Author Topic: rocket mass heater uses EIGHT TIMES less wood than a wood stove  (Read 15310 times)

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Offline paul wheaton

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I just now uploaded this.  Ernie Wisner talks about using four cords
of wood every winter using a wood stove.  And now he uses a half a
cord of wood each winter with a rocket mass heater.   Since he uses so
little wood, it ends up being just tree trimmings from his urban lot.

http://www.youtube.com/paulwheaton12#p/u/0/_jfag47dRCs

Offline NM_Shooter

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #1 on: August 17, 2010, 03:40:15 PM »
Do you think this might be because the exhaust gasses are cooled by the rocket heater mass, and most of the heat is kept in the building?

I wonder if it would be possible to make a "mini" rocket mass heater.  If I could figure out a way to build one in a 1'X1'X6' space....My cabin is so small, that a regular wood stove would blast me out.

Probably not... unless I wanted to be constantly feeding it small pieces of wood....  Hmmmmm......
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Offline paul wheaton

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #2 on: August 17, 2010, 06:20:25 PM »
Quote
Do you think this might be because the exhaust gasses are cooled by the rocket heater mass, and most of the heat is kept in the building?

Yup!  Exactly!

A traditional wood stove is pumping heat out of the house in order to get the smoke out.

Offline MountainDon

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #3 on: August 17, 2010, 06:35:52 PM »


A traditional wood stove is pumping heat out of the house in order to get the smoke out.


Depends on how traditional the traditional stove is. If it is outfitted with an outside fresh air inlet no air from the interior will be leaving the cabin and no cold air is being unnecessarily introduced to the interior. It'll still use more wood than a rocket, but the air inlet works very well. Especially nice when fitted with a air inlet booster fan for getting a fire going.
Just because something has been done and has not failed, doesnít mean it is good design.

If you didn't have the time or money to do it right in the first place, when will you get the time and money to fix it?

Offline paul wheaton

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #4 on: August 17, 2010, 07:08:40 PM »
Actually, I'm thinking about the 600 degree smoke going out the top of the chimney as opposed to the 100 degree exhaust dribbling out of the vent for a rocket mass heater. 

As for air intake, I used to be of the camp you are talking about.  But when I attended the RMH workshop, there were five instructors there having a bit of an instructor re-union.  They tried really hard to not roll their eyes.  After about 20 minutes I could see their point.  Now that a lot of time has passed, it makes even more sense.

First, a rocket mass heater can be set up with an external air intake - no problem. 

Second ....  well ...   it would take 20 minutes to say it I suppose.  Let me just say that they convinced me with stuff like "do you want to live in a ziplock bag?" and then the reminded me about radiant heat and conductive heat.  And then the told me about a place in sweden that had a three inch gap to the outside all around the house, but everybody felt plenty warm inside.  They also said "do you want to burn stale air or do you want to burn fresh air?"

That all probably just sounds crazy - as it did to me the first time I heard it.

Offline Ernest T. Bass

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #5 on: August 17, 2010, 07:45:11 PM »
I wonder if it would be possible to make a "mini" rocket mass heater.  If I could figure out a way to build one in a 1'X1'X6' space....My cabin is so small, that a regular wood stove would blast me out.

Probably not... unless I wanted to be constantly feeding it small pieces of wood....  Hmmmmm......

You're right about constant feeding... I've seen many "pocket rockets", but it's a full-time job sitting over them with a pile of sticks.

A traditional rocket mass heater really takes very little space, though, when you consider that the mass is in a very usable form (such as a bench), and you need a place to sit anyway. It's a multi-tasking stove.. :)

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Offline paul wheaton

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #6 on: September 13, 2011, 11:21:02 AM »
Moments ago I uploaded a video where a rocket mass heater combustion chamber is mounted on top of a conventional wood stove. 
 
Ernie Wisner has built over 700 rocket mass heaters and this is his third hybrid.  In three minutes he covers a lot of detail about the efficiencies of conventional wood stoves, rocket mass heaters and hybrids.  Including some enlightening information on how rocket mass heaters can heat a space using 90% less wood.

http://www.richsoil.com/rocket-stove-mass-heater.jsp#hybrid


Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #7 on: September 13, 2011, 12:13:29 PM »
I understand the physics, but I am a little off on some safety issues.  There is no way to get a 100% burn.  So whatever volatile gases (creosote), no matter how small, cool as the smoke cools in the chamber collect.  Is there a way to clean the piping?  What happens if there is a fire in the pipe?


I was looking for an environmentally friendly way to give supplemental slow release heat in a greenhouse.

Offline paul wheaton

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #8 on: September 13, 2011, 12:20:04 PM »
Quote
There is no way to get a 100% burn.

I'm confused.  What is your point here?

Quote
smoke cools in the chamber collect.

I'm confused again.  What are you talking about here? 

Quote
I was looking for an environmentally friendly way to give supplemental slow release heat in a greenhouse.

Have you seen this:





Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #9 on: September 13, 2011, 12:39:13 PM »
Whatever flammable gases (creosote) that don't burn, and will collect as they cool. All be it a small amount of gases, in a normal wood stove or fireplace, the pipe or chimney can be cleaned with a brush.  How is this dealt with in a rocket stove?  It looks like once it is built, it is a solid piece and you would have to destroy it to clean it.

Offline paul wheaton

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #10 on: September 13, 2011, 12:46:10 PM »
For a rocket mass heater:

1)  zero creosote.  for the thousands built, there has yet to be a report of creosote in one.

2)  I have yet to see one that doens't have a way to clean out the ash. 


Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #11 on: September 13, 2011, 12:57:41 PM »
How do you clean the piping?

Offline davidj

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #12 on: September 13, 2011, 01:07:03 PM »
Modern wood-burning stoves are supposed to have efficiencies in the 60%-70% range (although I'm guessing it's more like 50% in real life).  A rocket mass heater has got to be a lot less than 100% efficient, so at best it could use maybe 2/3 of the wood of a modern stove.  So it may use 1/8 of the wood of a really inefficient, inappropriately-installed stove but it's unlikely to be much better than a modern stove that's been installed sensibly (i.e. away from draughts and in the part of the house you are trying to heat).

Offline Native_NM

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #13 on: September 13, 2011, 01:15:10 PM »
We built a bio-mass stove for a science project a few years back using old steel cans, an old computer fan, rivets, and some other common junk. Note that the new paint cans, like the unused ones from Lowes melt. You need steel. If you have a welder you could put one together pretty quick.
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Offline paul wheaton

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #14 on: September 13, 2011, 01:20:30 PM »
I have the footage for another video along those lines. 

Basically it starts off with:  the whole 75% efficient thing turns out to be one big steaming pile of lies.  First, the scale goes to 116%.  Sometimes higher - this is because of the amount of heat that is allowed, in the system, to go up the chimney.  Next, the labs are competing with each other to do the ratings for the stove manufacturers.   So the numbers are mysteriously higher at some labs than others.  And so then the manufacturers will only use the labs that give the higher numbers.  The labs with ethics go out of business.   There is a lot more wackiness down that path.   Finally, the average wood stove owner tends to run their stove much less efficiently than what the labs do.  Probably a lot closer to 10% than the claimed 75%.

Next, the rocket mass heater has fewer ways to be operated inefficiently.  Plus, it doesn't try to push the exhaust out the roof, so it can extract more heat from the exhaust.  Conventional wood stoves usually have smoke leaving the roof at 300 to 600 degrees F.  Rocket mass heaters have exhaust leaving at about 90 to 110 degrees F.

And that's just the beginning. 

The bottom line is that folks that take out a conventional wood stove and replace it with a rocket mass heater, typically experience burning 80% to 90% less wood.




Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #15 on: September 13, 2011, 01:31:37 PM »
Modern EPA wood stoves have a re-burn feature similar to the concept of a rocket mass stove.  The purpose is to get a complete and hot burn of all the gases in wood.  I understand capturing the additional heat lost from the piping of the hot exhaust.

There is always some creosote, even if it is a small amount.  In your videos the description even states that the first 8 minutes or more of firing it is not efficient.  Then to trust my life to the idea that it will be 100% efficient in burning all volatile gases for the entire life of the stove would be fool hardy.  I watched many of your videos on the stove.  Some had vent pipes snaked all around a room with, from what I could see, no clean outs?  Do people assume there stove is 100% efficient all of the time?  I didn't see a way to clean the vent pipes addressed in any of the videos, just an ash clean out.

Edit:
I reread this and thought the tone could be misconstrued.  I am just curious how this is dealt with.
« Last Edit: September 13, 2011, 03:57:45 PM by Squirl »

Offline Ernest T. Bass

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #16 on: September 13, 2011, 04:07:37 PM »
In a good design, you have a cleanout located at every sharp bend. The ash buildup is so slow that you only should have to scoop/vacuum the pipes after 5-10 seasons. I wouldn't think that you could have a chimney fire in the thermal mass section of the system, (and it would be well contained if you have the pipe buried deep in cob), since the oxygen is consumed in the burn chamber. ??? We did have a small chimney fire at the base of our exterior vertical stack once, and it completely baffled me.. There was hardly any creosote buildup, and the temps are so low out there, I don't know how it started.. But all we had to do was cap up the stove and it snuffed right out. Wish we didn't have that vertical stack... You really would never experience creosote if you exited the building through the roof with a short section of insulated pipe.

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Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #17 on: September 13, 2011, 04:17:32 PM »
I could also see the massive savings in wood, depending upon the structure or individual.  In practice the way to get the 60% efficiency out of a wood stove is to burn it as hot as and fast as possible.  In practice most people don't do this.  In a modern well insulated home, the heat could easily become uncomfortable very quickly.  Most people I know try to get a slow burn out of there wood stove at night so that it releases heat all night.  This way it doesn't over power them in the beginning and leave them freezing in the wee hours of the morning.  The added benefit is they don't have to get up in the middle of the night to refuel.  I have been guilty of it too.  This is the least efficient way of burning wood and wastes a lot of its energy.  So the stove may be super efficient, but the users don't want that. 

In theory, and by testimonial, the higher thermal mass of a rocket stove is supposed to work as a heat sink, slowly dissipating the heat into the room over a long period.  By the design of burning as hot as possible for a more complete burn with the distribution method, in practice it is supposed to use far smaller amounts of wood. Masonry stoves are supposed to do the same thing, but without the gas re- burn chamber.  I would love to see more engineering data on these, other than mostly anecdotal.

Offline Ernest T. Bass

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #18 on: September 13, 2011, 05:27:25 PM »
Well, my testimonial for the past three winters we've used the stove is as follows; we burn 4 loggers' chords (4x4x8) of Aspen, which is about the lightest (least embodied energy)  wood there is.. Our cobwood home is about 1200 s.f., and although the walls are 16'' thick, we have developed quite a bit of draftiness due to the greenish logs we had to build with. On a calm, 10 degree winter night, we only lose a few degrees from the time the fire is put out (10ish) to the time we start it up again in the morning (8-9ish). Windy nights are a bit cooler, but most of the house stays around 65 degrees, and if that is too cold for ya' the bench is always very warm, sometimes too hot, for up to 3 days after the fire is put out.

It would be very interesting to compare that to a regular wood stove for a while, but we have no acceptable chimney for a test..

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Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #19 on: September 13, 2011, 06:15:07 PM »
We did have a small chimney fire at the base of our exterior vertical stack once, and it completely baffled me.. There was hardly any creosote buildup, and the temps are so low out there, I don't know how it started..

That is one of my main concerns.  There are many more volatile gases discharged from wood other than creosote.  Turpentine and Methanol are two that I can think of off the top of my head.  Many of these gases you would never have worry about in a normal wood stove.  The exhaust gets so cool out of a rocket stove it complicates things.  At the temperature of close to 100 degrees these and many other gases become liquids or solids, yet are still very flammable.  Even if it is a very small amount of inefficiency, there is something there.

Offline Ernest T. Bass

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #20 on: September 14, 2011, 03:59:49 AM »
Usually the exhaust is closer to 200 degrees, especially if the stove has been running for a while and the thermal battery is warmed up. Our exterior stack is currently a cheapy thinwall pipe that is 25' long (read: cold...).. A short section of insulated pipe would be ideal; I don't think that the gases would condense at all before leaving.

It wasn't the hot, raging chimney fire that you typically hear about... Just a few flames visible though some gaps and some ash flying high..

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Offline Alan Gage

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #21 on: September 14, 2011, 06:01:40 AM »
Squirl:

I've got the same worries about the rocket stoves as you. The reason to keep the chimney hot on a wood stove is to keep the gasses from cooling and condensing inside the chimney, which causes creosote to form. That's also why heat recovery units built into the chimney aren't recommended, they cool the gasses too much. If the exhaust was 600 degrees leaving the stove and 100 degrees out the chimney I'd be very worried about what was building up inside those walls.

Alan

Offline Squirl

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #22 on: September 14, 2011, 07:21:36 AM »
Yes, creosote boils at 400 degrees Celsius.  Because wood stove exhaust can hover between 300 and 600 degrees creosote is the main gas that can condense in a wood stove.  Because of this and the familiarity of wood stoves, it is what most people are familiar with.

Many of the videos and promotions of rocket mass heaters state the exhaust exits around 90 degrees F. Almost all other flammable oils and alcohols in wood condense by 150-160 degrees F.  Combine this with the greatest statement that I could find is a 90% efficiency of burn (anecdotal "Rocket Mass Heaters: Superefficient Woodstoves YOU can build" (Ianto Evans and Leslie Jackson, Cob Cottage Company p.80), and you have serious safety concerns.  Even if it were 99% efficient, that 1% can be alarming built up over time or just based on the contents of that 1%.  Anyone that has run a backyard still knows the flammability of a condensed alcohol (methanol).

I'm not saying that a wood stove is a better, but it takes these factors into consideration in the design.  It wastes the heat up an insulated pipe to cut down on condensation of flammables.  There are manufacturer guides and whole organizations devoted to the design of dealing with its drawbacks.  It also is a requirement that they have easy access to clean the pipes with a chimney sweep, and promote regular cleaning.  In a sense it trades efficiency for safety.

This is one criticism I have of many of the publications promoting rocket mass heaters.  They donít seem to address any of these issues.  Either they donít know or understand them, or they donít care.  Saying these issues donít exist would defy physics and sense.

Iím sorry if some of this is redundant, I was just trying to be clearer with my thoughts and concerns.

Offline JRR

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #23 on: September 14, 2011, 07:33:38 AM »
On the question of what air source to use for combustion air, fresh air or "stale" air, ... I would choose fresh air piped directly into the location of the heater.  Stale air should be removed through other proper ventilation devices and should not be dependent on the heating process.  The heating process should be as "stand alone" as possible.  Supplied combustion air in, heated exhaust out...with flows properly controlled.  Separate breathing air ventilation should be "as needed", heating season or not.

A tightly built, well insulated small residence that has been choosen to fit well with its environment ... will make any heat source seem to perform at its best.

A rocket mass stove certainly looks like a interesting, fun project ... not sure I have the craft skills to do a good job.  Think I will go with the store-bought heaters for now.

Offline Ernest T. Bass

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Re: rocket mass heater uses EIGHT TIMES less wood than a wood stove
« Reply #24 on: September 14, 2011, 07:44:35 AM »
Many rocket stoves have been in regular use for over a decade, and so far none of them have exploded... Like I said before, the chimney is buried at least 6'' deep in cob, so even if you did have a chimney fire, I can't see how it could be a serious danger. From my observations, very little if anything condenses on the pipe inside the house. The interior pipe is always very warm, being shrouded in hot thermal mass. The gasses don't condense until they hit a cooler surface.

We've used efficient air-tight wood stoves, crummy franklin stoves and old wood cook stoves, with insulated pipe and temporary thinwall pipe. Our lousy 25' feet of furnace duct (creosote magnet) that has been our temporary exterior stack for the last three years has had less buildup any of the other chimneys, about the same as burning a conventional stove hot through an insulated chimney.

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