to build) was 8 feet tall and could have run the riser that tall, but they put a wide space after about 3 feet to "slow the gases down" (thinking about that I have to wonder what room they expect it to fit in... none in my house). Their designs seem to be more continuous run, immediate heat, with no mass, but lots of radiating surface. They want to control speed of burn by feeding the fire slower so the burn can still be quite clean.

I personally don't like that idea, not because I don't think it would work, but because I don't like surfaces that get that hot. The Russian peasants had that one right. Hot surfaces, besides being a burning hazard also tend to burn the dust in the air. We live on the coast and it doesn't get that cold that we need to heat things up real quick or have a high cap heater. This is another reason I wish to try splitting the heat up all over the place and over a long time from a short burn.

I have found that a 400watt heater (5 fin oil filled electric on low) is enough to keep just about any one room in our house at whatever temp we like.... except the living room which is effectively connected to a hallway, kitchen and one bedroom that has no door yet and no heat really either. (the heat is turned off because we don't use that room right now) Even 1000 watts seems to take a while in there.
Bull norris


Joined: Feb 03, 2011
Posts: 50
Location: Chanute Kansas
The more i read the more questions i have, then i think about making a steel fire box .
should work right ,then i thought about a intake manafold the smother it is the harder to ajust the carb. when they extroude a manafould to enlarg the holes they make them smoother too. so the awncer to that is to regrind the inside of it .

So maybe i should case the out side and set the fire box out of fire brick and refrack cement? That way the inside of the fire box and flow will have the ruff texter that causes the syclone effect to form? 
                                      


Joined: Feb 08, 2011
Posts: 2
i have a friend that wants a rocket stove so they can produce wood charcoal to use as a soil additive. i was wondering if there is a special chamber in the rocket stove or how it can produce charcoal. also wondered if anyone knows about the efficiency of a masonry heater compared to a rocket stove. thanks
Len Ovens
pollinator

Joined: Aug 26, 2010
Posts: 1315
Location: Vancouver Island
    
  18
compostcanman wrote:
also wondered if anyone knows about the efficiency of a masonry heater compared to a rocket stove. thanks


I would say you don't want to go there   there is already a somewhat heated discussion going on about efficiency going on in another thread.

Seriously though, there has been a lot of testing going on with masonry stoves at http://mha-net.org/ I don't remember exactly where, but there are a number of posts about testing of various mass heaters in their lab. There is even someone in Europe experimenting with a masonry rocket design in a lab.... it does not look like any rocket stove I have ever seen, but the burn area is supposed to be based on the same principles. Mass heaters all work on the same idea, fast, short, clean and efficient burn... capture all the heat in a pile of mass to keep it from going up the flue and release it slowly.... be comfortable. The biggest difference is cost of install. RMHs are supposed to be cheap to install.
            


Joined: Dec 04, 2010
Posts: 79
I think this thread has created a level of confusion, equating a rocket mass heater with a rocket stove, which was what the original post concerned.  While a rmh can be used to cook with, it is not the same thing as a rocket stove, even though it uses the same technology.  And yes, a rocket stove can be made out of steel, many are and there are dozens of videos showing them on youtube.  No, rocket stoves generally do not create biochar, as they tend to burn the fuel to an ash state, not a charcoal one.  A wood gasification stove is what makes biochar, while giving off flame for cooking.
Gary Stauffer


Joined: Jun 27, 2013
Posts: 1
http://www.cotronics.com/catalog/44%20372%20%20375FT.pdf also http://www.cotronics.com/vo/cotr/fc_blankets.htm. I built the all steel rocket stove
from iwilltry. My wife would not let me use it in the house, so now it lays out in the yard. I have read that the steel chimney inside will not stand up to long
term high temperatures. So I was looking for a way to make it last longer if that was the case. Moldable ceramic wrap would kill two birds with one stone.
You could wrap your steel pipe, let it harden and if the steel tube deteriorates who cares. Because what is left is your thermal ceramic chimney. It does not seem to be that expensiveness. Having $500 to 600 a month electric bills this last winter makes me want to find an alternative heat source. So I want to build an outdoor version
for heating water for a water to air heat exchanger and perhaps in a green house setting. There is one guy on youtube that built a long feed tube in which he said he gets a 6 to 7 hour burn, he also has a how to prepare the wood video.
Ben Plummer
volunteer

Joined: Nov 15, 2012
Posts: 344
Location: Midcoast Maine, Zone 5b
    
  20
Gary, have you read Paul's article on cutting electric heat bills? My apartment has electric heat as well but don't mind it being 45°F inside, I love the cold

Edit: Lots of discussion here too.


"Always do right. This will gratify some people, and astonish the rest." - Mark Twain
 
 
subject: Rocket stove made out of steel.
 
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