12 rocket heaters - burning wood 5 to 10 times more efficiently

[Paul Wheaton]

So I posted a while back about this workshop I went to where we built one of these contraptions for about twenty bucks. While I was there, I managed to use my camera's video mode to try and capture a bunch of these that were already there. Well, I have a lot to learn about making the video thing work, but I did put what I could together.

YouTube

[Forerunner]

I thought the video was informative enough.
Pictures do speak volumes.
How do they clean the ash out other than the obvious, one small scoop at a time ?
I've been looking into wood gas technology in a wood stove that could be home made for some time. This looks like a very functional option to me.

[Paul Wheaton]

I know almost nothing about wood gas - I would like to know more.

As for ash, there are two spots to clean.

The first spot is where you do the burning. We had several instructors and some instructors preferred to make a little pit there and you clean the pit out once every couple of weeks. Others had no pit and thought you should clean it out each time you build a new fire.

The second spot is about a foot into the bench. Apparently, this is more for once every five years or so.

[Windy in Kansas]

Very interesting. When you first light the material in the feed does it smoke at all or is there always a draw just like the tall chimney of a home? It seems to me with the wood feed and combustion chamber and long bench heat mass it would take awhile to get some good draw flowing out the vent.

Quite interesting. Thanks for sharing the great video and narration.

[Paul Wheaton]

Did you see the guy with the wad of paper and tossing it down the chamber? If you watch my other videos, there will be a lot more explanation about it - I try to do a full summary of the workshop.

The key is that the combustion chamber is lined with metal surrounded by insulation. So it gets really hot, really fast. And then you have your draw.

Once the fire gets going, it sorta roars (a quiet roar) - hence the "rocket" term.

[Windy in Kansas]

I did see the fellow light the paper and figured it was to get it to draw but wasn't sure that would be enough.

These systems remind of home heaters in Korea. I've never seen them and have only been told about them---the heat is pulled along through passages in the floor as it goes to vent. Should make for nice winter time walking if not too hot.

[Forerunner]

I noticed that one unit used a water heater tank for the heat sink.
If the water heater was placed toward the end of the line, just before venting outside.... and plumbed... well, what an efficient use of wood heat.

We've looked into wood gas for some time now, procured most of the hardware to build several units for running different sized engines, and got bogged down in complexities and other priorities.
The complete combustion and much more efficient use of available BTUs in the wood really appeals to me. There are many youtube videos showing the simplest of wood gas design, for cooking a meal, to the most complex designs for running diesel engines. I wish some of the more mechanically inclined and imagination-rich among us were a bit closer geographically.
There is soooo much that could be done in building independence as a team.
One man only has so many hours and only two hands.
Forgive my whining.

[MELOC]

i can only imagine that with that much draft moving through the unit that there is a great air exchange with the outside. perhaps a good design consideration would be to include an air supply in the cob when building the exhaust. build the intake and exhaust all in one shot and terminate the intake so that the colder outside air can be taken in by the stove where the wood feeds in.

[SolarGary]

Thanks Paul -- good stuff.

I would like to add a wood heater to my shop to make use of the scrap wood.
I don't really have any good place for a conventional flue, and this has stopped me from adding the wood burner.

But, I noticed in the videos that some of the "exhausts" for these rocket stoves ended just outside the building, and sometimes very low to the ground. I did not see much sign of smoke staining or other problems from these unconventional exhausts.
So, I'm wondering if I can get away with such an exhaust arrangement if I build a rocket stove for my shop? Something in which the exhaust would exit through the wall and low toward the ground?
Is there something unique in the rocket stove design that makes a conventional flue unnecessary?

Gary

[Paul Wheaton]

Originally Posted by Windy in Kansas

I did see the fellow light the paper and figured it was to get it to draw but wasn't sure that would be enough.

These systems remind of home heaters in Korea. I've never seen them and have only been told about them---the heat is pulled along through passages in the floor as it goes to vent. Should make for nice winter time walking if not too hot.

There was a design on the internet where a fella routed a dryer vent tube to the wood feed before burying it in cob (mud). And then he had a BBQ or smoker lid that he could put over the wood feed. That way, all of the air would come from the outside - via the dryer tube.

So I saw this picture before going to the workshop, and I thought this would be an awesome question. Now, keep in mind that there are FIVE instructors at this event, because this class just happened to align with some sort of national pyro-instructor-convention .... so when you ask your question, it is actually fielded by five experts instead of just one ....

So I ask.

The answer took a good 15 to 20 minutes.

And I want to emphasize that these folks struck me as rather brilliant, so I'm still trying to digest this nugget ...

The consensus was:

A) You don't want to live in a ziplock bag. A lot of modern home designs are just messed up that way. I think this is a good point.

B) There is a lovely home in (I think it was ...) Sweden where there is a three inch opening along the roof line all the way around the house. You have a thermal mass heat system. Everybody is warm as toast. The focus on heat is more about radiant heat ant conductive heat. American heat is all about convective heat. I'm struggling to digest this one.

As an example: where we were sitting, it frosted outside and the building we were in was far from air tight. Visible gaps around windows, large (one inch or more) gaps around doors and a large hole for the cat to come in and out. It frosted outside every night. And we were warm as toast with one fire per day.

And a lot of the projects that we did were to "play with fire" and realize that we (well, maybe just me) really know very little about fire.

So ... adding an outdoor air intake ... the final story was, yes, you could. And it could help a little. But not enough (in their collective, unanimous, opinion) to be worth the hassle. Not even in the mountains of canada.

[Paul Wheaton]

Originally Posted by SolarGary

Thanks Paul -- good stuff.

I would like to add a wood heater to my shop to make use of the scrap wood.
I don't really have any good place for a conventional flue, and this has stopped me from adding the wood burner.

But, I noticed in the videos that some of the "exhausts" for these rocket stoves ended just outside the building, and sometimes very low to the ground. I did not see much sign of smoke staining or other problems from these unconventional exhausts.
So, I'm wondering if I can get away with such an exhaust arrangement if I build a rocket stove for my shop? Something in which the exhaust would exit through the wall and low toward the ground?
Is there something unique in the rocket stove design that makes a conventional flue unnecessary?

So my best video was from a large rocket mass heater in a building called "myrtle". And when the fire is running hard there, you wander outside and look at the exhaust. There's this little bit of steam sorta dribbling out. And it feels about room temp.

And you go back inside and look in the fire hole and there is this firestorm going on.

I'm told that what happens is that the combustion chamber reburns everything quite completely. Matter and gas are converted to heat. Not much is left afterward. In the combustion chamber, you can kind of hear that "rocket roar" and you know there is a lot of material going through there, but the combustion chamber is designed to be freaky hot. So what gas there is, is freaky expanded big. And then the heat is transferred to the mass, so the gas shrinks.

Interesting related tidbit: since nearly all of the rocket heaters there are made from salvaged stuff, they apparently ran out of 8 inch duct when making the mass heater for myrtle. So they used 8 inch duct for the first half and 7 inch duct for the second half. It's been running without a hassle for seven years. Ianto (the primary instructor and the author of the book) made this decision saying that by the time the air reached the seven inch duct it has already shrunk so much, it would make no difference. I think he mentioned that you could even use something smaller still.

The moral of the story is: yes, the dryer duct style exhaust should be fine. Keep in mind that what comes out of that is steam, CO2 and CO - so I wouldn't want to put it right next to a window or door.

[Paul Wheaton]

Originally Posted by SolarGary

Is there something unique in the rocket stove design that makes a conventional flue unnecessary?

Ooops, I forgot to address the last bit ...

I think your own observation answers that question.

If you had a conventional wood stove shooting the smoke out like a dryer vent, then your house would fill with smoke and the outside of your house would be blackened. So conventional wisdom is to keep the chimney exit above your roof line. That way, all of the pollutants/smoke will go away from you (and bother your neighbors?).

To call the output of a rocket mass heater "smoke" just seems less than accurate. "exhaust" seems more accurate. And when you look at the little trickle that comes out ... and the way it comes out, as a little trickle ... and it seems heavier than the surrounding air ... then it seems like a bad idea to try to push all of that up to the roof line. "Draining" it onto the ground seems wiser.

[horselogger]

Paul,youve sold me...now to figure out the bench mass in my wagon!!!!

[MELOC]

i would imagine the "experts" feel a bit defensive concerning their designs and the "technology" in general. it defies good logic to state that running an air intake while simultaneously building the exhaust is more trouble than it is worth. very few houses are totally air tight, so i think there is really no need for the added "fresh air" when considering the use of the rocket stove. it is logical to assume that supplying air from the outside will boost the effectiveness of the heater as one would be delivering the cold air brought in by the draw of the stove directly to the intake of the stove and not pulling it through any part of the house. i don't think the experts need to be defensive as the concept is good. nothing is ever perfect...at least not right away. then again. most of those cob houses looked pretty small, so maybe the rocket stoves generated more heat than needed and a little cold air is appreciated as there looks to be little regulation of the burn. if the draft were restricted, perhaps it would not burn so efficiently and the long, snaking exhaust would eventually have creosote issues. i dread to think of cleaning one of those if wet wood were used or the burn was not hot enough for any period of time. with the length of exhaust in most of those designs, i imagine there is lots of room to store and dry the fuel wood directly with the radiant heat.

[Paul Wheaton]

For those of you that think you might actually try this, I tried to compile a rather complete video showing the construction:

YouTube

And here is a somewhat more complete video about the exhaust:

YouTube

Also, a bunch of us have been talking about this for a long time at the permaculture forums at permies.com - and one of the instructors from this workshop has been popping in about every two weeks to visit with us. So if you're serious, there's a lot more info over there.

[Paul Wheaton]

Originally Posted by MELOC

i would imagine the "experts" feel a bit defensive concerning their designs and the "technology" in general.

I didn't get that impression. My feeling is that it was more like ... the five of them understood - and they realized it would be a while before I understood. And there were a lot of things that they did there that was a bit of a shocker to me in how fire and heat works.

The whole fire burning sideways thing is still a bit of a leap for me. Although I understand how it happens now. But how it happens with so little heat is amazing. And then there was the rumford thing - radiating heat from a small fire about 20 feet. The whole idea of a camfire is now blown away. And then holding your hand over the rumford fire - who-da thunk....

Originally Posted by MELOC

it defies good logic to state that running an air intake while simultaneously building the exhaust is more trouble than it is worth.

It defies logic with a limited information set. My impression is that when you add more information to the information set, then the logic works.

The key appears to be the value of inductive and radiant heat over convective heat. If your logic is based purely on convective heat, then you are right - it will not make sense.

Originally Posted by MELOC

very few houses are totally air tight, so i think there is really no need for the added "fresh air" when considering the use of the rocket stove.

Houses are getting more and more air tight because their heating and cooling systems are convective.

nothing is ever perfect...at least not right away.

True enough. And it would seem that there were many earlier rocket mass heaters that were later flushed as not as good.

And as long as we are talking about perfection ... if I haven't said it yet, I would like to express: I think this stuff is genius. I think Ianto is primary driving force behind this brilliant design. I hope he gets freaky rich.

[MELOC]

i think your post demonstrates how touchy folks can be even with the slightest suggestion for improvement or an objective discussion of the principles involved. that is the way of things i guess.

a leaky house is a leaky house and if you do your best to seal the cracks and eliminate air infiltration, it will retain more of the heat generated by any heating device. if you wish to sit on a warm bench all day, that is all well and good. once the heat radiates from the exhaust, the warmer air will move around the house via convection...like it or not.

radiant heat is all the rage these days. there are many forms of radiant heating...radiant heated floors are the first that come to mind. i think this system could be used in conjunction with radiant heated floors in areas away from the primary exhaust heated area via the use of radiant heated water tank like in the video you shot.

a fire burning sideways is no great mystery. if it doesn't rely on draft caused by rising hot air like in a chimney, the expanding gases will seek the path of least resistence and that will be the exhaust if the intake hole is smaller.

thermal mass is a great thing if you keep it warm. if it happens to get cold...it is more of a liability than an asset...just so people realize that.

i, too, hope mr. ianto gets freaky rich. i wish that for all good folks.

[Freeholder]

Hey, Paul, that 3" gap around the top of the walls -- how does that work when it's really cold out and the wind is blowing hard? I know some of the native tribes in this area (Eastern Oregon) used to just make brush walls for shelter (no roof) and have a small fire in the middle, and evidently did just fine with that. But I'm still having a hard time getting my head wrapped around that idea!

Kathleen

[Paul Wheaton]

Originally Posted by BlueJuniperFarm

Hey, Paul, that 3" gap around the top of the walls -- how does that work when it's really cold out and the wind is blowing hard? I know some of the native tribes in this area (Eastern Oregon) used to just make brush walls for shelter (no roof) and have a small fire in the middle, and evidently did just fine with that. But I'm still having a hard time getting my head wrapped around that idea!

Kathleen

I'm still trying to wrap my head around that.

Here is my best guess at this moment.

Suppose you have a mass heater in the room. And it is plenty warm. If you were to look at it with infra red vision, then it would look like a big red glowy thing. But without infra red vision, it just looks like a hunk of mass.

So the mass radiates heat to the walls, the furniture ... everything. So now everything is a bit glowy. And there you are standing between all of this glowy stuff and it gets its warm radiation on you too. So now you're glowy.

The mighty power of radiant heat.

So the air outside might be 20 below. And the air inside might be 50. But you feel warm because you are flooded with radiant heat (as opposed to warmed air).

And now to re-qualify: I'm not sure how factual this is. This is my best guess based on what little I know so far.

[Freeholder]

Your explanation makes sense -- I guess that, having been outside in some of our terrific winds (we are just in the eastern edge of the Cascades), I'm wondering about drafts with such a large gap at the top of the walls. However, I am totally convinced that fresh outdoor air is healthiest for us -- I think we'd get sick in the winter a lot less if we spent a lot more time outdoors instead of cooped up inside stuffy buildings.

I've got Ianto's rocket stoves book. Can't build one in the house, but have designated a corner in the garage for one! Did that before we even built the garage. The video (which I finally had time to watch) shows some work-benches with flues running through them, which solves one problem I had -- I was trying to figure out how to have a seat-height bench to run the flue through, and still have space for a workbench. And I think I've just solved another problem, too! I've been needing a chick brooder that didn't require electricity -- I think if I made one section of the bench low and the right size to fit my chick brooder (a chicken tractor), that would probably work just great! Might even figure out a way to incubate eggs, although temperature control would be a trick....But the ancient Egyptians incubated eggs in wood-fired ovens, somehow.

Kathleen