are rocket mass heaters "appropriate technology" given the availability of efficient woodstoves?

I will preface my post by stating that I have never built or operated a rocket stove or rocket mass heater.  My thoughts are based on reading and research here at permies.com and elsewhere.

I do have experience heating my home with wood in New Hampshire (Zone 5).  I have a Vermont Castings Dutchwest wood stove, and I'm planning on adding a Blaze King (http://www.blazeking.com/EN/wood-stoves.html) on the main living floor next year.

So I've been reading and following a lot of discussions here, and trying to understand why permies are enamored with rocket heaters, and whether they are truly "appropriate technology".

I see a lot of tinkering and R&D, which is great, but I haven't seen a lot of maturity in a "best practices" type of design.

Aesthetics are certainly in the eye of the beholder, but if I tried to bring a 50-gallon steel barrel and cob into my living room, my wife would have me sleeping outside with the dog

I see a lot of people burning up and corroding metal burn tubes, sending lots of toxic gick into the atmosphere.  It sounds like most folks have now seen the light in using refractory materials.

I see people experiencing puff-back of smoke into their living space, and at least one person nearly lighting his floor on fire.  There is a reason why insurance companies prohibit home-built woodburning appliances indoors, and why UL certifies stoves for safety after extensive testing.

I understand that rocket heaters should burn efficiently (have not seen any scientific testing to verify), but so do modern woodstoves with secondary burn technology (either reburn tubes or a platinum catalyst).

An Englander NC-30 woodstove (http://www.homedepot.com/p/Englander-2-400-sq-ft-Wood-Burning-Stove-30-NCH/100291302) can be had for $900 and burns at ~70% efficiency.  For those with a more liberal budget, a Blaze King operates at 85%+ efficiency and can burn one load of wood for 24 hours.  

Most woodburning enthusiasts (http://www.hearth.com/talk/forums/the-hearth-room-wood-stoves-and-fireplaces.6/) report burning 3-5 cords of wood per season to heat a typical home, depending on the climate.  A cord of red oak weighs 3500 lbs and contains 22 MBTU of energy (https://chimneysweeponline.com/howood.htm).  How long would it take you to gather 3500 lbs of twigs and sticks to feed your RMH?  I understand that twigs are a "free windfall", but at some point your time has some value.

I can see the advantage of rocket stoves as a substitute for open cooking fires. I understand that some extremely poor people do not have the means to buy an EPA woodstove.  I understand the appeal of a RMH in an outdoor application like heating a greenhouse where a design mistake doesn't lead to catastrophic loss of life.  But I can't understand why anyone with a better alternative would take the risk of building one of these things in their house.  What am I missing?

One thing you're missing is that rocket mass heaters (and masonry heaters in general) provide gentle heat for 8-24 hours after the fire is out, so even if a woodstove is operating at its lab-tested efficiency, which a properly built RMH will equal or exceed in operation, the RMH is delivering a large portion of heat while not operating. Thus, its daily efficiency is dramatically higher than its instantaneous operating efficiency.

Granted that the best modern woodstoves operated strictly to spec can be quite efficient, an RMH can be more efficient, and an RMH replacing an existing woodstove typically results in using 1/2 to 1/8 of the wood to achieve the same comfort in the same house, by actual reports. So gathering tons of twigs is not an issue. The typical RMH owner who previously used a woodstove will need to collect much less wood than before.

There is definitely a standard, reliable spec for the RMH (which needs to be adapted somewhat to each installation), but the vast number of tinkerers trying to reinvent the wheel tend to confuse the issue, with the ease of throwing an untested experiment up on youtube.

"Twigs" is rather an exaggeration, as the only use for them is generally as kindling. The RMH can use smaller wood than a woodstove, which makes harvest easier in some ways. Limbs that would be wasted in other cases can be cut and maybe split to 3-5" maximum dimensions, and young thinned trees from a woodlot are perfect RMH fuel. If you have modest needs, you may well be able to use deadfall branches and trimmings for your whole supply.

are rocket mass heaters "appropriate technology" given the availability of efficient woodstoves?

This is a really good question.  
The answer: yes.


Putting it really simply (because someone will be along in a moment with the technical stuff):

Appropriate Technology can be built and maintained by the user with no specialist training or equipment, using locally available materials.  These modern, efficient woodstoves require fancy materials, specialist training, and equipment to build and maintain.

RMH produce more heat for the amount of fuel used.

Most woodburning enthusiasts (http://www.hearth.com/talk/forums/the-hearth-room-wood-stoves-and-fireplaces.6/) report burning 3-5 cords of wood per season to heat a typical home, depending on the climate.  A cord of red oak weighs 3500 lbs and contains 22 MBTU of energy (https://chimneysweeponline.com/howood.htm).  How long would it take you to gather 3500 lbs of twigs and sticks to feed your RMH?  I understand that twigs are a "free windfall", but at some point your time has some value.  

RMH require considerably less fuel.  I don't know the numbers myself but I've seen people heat their home, in parts of the world that have real winters, for less than one cord of wood per winter.  Chopping three to five cords or gathering one?  Gathering windfalls can be done easily while doing other chores during the year.  Felling trees and chopping wood requires dedicated time.  



Now that said, I don't have a RMH... yet.  I'm still using a woodstove for a variety of reasons.  Partly insurance, party because we didn't know about RMH at the time, and a few other reasons.  I can see RMH in my future for the next house.

Glenn Herbert wrote:One thing you're missing is that rocket mass heaters (and masonry heaters in general) provide gentle heat for 8-24 hours after the fire is out, so even if a woodstove is operating at its lab-tested efficiency, which a properly built RMH will equal or exceed in operation, the RMH is delivering a large portion of heat while not operating. Thus, its daily efficiency is dramatically higher than its instantaneous operating efficiency.

Granted that the best modern woodstoves operated strictly to spec can be quite efficient, an RMH can be more efficient, and an RMH replacing an existing woodstove typically results in using 1/2 to 1/8 of the wood to achieve the same comfort in the same house, by actual reports. So gathering tons of twigs is not an issue. The typical RMH owner who previously used a woodstove will need to collect much less wood than before.

There is definitely a standard, reliable spec for the RMH (which needs to be adapted somewhat to each installation), but the vast number of tinkerers trying to reinvent the wheel tend to confuse the issue, with the ease of throwing an untested experiment up on youtube.

"Twigs" is rather an exaggeration, as the only use for them is generally as kindling. The RMH can use smaller wood than a woodstove, which makes harvest easier in some ways. Limbs that would be wasted in other cases can be cut and maybe split to 3-5" maximum dimensions, and young thinned trees from a woodlot are perfect RMH fuel. If you have modest needs, you may well be able to use deadfall branches and trimmings for your whole supply.

Yes the low steady output of a RMH is attractive.  Modern secondary-burn woodstoves acheive this effect by using a catalyst to burn off smoke when the fire is cut back to smoldering.  See page 4 of this PDF:  http://www.blazeking.com/EN/PDF/brochures/Broch_King_Princess.pdf
That is a 48-hour burn using softwood with a steady 150F flue temperature.  That is slow and low.

1/2 to 1/8 wood consumption is a remarkable claim.  Which type of woodstove were they replacing?  If it was an old potbelly stove, or 1970s Papa Bear, then I'm not surprised that a RMH cut their fuel consumption by 1/2-1/8.  If they were replacing a post-1993 EPA stove with secondary burn technology, I would love the see the writeup on that!

R Ranson wrote:

are rocket mass heaters "appropriate technology" given the availability of efficient woodstoves?

This is a really good question.  
The answer: yes.


Putting it really simply (because someone will be along in a moment with the technical stuff):

Appropriate Technology can be built and maintained by the user with no specialist training or equipment, using locally available materials.  These modern, efficient woodstoves require fancy materials, specialist training, and equipment to build and maintain.

Are we talking about some sort of Doomsday Preppers scenario, or state-of-the-world today?  The woodstoves I'm talking about are basically a cast iron box plus a platinum catalyst.  Others like the Englander NC-30 replace the catalyst with "secondary burn tubes".  They don't require any more specialist training to operate than any other new tool.  Basically, read and follow the manual, and pay attention to what you're doing.  The Blaze King has a thermostatically controlled air damper that meters out the heat while preventing overfires.  Cast iron woodstoves last for decades with essentially no maintenance besides replacing the catalyst once every five years.

If we're talking about some future scenario where iron and steel foundry and forge services are no longer available, then your refractory bricks and mortar will also be unavailable.

Davis Tyler wrote:

Are we talking about some sort of Doomsday Preppers scenario, or state-of-the-world today?  The woodstoves I'm talking about are basically a cast iron box plus a platinum catalyst.  Others like the Englander NC-30 replace the catalyst with "secondary burn tubes".  They don't require any more specialist training to operate than any other new tool.  Basically, read and follow the manual, and pay attention to what you're doing.  The Blaze King has a thermostatically controlled air damper that meters out the heat while preventing overfires.  Cast iron woodstoves last for decades with essentially no maintenance besides replacing the catalyst once every five years.

If we're talking about some future scenario where iron and steel foundry and forge services are no longer available, then your refractory bricks and mortar will also be unavailable.

I suspect, it's our definition of "appropriate technology" that's at odds.

"appropriate technology" as I understand it is easy for anyone in the world, regardless of social-economic status, to use locally available resources to make something useful.  

Building a cast iron stove from scratch wouldn't fit into my understanding of "appropriate technology" given the material has to be acquired (mined or salvaged) then put together in a way that is safe (fire in box needs some safety knowledge).  Now this is could be possible to do, and people have done it.  Building a Platinum catalyst on the other hand... do you see many rural African farmers able to do this (acquire the materials, have the skills and tools, &c) ?  


If you want to think about Appropriate Tech as a Doomsday Prepper scenario, feel free to do so if it helps.  I think of it as something useful to a subsistence farmer living in any part of the world (not just The West).  Appropriate Tech is available to all humans no matter location, resources, and skills.  Cast iron and platinum does not fit with my idea of this.  

My understanding of a RMH is that it can be adapted to be made with different materials depending on what's available - not all materials will have ideal results, but the design seems somewhat flexible to account for that.  



But like I said, I don't have first-hand experience with RMH yet so I can't say which would work better for me.
I think RMH qualify for "appropriate technology" whereas modern woodstoves do not.  That was the main question.
Which is "appropriate" for your situation depends entirely on what you want, your lifestyle, your set up, where you are in your life... and many other things.  I love my modern free standing woodstove, I think I would also like a RMH.

R Ranson wrote:

Davis Tyler wrote:

Are we talking about some sort of Doomsday Preppers scenario, or state-of-the-world today?  The woodstoves I'm talking about are basically a cast iron box plus a platinum catalyst.  Others like the Englander NC-30 replace the catalyst with "secondary burn tubes".  They don't require any more specialist training to operate than any other new tool.  Basically, read and follow the manual, and pay attention to what you're doing.  The Blaze King has a thermostatically controlled air damper that meters out the heat while preventing overfires.  Cast iron woodstoves last for decades with essentially no maintenance besides replacing the catalyst once every five years.

If we're talking about some future scenario where iron and steel foundry and forge services are no longer available, then your refractory bricks and mortar will also be unavailable.

I suspect, it's our definition of "appropriate technology" that's at odds.

"appropriate technology" as I understand it is easy for anyone in the world, regardless of social-economic status, to use locally available resources to make something useful.  

Building a cast iron stove from scratch wouldn't fit into my understanding of "appropriate technology" given the material has to be acquired (mined or salvaged) then put together in a way that is safe (fire in box needs some safety knowledge).  Now this is could be possible to do, and people have done it.  Building a Platinum catalyst on the other hand... do you see many rural African farmers able to do this (acquire the materials, have the skills and tools, &c) ?  


If you want to think about Appropriate Tech as a Doomsday Prepper scenario, feel free to do so if it helps.  I think of it as something useful to a subsistence farmer living in any part of the world (not just The West).  Appropriate Tech is available to all humans no matter location, resources, and skills.  Cast iron and platinum does not fit with my idea of this.  

My understanding of a RMH is that it can be adapted to be made with different materials depending on what's available - not all materials will have ideal results, but the design seems somewhat flexible to account for that.  



But like I said, I don't have first-hand experience with RMH yet so I can't say which would work better for me.
I think RMH qualify for "appropriate technology" whereas modern woodstoves do not.  That was the main question.
Which is "appropriate" for your situation depends entirely on what you want, your lifestyle, your set up, where you are in your life... and many other things.  I love my modern free standing woodstove, I think I would also like a RMH.

OK, I see where you're coming from.  My thoughts are that those able to purchase a UL/EPA-certified woodstove are unwise to be tinkering with homemade rocket heater designs that could potentially burn down their home or CO-poison the occupants.  Most permies.com posters fit this demographic; we are extremely affluent by global standards.

The African subsistence farmer example - yes s/he is financially constrained from purchasing a UL/EPA woodstove, so must "make do" with what's locally available.  I agree that s/he is not going to be building a platinum catalyst.  But there are other technologies required to build a rocket heater - either an arc-welder & 240VAC circuit, or refractory bricks and mortar produced in specialty kilns.  The temperatures achieved in a rocket burn tube are such that you can't just slop together some clay mixture, or use salvaged red clay bricks and hope the thing survives firing.  I'd rather have our hypothetical farmer burning wood in a low-efficiency open barrel rather than jeopardize his safety with the temperatures in a RMH.

I think it might be easy for some people to confuse rocket mass heaters with rocket stoves.  You can make a rocket stove for cooking out of junk, dirt, and rocks.  When it fails you just build another one.  But a rocket mass heater is a permanent fixture in the home and needs to be made of permanent materials such as refractory bricks, etc.

Tyler Ludens wrote:I think it might be easy for some people to confuse rocket mass heaters with rocket stoves.  You can make a rocket stove for cooking out of junk, dirt, and rocks.  When it fails you just build another one.  But a rocket mass heater is a permanent fixture in the home and needs to be made of permanent materials such as refractory bricks, etc.

Yes, and I'm sure I have been guilty of mixing the terms.

I totally agree that a rocket stove is appropriate tech, as a disposable cooking device.

I'm questioning whether a RMH is appropriate tech - it seems to me that those affluent enough to buy a woodstove should do so, and those who can't afford a woodstove probably can't afford the refractory materials required to safely build a RMH.

The main difference I see between the woodstove and the rocket mass heater is the mass of the heater provides a longer period of heating between firings compared to the woodstove.  So a RMH might be more appropriate in a locale where it gets cold and stays cold, and totally inappropriate (in my opinion) in a place where temperatures fluctuate a lot during the cold season, such as my locale.

As far as "appropriate technology" and being able to build with locally available materials, an RMH can be built with common soft red bricks; some of the original ones have lasted 20+ years so far. The cutting edge RMH with the best efficiency will be achieving temperatures that would degrade common brick faster than that, so the core might have to be repaired in 5-10 years or so. Also, there are areas subject to higher abrasion where hard firebricks are valuable.

So it sounds like as long as you don't try to make your rocket mass heater very efficient, you can get away with using locally available materials and be prepared to rebuild it periodically.

If you don't try for super high temperature burns, you can use locally available materials and expect it to last for decades in normal use. If you are rough with it, you may need to rebuild the feed area sooner.

That makes sense.  And it's not as if wood stoves don't require maintenance and repairs also - we had to replace some bricks in ours.  

So is the question: are RMHs made with ideal materials and technique, more efficient than modern cast iron 'effecient woodstoves'?

Or are we comparing modern woodstoves to the RMH made from local resources?  

Or is the question if the RMH appropriate for your specific situation?  


Before you choose one, check out this thread about what it means to say a wood stove is effecent and how rocket mass heaters fit with the numbers.

Tyler Ludens wrote:The main difference I see between the woodstove and the rocket mass heater is the mass of the heater provides a longer period of heating between firings compared to the woodstove.  So a RMH might be more appropriate in a locale where it gets cold and stays cold, and totally inappropriate (in my opinion) in a place where temperatures fluctuate a lot during the cold season, such as my locale.

The thermal mass dampening properties of RMH is quite desirable, but there are other, safer ways to achieve that same effect.

There are dozens of wood-burners on the Hearth.com forum who routinely report 12-24 hour burn times, in a variety of climates, using both softwood and hardwood.

http://www.hearth.com/talk/threads/simple-thread-blaze-king-princess-burn-times.99974/

http://www.hearth.com/talk/threads/advice-ideal-steel-or-blaze-king-princess.152006/

The bad old days of wood stove technology is behind us: overheating your room then waking up at 2 AM to reload.  The new generation of stoves has solved those performance issues with thoughtful engineering and data-driven decision making.  
This seems like a "solved problem" to me, and I can't for the life of me understand why someone any financial means would jeopardize the safety of their family to tinker around with oil drums, bricks and cob in their house.

The problem may be solved if high-tech industrial materials are reliably available, though even then I think the RMH can be more efficient on a seasonal basis.

"Tinkering" implies that there are not reliable models to follow for the RMH; building one by tried-and-true methods is hardly an unsafe proposition, any more than having any woodburning appliance inside the home. I note a thread about "puffback" in the hearth forum you mentioned; a good chimney is necessary for best operation of any appliance that is manually lit and fed.

Those wood stoves are very expensive!

I would ask, since you have obviously looked through the woodstove forums a lot, what reduction in wood use has been reported by people who have replaced an old woodstove with the latest efficient models?

If they have cut wood use in half or less, the fact would presumably be significant enough to mention.

Tyler Ludens wrote:Those wood stoves are very expensive!

The upfront cost of an efficient woodstove is certainly higher than the upfront cost of a homemade RMH.

$1000-3000 for the stove plus $2000 for the stainless chimney liner and installation if you don't DIY.  So let's say $5000 up front, compared to $500 for a DIY RMH.

That may be sticker shock to some, but the fuel cost and the value of my time far surpasses that over time.  To heat my home in New Hampshire with propane cost $1800 a year, every year.  Purchasing log length firewood and cutting and splitting it myself gets that down to $500 a year.  Switching from fossil fuel heat to wood heat saves you so much money over time, that the cost savings of a RMH vs woodstove is irrelevant in my view.  Especially if you have to rebuild the core every few years due to high temperature.

I tend to think longer term and my philosophy is to buy the highest quality tool for the job to decrease the maintenance and replacement cost over time.  As I mentioned a cast iron woodstove will last for decades with minimal maintenance.

Glenn Herbert wrote:I would ask, since you have obviously looked through the woodstove forums a lot, what reduction in wood use has been reported by people who have replaced an old woodstove with the latest efficient models?

If they have cut wood use in half or less, the fact would presumably be significant enough to mention.

Typically I see folks stating they cut their wood consumption from 5-7 cords with an old "smoke dragon" stove, to 3-5 cords with a modern efficient stove.  So 1/3 reduction.  Which is why my jaw drops when I see claims of 1/8 to 1/10 reductions for a RMH.  Extraordinary claims require extraordinary evidence.

What more extraordinary evidence can you get than actual reports of average wood use? Greater numbers of such reports are obviously desirable. Ernie and Erica Wisner, who have built or consulted on many hundreds of RMH builds in their career, would be in a good position to have gathered these reports. They reported for themselves that their first RMH reduced average wood use from 3-4 cords to less than 1/2 cord per year.

Davis Tyler wrote: Especially if you have to rebuild the core every few years due to high temperature.

Why would you need to rebuild it if you bought $500 worth of durable refractory materials (I'm guessing that is what the $500 is for since we were talking about building RMH with locally available materials except the core)?

R Ranson wrote:So is the question: are RMHs made with ideal materials and technique, more efficient than modern cast iron 'efficient woodstoves'?

Or are we comparing modern woodstoves to the RMH made from local resources?  

Or is the question if the RMH appropriate for your specific situation?  


Before you choose one, check out this thread about what it means to say a wood stove is efficient and how rocket mass heaters fit with the numbers.

I'm quite certain that a RMH is not appropriate for my specific situation, but I'm trying to understand a situation where it IS appropriate.

I'm having trouble imagining a scenario where cast iron is NOT available, but refractory bricks ARE available, so that eliminates the "ideal RMH vs. woodstove"

So let's compare "locally available RMH vs woodstove"

I see the thread you linked to, questioned test method validity for efficiency.  I have yet to see side-by-side comparison of a RMH vs. woodstove.  The Wood Stove Decathalon (winner Woodstock Soapstone of NH!) would have a been a great opportunity to do that comparison in a controlled setting, but it sounds like Dragon Heaters were not able to get a rocket prototype ready for the contest.  https://www.niftyhomestead.com/blog/wood-stove-decathlon/

Tyler Ludens wrote:

Davis Tyler wrote: Especially if you have to rebuild the core every few years due to high temperature.

Why would you need to rebuild it if you bought $500 worth of durable refractory materials (I'm guessing that is what the $500 is for since we were talking about building RMH with locally available materials except the core)?

well I guess that was based on previous comments and posts about RMH users tearing down and rebuilding their core - were they using red clay bricks instead of fire bricks?

Using all new material, the firebrick would cost around $200 depending on local availability, a good used 55 gallon drum may cost $20 if you can't find one free, ducting or stovepipe may run $50-200 depending on design. The big thing is an insulated chimney which is recommended for safety and posterity (in case a future owner decides to tear out the RMH and put in a woodstove), which will cost the same thousand or two no matter what system you have.

Glenn Herbert wrote:What more extraordinary evidence can you get than actual reports of average wood use? Greater numbers of such reports are obviously desirable. Ernie and Erica Wisner, who have built or consulted on many hundreds of RMH builds in their career, would be in a good position to have gathered these reports. They reported for themselves that their first RMH reduced average wood use from 3-4 cords to less than 1/2 cord per year.

Wow, I would love to burn 1/2 a cord the entire winter!  Do they heat 24/7 with their RMH?  Or just occasional use?  How many square feet is their house?

I think most core rebuilds were due to not following published best practices closely, either in configuration or materials. Some were using cast refractory in areas where the user was very rough with feeding and had heavy heating loads.

I don't recall seeing any core rebuilds of all-firebrick with external insulation, at least not due to deterioration.

Glenn Herbert wrote:Using all new material, the firebrick would cost around $200 depending on local availability, a good used 55 gallon drum may cost $20 if you can't find one free, ducting or stovepipe may run $50-200 depending on design. The big thing is an insulated chimney which is recommended for safety and posterity (in case a future owner decides to tear out the RMH and put in a woodstove), which will cost the same thousand or two no matter what system you have.

A RMH is certainly cheaper up front, no argument from me.  If the chimney is a wash comparison, then we're comparing a $300 RMH to a $3000 Blaze King.  If they both last for 20 years of service, that extra cost works out to $135 a year.  That is a trivially small line item in anyone's budget.  If they go with the Englander NC-30 ($900) instead of the primo Blaze King, the difference is only $30 a year.  The 20-year fuel cost far exceeds the appliance cost, which is why I'm so interested in the efficiency.  If I could get my wood consumption down to 1/2 a cord per year as stated earlier, that would drastically free up my wood-splitting time for fishing in the summer!

Glenn Herbert wrote:I think most core rebuilds were due to not following published best practices closely, either in configuration or materials. Some were using cast refractory in areas where the user was very rough with feeding and had heavy heating loads.

I don't recall seeing any core rebuilds of all-firebrick with external insulation, at least not due to deterioration.

Yes with all the different build threads it's hard to keep straight on which ones are a re-design due to inherent flaws, and which ones are due to degradation over time.

It looks like a properly constructed RMH is still a better deal than a new efficient wood stove.   $3000 is not a trivial amount of money if your annual income is small.

Tyler Ludens wrote:It looks like a properly constructed RMH is still a better deal than a new efficient wood stove.   $3000 is not a trivial amount of money if your annual income is small.

Indeed each household has a different threshold of how much a "better deal" means to them.  For the same reason I don't drive 10 miles out of my way to save $0.02/gallon on gasoline, that may make financial sense to some folks.  
But I also reject the short-term thinking of so many in our society; buying a cheaper tool upfront doesn't necessarily save money over its lifetime.  Or buying an inkjet printer without considering that the lifetime refill cost of the ink is 10x the cost of the printer.

That was in an attached garage apartment in Portland, Oregon. The climate is not extreme, but I do believe the RMH was their only heat source. They have since moved to northern Washington, almost in sight of Canada, in the mountains. They have described their wood use for the first season there, but I haven't seen totals over a couple of years yet. Of course, they can't compare to before because they didn't heat the new place with wood before.

I'm still not seeing how a $300 properly constructed RMH which (let's say) uses the same amount of wood as a $3000 wood stove isn't a better deal for those whom the RMH is appropriate.

I'm not seeing what the implied "short-term thinking" is with the RMH, which looks quite permanent to me.

I think that was the very first RMH video I watched!  Got me interested in understanding the tech behind it.  So, OK 1/2 a cord of wood for a garage on the Pacific coast is not that impressive.  I have a 2400 square foot home I'm heating in NH when I talk about 3-5 cords.  It routinely hits -10F here, sometimes -20F.  It would be interesting to see how they fare with heating an entire home in their new location

Decreased wood consumption is a byproduct of greater seasonal efficiency, but one of the best drivers to actual impetus to build one.

Another factor to consider is that an RMH does not have to include a 55 gallon drum. There are tested configurations that look more like traditional masonry heaters. These may cost more in materials and skill to build than cob & barrel RMHs.

You may remember that they talk about their wood use per day, and figure about a cord for four months of deep winter in the mountains near the Canadian border. This is for their pre-existing small house, construction standards unknown but probably standard stick-framing at best.

Glenn Herbert wrote:Decreased wood consumption is a byproduct of greater seasonal efficiency, but one of the best drivers to actual impetus to build one.

Another factor to consider is that an RMH does not have to include a 55 gallon drum. There are tested configurations that look more like traditional masonry heaters. These may cost more in materials and skill to build than cob & barrel RMHs.

yes, the Dragon Heater and other bell designs are much more visually appealing.  Unfortunately I don't think any of the mass designs would work for a retrofit in existing homes around here.  We all have basements, and there's no way standard flooring would support the additional several thousand pounds of cob or masonry it wasn't designed for.

From six years ago:



Same house with the same size of footprint and same insulation.  Same people with the same level of comfort desires.  Same wood.  Same wood harvesting and storing habits.  

4 cords to "about half a cord".  

This is just one example.  

My brother ran a conventional wood stove in an uninsulated 400 square foot uninsulated (unfinished) wofati and then ran a rocket mass heater in a 900 square foot uninsulated (unfinished) wofati and reported that he was burning about 1/5th the wood for the larger space.

I think I have heard from about two dozen people that have replaced a conventional wood stove with a rocket mass heater and the reports have been anything from 1/4 of the wood to 1/10th.    And the 1/4 of the wood was just once - and it seems that the wood stove that was replaced was a "high efficiency wood stove".  

I think a lot of this is mentioned throughout threads in this forum.  And I think there are a lot of threads here that go over details on why.  You are not the first to ask.

The "why" or "how" is related in many ways.   Some argue that it is mostly to do with the mass.  Others argue that it has to do with the very complete burn and the exhaust temp being around 100 degrees.

We're in montana.  I run my rocket mass heater about every other day most of the winter.  The wood feed is 7.25 inches by 4.75 inches - it doesn't hold much wood.  I burn for about an hour and a half on the days that I do burn.   The house is a double wide - about 1400 square feet.    I remember running a conventional wood stove in a house that was almost identical eight years ago (three bedroom, newer double wide, about the same size) - before I learned about rocket mass heaters.   I suspect I burned eight to ten times more wood and that was in the seattle area.  And even then, when the fire went out, it seems to get damn cold really fast.  

So, is this an appropriate technology?   Fuck, yes.   Most of these are built dominantly with cob, duct and a barrel.  The better ones are featuring some firebrick and durablanket. The designs are getting simpler and better every year.  The functionality is getting better every year.  Current rocket mass heater designs are excellent and we still have oodles of room for optimization.  We're just getting started.  

Davis Tyler wrote: bell designs are much more visually appealing.

I think these look pretty damn good:









Here is a thread on beautiful rocket mass heaters:

https://permies.com/t/40573/rocket-mass-heaters/beautiful-rocket-mass-heaters