Glenn Littman

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.

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.

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.  

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.  

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.

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.

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.

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.

Maybe I missed seeing it but surprised nobody has mentioned DMSO. Safe and available over the counter if you know where to get it. Do a search for A Midwestern Doctor on Substack. He has done exhaustive research on the many uses of DMSO. It transformed my wife's life due to arthritis in the spine. Now she's as spry as she was 30 years ago. We buy pharmaceutical grade from the DMSO Store. Their website is that name with a dot com after it. They have a ton of great info on their website too.

A few facts to add...
DMSO has been around for many decades for medicinal use.
In the 80's and perhaps 90's you could buy it at your local pharmacy but big pharma had other ideas since it was cutting into their profits.
It is still used extensively in veterinary applications for inflammation. You can buy it at your local Tractor Supply if you want, we prefer the products offered by the DMSO Store as they offer is in various strengths and mixtures (liquid, gel, roll-on, mix with aloe for example).

Do you own research but take this one seriously. We know of lots of people who have had tremendous success with it.

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.