Are you interested in building a rocket mass heater? What is holding you back?

Several of us who have built heaters as discussed on this forum have wondered why so few people seem to build one for themselves, considering the many benefits that have been documented for these systems. It would seem like there would be many more builds, or perhaps there are but you're not sharing your experience with the community. For those of us who have built systems, we recognize that this is not an insignificant undertaking but the benefits are significant in comparison to the effort. And, as I like to say with so many of our significant projects around the homestead, "a one time effort for a lifetime of payback".

If you have been lurking on these pages or simply wishing to have a rocket mass heater, I think it would be interesting to the community to understand what has kept you from moving from the wishing phase to the building phase. Don't be shy or feel uncomfortable sharing your reluctance. Perhaps the folks here can help turn your "wants" into "haves". What is holding you back?

I don't want to give up the space in my house. I don't want to wrestle with insurance.

I *do* want to try putting one in a greenhouse, but it's a project that's a few years out.

ETA: we might convert a barn to a guesthouse and that's pretty likely to have a RMH-bench/bed but it's probably more years out than the greenhouse.

Space constraints is a real potential issue Christopher, so no argument there. However this can perhaps be overcome with enough motivation. You'll see some examples with folks posting their builds in a tiny home, but it definitely takes some effort to reprioritize floor plans.

As for the insurance argument, this one has me scratching my head a bit. I'm curious if anyone on the forum knows of anyone that actually had a refusal to insure because they have an RMH in their home. Especially, with a batch box design where the combustion chamber is fully enclosed like any wood stove and a double skin bell that provides assurance of sealing. I suppose some folks don't want to take the risk of a denied insurance claim and if that is their fear it is inappropriate for others to judge how they balance risk in their lives. From experience, I can confidently say that my RMH is much less of a safety threat than my traditional wood stoves. I can provide reasons if anyone is interested but I'll keep this brief.

As for the greenhouse, definitely keep this on your to-do list. Extending your growing season is of huge benefit and a properly designed batch box RMH in a greenhouse would be stellar.

I get somewhat confused on the longevity of the different parts for the different types.

I can't quite wrap my head around what I'll need to replace/clean, about how often it will need replaced. That seems to depend on the type of RMH. I have read through your masonry build in the large building, the Wisner's materials, Thomas Rubino's projects, etc. I have seen where some of the metal components need to be replaced and some of the bricks crack and need replaced, etc.

I understand a factor of that would be climate dependent due to the amount it would be used (wood burned). The larger masonry heaters seem like what I would work up to in my residence but practice building one on in my accessory building.

Excellent post Glenn,

I would like to add to this a bit.

1) any in the Midwest with basements, you have great potential room, if you have a chimney that goes with the house.   Heat rises nicely. and yes it might not be a center piece, I think many will enjoy this especially if you have a window on your door ( easily done )

2) insurance- I bet I have heard 100 times from home owners that they feel they could not get insurance if they have this built by themselves or a contractor, So I ask why?  other than hearing  " because that is what I heard "  I have yet to have someone tell me, this is a non-insurable unit, causing the house to be uninsurable.   Better yet, show me.  So I have to presently believe this is a classic case of  Somebody said that somebody said, that somebody said, it can't be done.  Which generally plays out that it was never a fact, I don't mind to be proven wrong, but simply have not seen this yet.

  To back this up further, for every reason that a classic wood stove with smoldering fires that might be producing a potential chimney fire, I can show you why that is simply not true with a rocket mass stove.

3) Time, everyone seems to be short on time, but, for every hour spent gathering wood, for a Rocket Stove, you can get 2-5 times more usable heat, plus if you add in all the easy to gather woods ( throw away boxes at nearly every lumber yard, construction sites, storm damaged areas,  that are in fact FREE )

This of course is just food for thoughts, but the 1st step of any job, is starting, whether that is in the planning, cleaning, or constructing. But, if you don't have time to do it right, when are you going to have time to do it over?   Might as well, start with a great plan and follow through.  Tom of Dragon Tech just showed what got done with a great mission and determination.

Best of success!

Jackson Bradley wrote:I get somewhat confused on the longevity of the different parts for the different types.

I can't quite wrap my head around what I'll need to replace/clean, about how often it will need replaced. That seems to depend on the type of RMH. I have read through your masonry build in the large building, the Wisner's materials, Thomas Rubino's projects, etc. I have seen where some of the metal components need to be replaced and some of the bricks crack and need replaced, etc.

I understand a factor of that would be climate dependent due to the amount it would be used (wood burned). The larger masonry heaters seem like what I would work up to in my residence but practice building one on in my accessory building.

Longevity?  Lets flip this a bit, and look at the numbers,  so we have a outdoor stove that cost 10 times more and burns 10 times more wood (ok, lets cut that to 5 times more wood)  and still needs repairs after a few years, at the mercy of electricity and dealership cost.  vs rocket mass stoves that probably and possibly need some general maintained cost of a few dollars per year

Tom has clearly shown what hard burning, looks like and how he handled it.

I am on my third year of burning, and while I think I can detect some future repairs, they appear to be minor. and while I am trying to get a better video, I can honestly say, no hotter fire has burned in a standard wood stove.  I can heat a 1/2 round rod, to deep red in less than 3 minutes and cherry red in 7 minutes so hot it is.   I have been a proponent of building a stove where the entire core burn unit could be replaced without touching the bell. This was not hard but has not been needed. But I probably will do this again. Just takes a bit of planning.

Jackson Bradley wrote:I get somewhat confused on the longevity of the different parts for the different types.

I can't quite wrap my head around what I'll need to replace/clean, about how often it will need replaced. That seems to depend on the type of RMH. I have read through your masonry build in the large building, the Wisner's materials, Thomas Rubino's projects, etc. I have seen where some of the metal components need to be replaced and some of the bricks crack and need replaced, etc.

I understand a factor of that would be climate dependent due to the amount it would be used (wood burned). The larger masonry heaters seem like what I would work up to in my residence but practice building one on in my accessory building.

Good question Jackson and one that has some experience behind it to back it up. You are correct that metal pieces can require replacement, however testing and development over the past number of years have greatly improved this area.

For the original Peter van den Berg batch rocket design (not a Shorty core) the one component with the greatest potential for degradation is the secondary air tube at the rear of the combustion chamber where the secondary air is being introduced to the flame just as it enters the riser. The intensity of the flame at this location will cause spalling to the metal if you use standard steel tubing. Some of the pioneering guys (I'm not sure who gets the credit for this) researched and found special alloy tubing (called RA330) that resists high temperature, low oxygen spalling. I am now on my 3rd year with my batch rocket, my tube looks perfect and living high in the mountains our heating season starts in early October through end of May and we burn 2-4 batches every day.

The brick in the combustion core is also subject to degradation due to the environment. This has also been addressed with the use of good quality refractory brick for the core and using a high temp ceramic coating on the brick after construction. The brick in my combustion core looks like the day I built it.

Those are the 2 areas that could require future maintenance. The secondary air tube is something that Tom Rubino from Dragon Tech sells that has a replaceable RA330 tube so this is simple and can be accomplished in 10 seconds. Rebuilding the core is a different matter but I suspect that the answer here is to reapply the ceramic coating after a number of years, which you just paint on, and this will protect your brick.

For all of you considering a RMH, my suggestion is to focus on either of the 2 designs by Peter van den Berg - Batch Rocket (original design) or Shorty core. I can't see any reason to build a J tube design except for some special corner cases. Tom Rubino would be a great resource to discuss the pluses and minuses of this decision since he has over a decade of experience with all of the designs.

I am one of those guys who is held back by insurance fears.
My fears are furthered because I want I avoid scrutiny from my insurance company.
I have a visceral fear of any kind of consequential  paperwork, and I have been following news of people losing their house insurance from simple drone flybys.
All that fear is reason enough for me to focus on other life challenges.
Not reasonable, but there it is.

The only cost that concerns me is the flue.
Although rmh can operate without a chimney, they work much better with one.
Although they can use single wall stove pipe or maybe even even HVAC duct, local codes might call for something much  more expensive.
My own home has one chimney with two flues.
One was made for the new fangled first floor gas fireplace, the other for the basement coal furnace.
Either should be fine as is for rocket stove exhaust, but I'm pretty sure local code calls for a stainless steel liner.

Before I found permies I learned about thermal mass heaters that were located in the cellard of off grid cabins.
It was just the basic barrel stove, buried in literal tons of rock and earth and burned as hot/ quickly  as possible.
I have considered doing this with the cheapest conventional woodstove that will pass regulatory inspection.

Hey William... you need to do what is right for your situation so if the insurance consideration is a show stopper then, as you say, you should just put your energy into other life stuff.

The only other comment I'll make is in regards to the chimney/stovepipe for the benefit of others reading this. The stove pipe diameter is critical to properly size the rest of the system. When you read on this forum or on Peter van Den Berg's website and see reference to a 6" system, or whatever other size, this refers to the stovepipe size. For the system to burn and flow properly you MUST size everything based on the stovepipe size used. Most common is a 6" pipe and therefore the specs for a 6" system.

My only other comment is to use actual steel stovepipe. Definitely not HVAC duct. Whether you use single wall or double wall is your choice as well as any building code requirements in your area. Most building code will require double wall, at least where the pipe meets the ceiling and roof. I happen to live in a remote county with no building code and no inspectors. Since my exhaust temperatures right at the connection to the heater never sees more than 250 F and that cools significantly as it goes up the pipe... and the fact that my ceilings are 14' before the stove pipe touches anything I used single wall pipe.

Hey Glenn. Nice idea this thread.
What’s stopping me at the moment to build one at my home is:
a) I don’t own the house just yet
b) the position of the existing exhaust is not ideal for integrating the RMH into the existing floor heat system. Also that’s probably quite complicated and maybe not even such a great idea

Not in my case, but would be if I still lived in Germany. RMHs are basically outlawed by now because all wood burning devices need to pass a specific test to prove they are burning clean (dust particles). And proving that of way too costly to do for a self built stove.

Ha, well, given the number of photos of my Shop Dragon, I should admit I am not up to code... if we had such a new-fangled thing.

In my shop, I exit the bell using 8" black steel stove pipe, just above the bypass, I switch to 5' sections of HVAC  (OH NO)...
With the help of a couple of flex 90's, I exit the ceiling through a Class-A roof jack.  At least I have that much concern right...
It has been this way since I built the first version of "Shop Dragon."
The original 6" metalbestos pipe was not large enough for my 8" J-Tube, and it has been leaning against the shop wall for several years, awaiting a new use.
I'm happy to say I used that insulated pipe this year on my Studio Dragon Build.

Ruh-Roh Tom… so much for telling folks to not use HVAC duct 😳. No problem in this case. Your shop dragon is super steam-punk so it’s ideal. Love the over-head pulley for pulling the barrel to load Mad Max villains when they show up at your shop.

Currently, I can add a reason, that I do not own the house I am living in. But previously, when I did have a house, it was mostly money.

For me it was primarily the costs. Most realistic estimates that I saw, was near $2000 to build one myself. Both houses I owned were up off the ground, so I would have had to add additional supports for additional costs. And perhaps the greatest thing... I was not confident enough that I could build it myself, so that made me feel as though I would need to hire at least a consultant, if not a mason of some kind. And that added even more costs. So while two grand was a big hurdle... the other costs seemed insurmountable.

I agree,  specifically with William, about insurance costs and  related bedevilment.  My old style wood burner is inefficient,  yes, but can handle a 32 inch log. At least my sawing and splitting is half the effort of many folks here.   And my home insurance only covers this type, not any hand made stove.  The company could cancel my policy at any time for any reason....happened to a friend less than a mile away.  New Mexico has rough weather at times, I know, but in GA insurance companies are averse to our 100 year old house enduring  hazards like hurricanes, tornados and tropical storms from the Atlantic and the Gulf of Mexico.  

For example, Helene did some damage with winds but more with rain, 20 inches very quickly in parts of the South.  North Carolina still is digging out in their mountains.  Since that storm many companies just dropped coverage for entire counties in GA.  Less competition has led to price boosts and worse.  

Insurers will not spend time on understanding the merits of rmh design, their cookie cutter, one size fits all approach leads to more corporate profits.  So I will stay with a stove we have, rather than spend effort and 2k$ on a great batch box..... that might cost us more than $ down the line.  And William is on the mark. Drone surveillance has increased scrutiny of home owners exponentially.  Big brother is watching.....

Matt McSpadden wrote:Currently, I can add a reason, that I do not own the house I am living in. But previously, when I did have a house, it was mostly money.

For me it was primarily the costs. Most realistic estimates that I saw, was near $2000 to build one myself. Both houses I owned were up off the ground, so I would have had to add additional supports for additional costs. And perhaps the greatest thing... I was not confident enough that I could build it myself, so that made me feel as though I would need to hire at least a consultant, if not a mason of some kind. And that added even more costs. So while two grand was a big hurdle... the other costs seemed insurmountable.

Excellent points for awareness Matt. You are right about costs if you want to build with new, high quality materials. Costs can be reduced some if the builder takes their time to source used clay and refractory brick. My post in my signature includes a materials list that is helpful for someone wanting to build Peter's original batch rocket design. A Peter Shorty Core will reduce cost some since you don't have the height of the riser to enclose with the bell.

You also make an excellent point on skills. Everyone was blessed with different skill sets when they were born. Some are comfortable jumping in to any type of project, regardless of prior experience and learning on the job. Others are not so inclined to building stuff and all of the various skills that go into it. Building an RMH includes general project planning and layout, masonry, metalworking & welding and installing a thru-the-building stovepipe (unless you use an existing one). Personally, I had never laid a single brick when I began mine but prior experience never held me back from jumping into a project with both feet. Each person needs to decide for themselves where their comfort level is for a project like this and how well they remain committed to a project as they need to problem solve things as they arise. For me, I love the challenge and I always say that every problem or issue that may arise has a solution if you approach it in that manner. For folks that have desire and some level of skills but perhaps don't have the equipment they may need to factor in the cost of farming out some of the build, like fabricating the door.

The other piece that you haven't mentioned that fits in nicely here is the time commitment. This is not a minor undertaking. My build took me roughly 3 months but being a compulsive perfectionist with my projects and having the luxury of being retired I took my good old time. Tom Rubino can comment on expected time frames to build since he has built so many over the years, both by himself and with the help of Gerry Parent (that's an advertisement for you Gerry in case you want to hire out as a professional batch rocket scientist). So, the builder needs to plan their time accordingly and have the available time to commit.

Rico Loma wrote:I agree,  specifically with William, about insurance costs and  related bedevilment.  My old style wood burner is inefficient,  yes, but can handle a 32 inch log. At least my sawing and splitting is half the effort of many folks here.   And my home insurance only covers this type, not any hand made stove.  The company could cancel my policy at any time for any reason....happened to a friend less than a mile away.  New Mexico has rough weather at times, I know, but in GA insurance companies are averse to our 100 year old house enduring  hazards like hurricanes, tornados and tropical storms from the Atlantic and the Gulf of Mexico.

The whole area of insurability of a building with an RMH is something each person needs to assess for themselves. I won't try to convince anyone that this doesn't need to be considered. Maybe in some parts of the country insurance companies send out representatives to inspect properties for insurability. I have no experience with insurance companies being intrusive to my life so I go on living the way I chose.  

Glenn Littman wrote:  

The other piece that you haven't mentioned that fits in nicely here is the time commitment. This is not a minor undertaking. My build took me roughly 3 months but being a compulsive perfectionist with my projects and having the luxury of being retired I took my good old time. Tom Rubino can comment on expected time frames to build since he has built so many over the years, both by himself and with the help of Gerry Parent (that's an advertisement for you Gerry in case you want to hire out as a professional batch rocket scientist). So, the builder needs to plan their time accordingly and have the available time to commit.

I will jump in with what I consider some time saving but in no means lowering the quality of your build.  

1) scrounging  for material-  with the avenue presented to us, are all kinds of companies looking to get rid of waste ( bricks ) almost any brick yard has toppled over pallets, some have been there for years or months.  you may end up with 3 different colors, but if your doing a double skin bell it don't matter, as no one will see the inner bell.  I was patient, and ended up with enough bricks for 4  7" sized bells ( big ) for $0.10 each,   Yes that is 10 cents. And they were brand new and still on a pallet.   It can happen.

2) A trick)-- There is so much info out there and what I would call  "new tricks" that simply should not be ignored.  The double corner laser lines are one of them letting you do two directions and know they are square in two directions at the same time.  

   b trick)== knowing that if your two corners, back left to front right and back right to front left have exactly the same measurement the the likely hood of your bell is square is high. Always good.

   c trick)-- Preplanning your bell size so you have NO cut bricks per side, lets you build one tier at a time very quickly.  And having all the bricks for this one tier ready to place, you can then plan for the time involved per level.   Suddenly you can say, " this tier takes 1 hour, and I have 1 and 1/2 hour, might as well do it"

  d trick)-- if you build a tension frame for the burn unit,  the same squareness rules apply, just makes everything else work so much easier  Measure twice, tack once. check and weld.

  e trick)--  if you know your 4 corners are square, you can actually set those 4 bricks in place,  and place the balance between these 4 corners. A big time saver again.  Remember  Brick length + joint thickness equals total length per unit. You can fudge a bit as needed between the 4 corners.

   f trick)--  spring loaded string lines between the set corners, really can speed things up.  A lot of the best brick layers in the world use the guide strings for good reason.

Hope some of these will apply to your build, a person doesn't have to use all or even any, but most will help.   Even the best, can't often just build on the fly without checks on his or her progress.

Best of success.
   

Our reasons have only to do with cost and our labor and our age.
The floor would need a redo for support....there's a tile lined brick chimney straight up through a metal roof that would work.

The thought of beginning another big project is beyond us now but back in the seventies, if we had heard about rocket heaters, it would probably have happened, as back then we were attempting all sorts of time consuming low cost things .

We've always lived out in the woods where there were no codes although I guess we never really thought much about it or even asked?
Now that we are on the edge of this small rural town I don't think anyone would care or notice...we've never thought to ask anyone about any of our building projects here.

And , we've never had house insurance so no limits there.

So, really, it comes down to money and energy.

I don't know what a rocket mass heater is. In some photos/videos I see basically a small fire box and a vertical chimney, with a good amount of masonry material to absorb and hold heat.  The idea being a small hot fire with the heat being stored to be slowly released in the house. That makes a lot of sense and is basically the same idea as what used to be called a Russian fireplace. The small hot fires as opposed to big smoldering ones, release far less smoke and are much safer because it doesn't build up creosote in the chimney. Also, better because the nice hot fire and chimney are more resistant to a back draft.

In other photos/videos I see old metal barrels that might be fine for burning trash outside, but I would never have such a thing in my house. Also, often times with elaborate constructions apparently designed to make hot gasses move horizontally, rather than vertically. Some of those are very nicely constructed with nice ascetics but still it violates the laws of nature to move hot gas horizontally. I've seen discussions and images of using fans to help the flow and small secondary fire chambers to heat the upper part before a fire can even be started in the primary burn chamber and I haven't seen any creditable indication that moving hot gas horizontally without some extra forces applied is possible.  An electrically assisted draft and preheating a chimney are not in my opinion effective and worse, quite unsafe.

I think the first example where a small hot fire heats a thermal mass is a good way to go and is basically what I have now except it consists of a small Vermont Castings stove with heat shields removed, placed in a brick surround on a brick on concrete floor. I build a small hot fire in the evening and usually let it go out at bedtime. It works very well. If I was to build a new house I might go with something even more like the traditional Russian fireplace maybe even with some baffles in the chimney to extract more heat but not anything that would significantly restrict the upward draft and absolutely no heated benches.

Basically, I just don't believe a lot of the documentation on the effectiveness and safety of some rocket mass heaters.

Hi Judith;
I sure hear you about the energy thing for starting new projects.
Some projects get quite a bit of "pre-work" thought simply because they are tiring to even think about, much less actually work on...
As Scott mentioned, most materials can be obtained at low cost, though you still need to load, haul, and unload them.
The metal pieces must be fabricated or bought.
And yes, the floor must be strong enough.

The actual construction can be very casual, and clay mortar is an absolute joy to work with, very forgiving. (Well, the sand gets irritating after a few days)
Glenn in Colorado amassed his supplies and then, working almost alone, took three months to build a masterpiece.
His wife was there and also pitched in with the large job of cleaning bricks behind him.

Last season, Gerry and I took 10 days to build a single skin Shorty Core in a tiny old cabin.
This season, it took us 10 days to build a double skin first-generation Batchbox on the concrete floor in Liz's art studio.
We had almost all the materials on hand and knew where to get more quickly if we ran short. (We did!)


Even if the construction takes 6 months or more, the result is, quite frankly, beyond belief!
With one fire a day or in the middle of winter, two fires, your home will be so gently heated that it will seem like a central heating system.
Warm but only hot if you sit near the metal door.
Your home will be evenly heated, no matter how old or funky it is. (Every building here is old and funky)
Your wood cutting and splitting will drop to a casual job; nothing like trying to feed a standard box stove.

However you acquire them, having the materials on site is the key to building a masonry batchbox stove.

On the subject of insurance.
For some lucky folks like Glenn, myself, and, it sounds like, Judith as well, it is a non-issue.
For others, it is a deal breaker.
Masonry stoves are insurable in most states (Georgia included). There are very clear rules (IBC), and most insurance companies have a policy in place regarding them.
As far as drone observation, it is real, and they even do it here in the boonies for tax assessment.
However, if you already have a wood-burning chimney and a 32" wood gobbler... the drone won't know you changed anything.

Now, I admit to being biased here, BUT in my opinion, the insurance company will always look for a reason to deny any claim.
They are nice, happy folks while selling you coverage, not so nice if they think you would like some of that money back...

A double-skin masonry stove is about as safe as it can be; it will not start a fire in your house, nor will it fill your home with the silent killer (carbon monoxide).

I have a hard time imagining a RMH costing so much, but then again, I am a compulsive scrounger.
I also put function over form, so ugly is very subjective.
A stove that's a little crooked or made with bits of salvage that saves my family a bunch of money is gonna look beautiful.

Absolutely, William!
It is all in the eye of the beholder and in the eye of the guy holding the checkbook!
When I say it can cost $1500-2000, I'm talking top-of-the-line, new everything, with no scrounging.
With some good scrounging, you could keep costs around $500 or less.
Many folks like yourself value reusing or repurposing old items.
They take pride in doing so.
Others want a showpiece they can brag about... (Notice all of my posts...)
Spending money to ease my workload and create a work of art is worth it to me.

No question William that a RMH can be built for a lot less money than my build if you are willing to invest the time to scrounge. No question that clay brick and firebrick can be scrounged at a fraction of what I paid. This is an important point for prospective builders to understand. It is also worthwhile for folks that for any number of reasons may not want to go that route, so everyone gets to chose their preferred course of action.  

I'll explain my motivation in this and other critical infrastructure projects that I have built over the years. I try to build the best possible quality that I can to produce the longest lasting end result. In 10 to 15 years I am pretty sure I will not have the strength and motivation that I have today. So for projects such as this our saying is "buy once, cry once". Fortunately we are able to budget accordingly for our critical projects like this one, as well as our power plant, our water source and so forth.  

Mark Reed wrote:I don't know what a rocket mass heater is... <edited post... you can read his entire post above>

...Basically, I just don't believe a lot of the documentation on the effectiveness and safety of some rocket mass heaters.  

Mark, your post has me puzzled. You being by stating that you don’t know what a rocket mass heater is and you close by saying that that you don’t believe some of the documentation on effectiveness and safety.

I will agree with you on some of the statements made on effectiveness. There are prominent folks on Permies who for many years push the notion that a RMH will consume 1/10 the wood of a traditional wood stove. I think this is a huge disservice to the community as I think this is grossly overstated. I suppose you could come up with a scenario of comparison of a really inefficient wood stove an perhaps arrive at this difference but that doesn’t compare to my years of experience with a bunch of different stoves. I don’t know the range of actual reduction in wood consumption but I know for a fact that it isn’t 1/10 but it is measurably much better.  I just don’t have the time or desire to try to measure the difference.

Where I take exception is the statement about safety, although you added an ambiguous qualifier of “some rocket mass heaters”. I don’t know what this means. We are super safety conscience and this was an area we looked at carefully before committing to our build. There are a number of key areas where I contend that an RMH is safer than a traditional wood stove if you build in a proven manner as has been well documented in this forum. So before people reading thread this get concerned about safety I would respectfully ask for you to please add some detail and reasoning to your statement about safety. I’m not trying to be argumentative, I just get concerned when statements are made about safety with no facts to allow people to assess the basis for your concern.

I said I don't know what a rocket mass heater is because I see basically two different things and I never know for sure what the person is talking about. One is the simple, small sized fire box surrounded by or buried in a heat absorbing and fireproof material like brick or stone and with a vertical exhaust.  That is basically what I have with my wood stove except since mine is an actual stove and the heat absorbing material isn't in direct contact with the fire mine isn't quite as efficient as it might be if it was a true masonry type construction. Still, I really do burn 1/10 or less wood than most people I know who have big wood stoves and my house stays more comfortably warm than theirs. I don't have the periods of damn it's hot in here, alternating with its freezing, put in more wood.

The other is the type with the horizontal flow of hot exhaust. I've seen a number of videos and designs on that but never one that I would trust to operate safely. I don't see why, even if you did nice small, clean burns that creosote would not eventually build up in something like that, leading to the danger of fire. I also suspect they would be very highly prone to back drafts, when a gust of wind hit the chimney just right and blows the smoke back down. Or just flat leaking exhaust through any possible gaps or even back out through the actual air intake as a fire is in its last stages and cooling off. It seems to me that a combination of cooling but remaining coals, combined with a cool chimney and the expectation of horizontal exhaust is a recipe for disaster. Videos and drawings I've seen with the extra fire box to heat the chimney and initiate a draft, not to mention the ones using electric fans lead me to believe that people have attempted to address this issue, but I have never seen anything that solves it to my satisfaction. I don't believe anything other than a hot vertical chimney, by working with the laws of physics, rather than against can continue a safe draw as the fire dies out. Extracting most all of the heat and releasing just cool smoke sounds great, I just don't believe it works.

The facts as I understand them are that hot air/gasses are lighter than cool air, so they naturally move vertically, not horizontally, and I don't know of anything other than the extra little fire box or an electric fan can change that and both of those are subject to failure. It's just my opinion that it creates an unsafe situation. I would love to be proved wrong on that because saving every bit of heat in the house rather than using some of it to exhaust the gasses would be pretty sweet.  

I don't know when someone writes about a rocket mass heater which thing they are talking about. If it's first one, I think that's great, if the second one, not so much.

Mark, you make an interesting point.


As I've followed the development over the years I've seen the consensus around chimneys shift.
In the early days many people insisted that a normal vertical chimney was not needed,now most people involved in the making of rocket mass heater seem to think it is best practice to use these kinds of chimneys.

Rocket mass heaters with good chimneys can still be subject to back draft.
A good insulated chimney is one thing that helps prevent that.
Heating that chimney also helps.
Most current designs call for a "bypass" that routes the exhaust gasses  directly to the chimney, "bypassing " the horizontal piping.
Once the chimney is sufficient hot , the bypass is closed and the exhaust gassed are drawn through horizontal flues that are buried in the thermal mass.
Once the thermal mass is hot enough, the heat it gives off is usually enough to create draft, meaning there might not be a reason to use the bypass except for when the mass is stone cold.


You mentioned the a simple firebox  in one place and the inevitability of  creosote production  in another.
The dimensions, geometry and material  of a properly built rocket mass heater create a burn intense enough that no creosote is produced.
Most of the research around rocket mass heaters centers on perfecting this part of the design, known as the core.
It's simple enough , in execution, if you follow the formulas laid out, but the designs themselves are not simple.



Because the fires in rocket mass heaters burn hot,fast, and clean, there are not a lot of points where hot coals are sitting unattended.
Because there is no thermal mass around a conventional wood stove, users seem to favor low smoldering fires for unattended heat.
In either case, hots coals and a back draft would be a disaster, but a rocket mass heater doesn't depend on smoldering
fires for long lasting heat.

Your posts make me think you might not know how the "bell"  form of thermal mass.
I've recently tried to explain the principles of how a bell works and  seemed to fumble it rather badly , so I'll leave that to someone else.

Hi Mark;
In reality, hardly anyone is building a piped mass anymore.
Too much maintenance cleaning of pipes each season is the primary reason.
Stratification chambers require almost no maintenance.
I will no longer be building using a piped solid mass.

Check out this thread to learn all about them.
https://permies.com/t/270559/Stratification-chambers-Bells-explained

In the early days many people insisted that a normal vertical chimney was not needed,now most people involved in the making of rocket mass heater seem to think it is best practice to use these kinds of chimneys.

It's good to know that most people are now realizing a vertical chimney is necessary to achieve a good safe draft. I think you need a good draft just to start a fire, if you don't want a face full of smoke, and it needs to get hot fast to heat the chimney and establish the draw. I think if you remove the issue of horizontal movement and extracting ALL of the heat before exhaust then you have removed any issues I might have.  

Yes, I suppose some might see it as wasting heat, but it is critical that the chimney itself gets hot and stays hot.  Our masonry chimney, not on an outside wall, itself, works as a mass heater. My little wood stove puts off a pretty good roar if it's cold enough outside to warrant it. So, I guess I already have a rocket mass heater, of sorts.

Worry about horizontal movement of hot air is a red herring. As long as there is a good push or pull of draft from a vertical element in the system, a certain amount of horizontal movement is easy to reliably maintain. These factors have been experimentally established; early trials often pushed too close to the limits for reliability, and there are always amateurs who think they are going to do something different and better who end up smoking themselves out, and those who don't understand that certain elements of a system have to be done just so in order to work. I think publicity from those people has done great harm to the RMH movement.

A good draft can be established in ordinary conditions with only a 150-200F stack temperature. A rocket mass heater combustion core is specially designed to burn essentially all combustible gases so that there is no creosote at all, and thus no need to keep a chimney very hot. A hot fire in a wood stove is good, but does not approach the performance of a J-tube or batch box core built according to published specs.

the concept of the heated flow of gas out the riser and into bell and benchs , then up out the flue has been explained by Matt Remine of Walker Stove s ----and others ----check out his youtube clips ----as to why it works ----its not really just that hot air rises  ----its flow or movement of the hot gas from high to low pressure and how this is happening as the hot gas gives up its heat into the masonry ---as far as i can make out. My biggest motivation for building my stove was safety---- no night time smouldering fire inside my cottage ---no carbon monoxide---no creosote chimney fire---its a relief now to go to bed  with out this on my mind ---as i have done for decades---its a huge plus for me---all the rest of the stoves positives are a bonus on top of this

Glenn Herbert wrote:Worry about horizontal movement of hot air is a red herring. As long as there is a good push or pull of draft from a vertical element in the system, a certain amount of horizontal movement is easy to reliably maintain. These factors have been experimentally established; early trials often pushed too close to the limits for reliability, and there are always amateurs who think they are going to do something different and better who end up smoking themselves out, and those who don't understand that certain elements of a system have to be done just so in order to work. I think publicity from those people has done great harm to the RMH movement.

A good draft can be established in ordinary conditions with only a 150-200F stack temperature. A rocket mass heater combustion core is specially designed to burn essentially all combustible gases so that there is no creosote at all, and thus no need to keep a chimney very hot. A hot fire in a wood stove is good, but does not approach the performance of a J-tube or batch box core built according to published specs.

The situation of vertical chimney and horizontal ducting feels like an equivalent to a siphon... If you just look at one part of the system, taken out of context, it seems in defiance of the laws of nature ("water flowing upwards/hot gases flowing horizontally? Ridiculous!") but if you factor in the other part of the system, that pulls the water/hot gas through the first part, it all makes sense. Just a random thought.

To answer the original questions: Yes, we're interested in building a rmh, and what's holding us back is just a slight (and hopefully temporary) lack of house...

Thank you Mark for providing additional color around your concerns and thank you William and Tom for further filling in details.

Reflecting back on my journey when I began educating myself on the technology… there was a lot of confusion about terminology and designs so I thought I would break things down a bit here for those folks reading this who are still getting up to speed on RMH technology evolution and terminology…

The references to horizontal chimneys and piped mass relate to the older design whereby a J tube combustion area at the start of the system is where you add a few pieces of relatively small wood and requires the person to tend to the fire pretty closely to continue adding wood. It typically then directs the heat into a steel drum that acts as an immediate heat radiating device, Internally the heat is then routed through horizontal pipes that are embedded in a mass, typically in the shape of a bench that you can sit on and get the benefit of a nice warm seat. The gases are cooling the whole time, giving up the heat to the mass, so the size of the whole ducted system is critical to ensure there is still enough heat the create a draft once it makes the transition to the vertical chimney.

The current favored design was in large part developed and refined over more than a decade by dutchman Peter van den Berg. His work is published in detail on his website batchrocket.eu. The designation Batch Box or Batch Rocket refers to the idea of an enclosed combustion box that will accept a good size batch of wood, the door is closed and the batch will typically burn unattended for about 90 minutes. The combustion box features a venturi at the rear along with a secondary air inlet. The combination of these 2 features creates a flame that is substantially hotter than any standard wood stove. It is common to see temperatures after the venturi in the 1,500F range which is actually burning the smoke that would typically dump out your chimney with a standard wood stove. Due to this secondary high temperature burn nearly 100% of all combustible material is then consumed creating a much cleaner burn than standard stoves and also burning ANY creosote. The heat at this point is dumped into a stratification chamber (also referred to as a bell) which is typically a masonry mass. The bell is simply an enclosed space that allows the heat from the combustion to soak into the interior walls of the bell which in turn warms the exterior walls of the bell. Think of it as a thermal battery where the masonry stores the heat and slowly radiates it into the room. For anyone with experience with radiant heat floors you’ll immediately appreciate the benefits of a radiant heat system. The combustion heat is absorbed by the mass as it moves through the bell to the chimney exit that is essentially at floor level. To quantify the mass, my system has 7,000 lbs of double wall brick mass. Once the heat transits down the bell to the chimney exit we are typically seeing chimney exit temps at the floor level of 175 to 225F. I’ve never measured the temperature of the exhaust out my 20’ tall chimney but it is pretty cool with zero smoke and just a bit of moisture. As William pointed out, many of us have a chimney bypass that exits the bell into the chimney pipe at roughly the top of the bell. This is used only when the system has zero heat in it at the beginning of the heating season. After that there is no need. When it’s time to burn a batch the air intakes are opened up and the heat resident in the mass immediately initiated the draw up the chimney. The reason it is called a rocket, or in the case of Tom his herd of dragons, is due to the amount of draft created by the system causes the intense burn and a rocket or dragon roar is heard.

In my experience I have never had and don’t ever expect to experience backdraft. With a warm system backdrafting is basically impossible and the bypass eliminates this possibility when the system is cold.

Some of your description and logic seems pretty sound, and interesting. Actually, the whole part about your own set up sounds pretty interesting. I wouldn't mind seeing such a contraption in person. Maybe the parts that I'm still not sold on would fall in line. I still would not want one myself, at least not in our current house. Here, the little free standing Vermont Castings does fine.

I guess I do cheat a bit.
The house is small, well insulated and partially underground with big south windows. As mentioned before the bricks around and under as well as the chimney itself serve as heat sinks.  They absorb heat from the stove and the windows both. It was all built that way on purpose. The stove has a double wall and a lever to close the chimney forcing smoke out through the coals at the bottom. *That only works while the chimney is hot, but I'm fine with that. Once the chimney is hot, the draft continues until all coals are fully cooled.  A little smoke comes out of the chimney when I start a fire but almost nothing visible comes out once that lever is set. I just load it up based on, do I want it to roar for 1/2 an hour or do I want it roar for two hours. I also have an abundant free source of seasoned black locust firewood, basically in the yard. That was also by design. I guess by volume I burn about fifty cubic feet of actual firewood in a season.  Black locust as a bonus drops a lot of sticks which we use for small quick fires when it isn't cold enough outside for a real fire.