rocket mass heater

[paul wheaton]

So there seems to be good responses to my stuff in this forum.  So I have another thing to share.  (I searched the forum and found some mentions, but no dedicated thread)

I have now attended two rocket mass heater workshops.  I have to say that these contraptions are rather amazing.  Definitely the next generation of heating your home with wood.

The perks:

Heat your home with five times less wood.

Cleaner, more complete burn.

You can build one in a weekend (I'm about to try an experiment and build one in less than a day)

If you scrounge for parts, you might build one for as little as $20. 

The heat from one fire can last for days.

I've mashed all of my notes together into one page on rocket mass heaters.

Anybody else a fan of these contraptions?

[RainDog]

 I just don't quite understand how they manage to draw through the combustion chamber, all that horizontal pipe, as well as the four 90s I count in your construction photos.

I don't doubt it works, I just don't get it. Especially considering that people have problems with the draw in wood stoves without having near that degree of resistance in the exhaust.

A light bulb never did appear above my head concerning that. It remains an empty space. Not my head, but the space above it. Okay... maybe both sometimes.

Oh, and how do you clean the firebox of ashes?

[MountainDon]

Raindog, you are not alone in your puzzlement. I wonder about starting the fire without having smoke in the room aas well as how it can draw around so many corners. 

[bayview]

   I'm the first to admit that I don't have any knowledge about this, BUT . . .

   Wouldn't a slow burning fire in all that flue material cause unwanted creasol?  Seems like a disaster waiting to happen . . . (?)


/

[Ernest T. Bass]

It's actually a very fast burning fire; that's why it's so efficient and clean. Most of the heat is absorbed by the thermal mass that the flue is typically run through before it exits the building. Our rocket stove-heated cob bench was still quite warm all day today, even though we haven't run a fire in over 24 hours. (It was a sunny day in the 40's, and the house stayed at about 67 with no additional heat..)

A properly designed stove does not smoke at all when you're starting a fire.. Our stove was an afterthought and we were forced to put the whole vertical stack outside. We also run our pipe about 20' through the floor, which is lower than your typical through-a-long-bench run. Those two factors make the stove a little difficult to start every once in a while, but we never have a problem once it's going.

[glenn kangiser]

Thanks for continuing to contribute on this forum also, Paul.  People here like to learn - and some haven't ever heard of this stuff.

Some like Andrew are already using it though.

How often do you have to feed it, Andrew - Is it a time consuming process?

My stack wasn't tall enough and I wanted the room so I took it out.  I was getting downdrafts due to the low chimney.  I had to extend it for the Franklin stove too.

I know the fast fire with small dry wood is the key and the creosote becomes part of the fuel rather than coating the inside of the pipes./

[NM_Shooter]

Is the open fire inside or outside?  It would seem that in order for the thing to start to draw, it would initially have to be warm.  If it didn't draw well, then why wouldn't startup be a problem?

I really like the huge amount of cob that is used for thermal mass.  Because of how slowly that heats up, it would be fairly easy to control the the amount of heat.  Once it gets to the right temp, extinguish your small fire.  However, the flip side would be that in order to get it and the house up to temp, you would need to tend the fire for a long time.

[paul wheaton]

Usually, when you go start a fire, the combustion chamber is still a little warm - so there is already a slight draw.  But if the combustion chamber is really cold, wad up a piece of paper, light it and toss it in under the combustion chamber.  Since it is insulated, that little bit of heat usually gets a little draw going.  Then when you start the rest of the fire, the first bit of smoke goes up the combustion chamber (instead of in your face) - and that smoke warms it further.

I don't doubt it works, I just don't get it.

Which is why the book is of such great value.

Oh, and how do you clean the firebox of ashes?

It depends on which instructor you have!  My first workshop had five instructors so you got to hear some debate on the issue.  Some people reach down and pull out a handful of ashes just before every burn.  Some like to make a little pit and pull the ashes out every ten burns or so.  Others like to shop vac the bottom.

I wonder about starting the fire without having smoke in the room aas well as how it can draw around so many corners.

Silly rabbit, trix are for kids!

The only draw is in the combustion chamber.  It is designed to have a draw equivalent to a 20 foot tall smoke stack fully heated - and then compressed into two feet. 

So everything beyond the top of the combustion chamber is a push.  So the behavior is more like a dryer vent than a chimney.

Wouldn't a slow burning fire in all that flue material cause unwanted creasol?

I had the same question.  And I was directed to a system that had seen heavy use for seven years and told that when they opened it up recently, they found some ash, but no moisture and no creosote.  The explanation is the burn is so hot and so complete that after the first minute the exhaust is only steam and CO2. 

Our rocket stove-heated cob bench was still quite warm all day today, even though we haven't run a fire in over 24 hours.

It might even still be too warm to sit on directly.  ??

Thanks for continuing to contribute on this forum also, Paul.

Always happy to help out where folks seem to appreciate what i have to share.  If there is no objection, i'll try to stop by whenever I have a new video or new article that might be relevant to these forums.

Is the open fire inside or outside?

Yes.

(sorry, couldn't resist)

Most folks have the feed tube indoors.  So while standing over it you can look in and see the flames running away from you.

If you look at my rocket mass heater article, you will see a video or two that shows the feed tube outside.

Once it gets to the right temp, extinguish your small fire.

Nobody extinguishes the fire. 

The thing is that the mass heats up very slowly.  So when the fire needs more wood you sorta think "nah, that seems warm enough" and then you let the rest of the wood burn and then you're done.

[Ernest T. Bass]

The fire is inside.. The flames draw themselves horizontally through the burn chamber. Here's a sketch I found:

Even with the lid off of the feed chamber we never get smoke in the house once the fire is going, unless a piece of wood gets 'hung up' and doesn't feed itself, which doesn't happen very often.

We mostly burn Aspen, and we split it pretty small. We feed a couple pieces with about a fist-sized cross section every half-hour or so.. If you pack it full of hardwood it'll go about an hour and a half. Some people are turned off by the amount of attention it requires, but it only takes a few seconds and we're always home. One of those 'not for everybody' type of things, but it seems that most everybody who it IS for like it a lot.. Our neighbor has one of those smokey, expensive outdoor boilers and he's very proud of it.. But he never looks at our firewood pile without saying "Are you sure that's enough?".

[Ernest T. Bass]

Our rocket stove-heated cob bench was still quite warm all day today, even though we haven't run a fire in over 24 hours.

It might even still be too warm to sit on directly.  ??

We've got our flue 7 inches deep, so it takes a full day of burning before it gets too hot to sit on without a cushion. It's usually very comfortable the next morning.. With a few cushions on it it'll stay warm for three days.

[RainDog]

 

The only draw is in the combustion chamber.  It is designed to have a draw equivalent to a 20 foot tall smoke stack fully heated - and then compressed into two feet.  

So everything beyond the top of the combustion chamber is a push.  So the behavior is more like a dryer vent than a chimney.

Gotcha.


Tell ya one thing for sure, it's difficult finding information on rocket stoves from anyone who isn't ideologically invested in promoting and praising them.

Here's my take considering what I could dig up:

Rocket stoves can be a positive and viable alternative in the developing world, where there is not access to materials needed for other firebox and flue run systems and wood for fuel is at a premium, to the widely used three rock fire in an enclosed area, which is highly inefficient and causes major health problems. They can be built with found items and materials and require no great craft to build, and are therefore attractive to those who, out of need or inclination, wish to build for themselves.

Other than that, they suffer all the same downsides, and aren't superior to other wood burning heating systems, which can be of equal or greater efficiency and burn just as cleanly if well designed.

The thermal mass, granted, is pretty darned large, but that's nothing particularly new, as masonry wood burners use mass as well to contain and evenly release heat over time as a rule.

Fair enough, or am I being overly critical? Is that about the gist of it?

[glenn kangiser]

Paul, please continue to bring relevant content to the forum anytime you have time to do it.  

We have people interested in all sorts of things here... some like alternative, some like conventional, but if they see different ideas even if they don't use them immediately, at least they know and who knows when that knowledge may prove useful. Not having knowledge is more of a problem than having too much.

I was originally invited here to answer questions about my underground cabin, even though most here build above ground,  

Thanks for taking the time to share with us.

[RainDog]

Paul, please continue to bring relevant content to the forum anytime you have time to do it.  

Absolutely. I second that.

Thanks

[NM_Shooter]

Andrew, can you post some pictures of yours? 

I'm confused as to how a 55 gallon drum with a "plasma reburn section" can hold up to the heat over time.  Maybe plasma is not the right term?

I guess that the efficiency of this thing actually comes from the fact that the larger thermal mass sucks the heat out of the exhaust before it exits. 

Does it draw very, very heavily?

(Mtn Don, I'm thinking of the firepit at the Rudy's on Carlisle.... I often wondered as to why that does not smoke the place up)

[Ernest T. Bass]

Yeah, I didn't get the "plasma reburn" thing either.. The barrel does get quite hot, but it's not supposed to burn out nearly as fast as your typical barrel stove, since most of the oxygen is consumed by the fire before it reaches the metal. Ianto's is many years old and still cooking. And, it's not exactly hard to replace if it does burn up.

It does draw quite heavily, hence the term 'rocket stove'. It has a bit of a rumble from the flames when it's running good. You can tell when it's time to stoke it just from the sound, even if you're on the other end of the room. The good draw also makes it very easy to start a fire; we hardly ever use kindling. Just a couple small and dry pieces of popple, a half a sheet of newspaper and the rocket takes off.

I'll look for pics.. Not sure if we have any besides what's on the blog.

[Ernest T. Bass]

Couldn't find any new pics besides the construction ones:







The bench looks a bit nicer than this now after drying out/getting the stones cleaned/getting oiled..

We actually gave the stove a little overhaul this past fall. Many of the old bricks we had used were falling apart due to the heat stress, so we replaced them with some firebrick and a chunk of clay chimney liner for the heat riser. We also took out the clay/sawdust insulation we had used and replaced it with vermiculite. Seems to work a little better, but it would have been fine before if not for the inferior brick..

[NM_Shooter]

Very interesting.  Thanks for the pix.  I am amazed by the length of the "flue"! 

I wonder if there would be some way to use a small fan in a seperate chamber to assist with starting airflow when the unit is cold. 

I'm really intrigued by this.  Not a solution for my cabin, but what a neat idea to harvest all that heat that normally escapes up the chimney.   I am a little spooked by it though.  If the barrel fails, things will get dangerously ugly in a hurry. 

Do you have any sort of a way to measure the temperture on the very top of the barrel?  I'd be curious to know how hot it gets when it is up and running. 

In the first picture, is that your firepit that is well off to the right?  What made you guys position it that far away?

Also... how did your family learn about these types of stoves?


Thanks... -f-

[Redoverfarm]

Frank I am like you.  People used to make oil drum heaters all the time.  There was one stacked on top of the other.  In fact there was kit's available.

http://www.northerntool.com/webapp/wcs/stores/servlet/category_6970_122+770342+821214

I remember that they (the drums) didn't last that long and they would glow red from the heat.  I think it is a really cool idea but if I were building I would have to have a more stable firebox.  

[Ernest T. Bass]

Very interesting.  Thanks for the pix.  I am amazed by the length of the "flue"! 

They say to keep an 8'' diameter flue under 30' long.. I think we are about 27' with 3 90's and a couple 45's... In hindsight we should have put another cleanout somewhere down the line. You're only supposed to have to vacuum the ash out every 5-10 years before it effects the performance of the stove, but we're either going to have to make a very long hose for the shop-vac or cut out another cleanout.

I wonder if there would be some way to use a small fan in a seperate chamber to assist with starting airflow when the unit is cold. 

Like Paul said, throwing a piece of burning newspaper to the back of the burn chamber will usually get it going. Worst case scenario, you have to stick a little burning paper in the cleanout at the base of the vertical stack. It would have been much better if we could have kept the vertical stack inside the house for the most part.. I don't think there would ever be cold start problems. Also, thin stovepipe could be used without any problems, and it would radiate more heat back into the house. We're stuck with a 25' stack outside that should be insulated ideally, and weatherproof..

I'm really intrigued by this.  Not a solution for my cabin, but what a neat idea to harvest all that heat that normally escapes up the chimney.   

I'll admit that playing with fire can get kind of addicting.. It's made me a bit of a pyromaniac..

I am a little spooked by it though.  If the barrel fails, things will get dangerously ugly in a hurry. 

Unless the brick structure of the stove collapses, I don't know how it could suddenly fail.. If you go all out and build the whole thing with firebrick and refractory cement it would be pretty foolproof.. If you leave the lid on the feed chamber with just the center hole open for air, even if the draw were to reverse the fire would choke itself out and you just end up with a house full of smoke.

Do you have any sort of a way to measure the temperture on the very top of the barrel?  I'd be curious to know how hot it gets when it is up and running. 

No.. We have a copper water heating coil over the top of the stove with a secondary 'lid', so the actual exposed top never gets glowing hot or anything. When it's going strong you can hold your hand about 4'' over the top without much discomfort. Without the water coil it would probably be more like 8-10''

In the first picture, is that your firepit that is well off to the right?  What made you guys position it that far away?

I think that's just the lid to the feed barrel sitting on the ground.. It's in its proper location in the second pic.

Also... how did your family learn about these types of stoves?

I think my mom found them on the internet.. There's more stuff available online now. We built the thing according to this book:

It's a fun thing to experiment with.. My mom wants one that she can bake in, and I'd like to make one that's more specifically designed to heat water.

[paul wheaton]

Here's my take considering what I could dig up:

Rocket stoves can be a positive and viable alternative in the developing world, where there is not access to materials needed for other firebox and flue run systems and wood for fuel is at a premium, to the widely used three rock fire in an enclosed area, which is highly inefficient and causes major health problems. They can be built with found items and materials and require no great craft to build, and are therefore attractive to those who, out of need or inclination, wish to build for themselves.

Other than that, they suffer all the same downsides, and aren't superior to other wood burning heating systems, which can be of equal or greater efficiency and burn just as cleanly if well designed.

Oh man, my eyes are rolling so hard I'm getting a headache. 

And now my headache is worse because I'm beating my head against a wall. 

I had to go outside and count to ten, ten times. 

Deep breaths ....   okay ....

First, we have two different contraptions.   A rocket stove is for cooking food.  And while it has been utterly amazing in third world countries, I think it would be equally amazing in any home that chooses to cook with wood.  Far cleaner and far less wood. 

A rocket mass heater is a different contraption.

Like you said, a rocket mass heater uses a mass.  Nothing new there.  But there are many things that are new.  Things that are not part of any other wood burning contraption.  If nothing else, a RMH can be built in a weekend for about twenty bucks.  A russian heater is more like a month and at least ten grand. 

It is true that the very best designed modern wood stove when used with the draft ALWAYS wide open might be able to come really close to a RMH in cleanliness.  But a RMH will keep three to ten times more heat in the house. 

So, based on my knowledge, I would have to say that it is my opinion that when it comes to cleanliness, the RMH is generally superior.  And when it comes to heating efficiency and startup cost, the RMH is far, far, far superior. 

And, I need to point out, that this opinion is radically different from the opinion you conveyed.

[paul wheaton]

I'm confused as to how a 55 gallon drum with a "plasma reburn section" can hold up to the heat over time.  Maybe plasma is not the right term?

When I think of "plasma" I think of molten lava.  I suppose I could go and spend some time looking up the word "plasma", but I think if I owned that drawing, I would change that word.

Basically, the two foot tall combustion chamber is insulated and right above the fire.  So it gets freaky hot in there.  Plus, the smoke and flames and air are getting very thoroughly mixed.  Combine the mix and the freaky hot temp and your smoke gets completely burned.   

[paul wheaton]

I am amazed by the length of the "flue"! 

Rather than "flue" I usually go with the word "exhaust" or "exhaust pipe" or "duct".  After all, at this point it is more like a dryer duct than a chimney.

[frazoo]

I wonder what kind of btu's the 55 gal model puts out, and would a smaller 35 gal drum work? 

frazoo

[Don_P]

I'm not convinced that this isn't a draft dependent burner. If you are able on the next build try building it with a temporary horizontal exhaust below the burn tunnel and see if the fire "knows" where the exhaust is after its warmed up. Here nor there, I'm just trying to understand.

I believe a flame is one form of plasma.
Water vapor aside, smoke is mostly inefficient combustion, unburnt fuel that either didn't have sufficient oxygen to burn or some of the fuel was carried away from the heat before it could ignite. It looks like the reburn chamber is similar to most modern woodstoves, the smoke is mixed with air and given a place to concentrate heat and burn.

Benjamin Franklin invented a downdraft woodburner that was later improved upon and attributed to Charles Willson Peale, "The Smoke Eater". The smoke was returned back through the fire, ran horizontally at floor level, past an observation port, and then to a chimney where it exhausted only warm air.


I think my old Sotz barrel stove is in its 5th barrel and needs a new one, I've been using the door and flue flange for about 30 years, it does go out on jobsites in the weather at times. This one is a single barrel although a shop I rented in the 80's had a double barrel. I don't believe we were getting combustion in the upper barrel. I think if it had a larger intake or an intake on the upper barrel and some form of liner it might burn up there but the heat needs to be able to rise enough to ignite the unburnt gasses. Our second barrel was basically cooling the smoke indoors instead of in the flue. I recall it being a creosote maker. That is the only setup of one I have any experience with though.

I went googling and found this pic of a Hungarian bubos kemence. The white domed structure is the "barrel"

[Redoverfarm]

But Don for almost instant heat in a unfinished area they were hard to beat.  Good shop furnace if you left them wide open.  I always remembering to use sand in the bottom of the barrel to reduce them from burning the barrel's out.