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Credit: Mike Ewing

Paul makes a spur of the moment podcast with Ernie Wisner about rocket mass heaters (RMHs). They are joined by Erica Wisner a little past the halfway mark.

Paul starts off with news that Erica's latest shipable core attempt has cracked and they are now doing a post-mortem. Erica is now obsessed with creating the world's most awesome shippable core.

Paul's pebble-style RMH in the office has been getting mostly daily use for 3 weeks and is working well but they have some ideas for improvements. One thing they note is that it has a smaller mass than a typical cob RMH, so it warms up and cools down quicker. Maybe something more suitable for a weekend cabin, where a cob RMH would take too long to warm up. They talk about pocket rockets and how well they do what they do, which is radiant heating.

Ernie spoke about folks who complain that RMHs need a lot of tending (which they don't). Paul ascribed this to corporate trolls, but Ernie thinks it may have more to do with people who are used to L-tube rocket stoves assuming that rocket mass heaters (with J-tubes) suffer the same problem. Side discussions ensue about L-tube stove deficiencies and underhanded marketing tactics.

Paul brings the discussion back to talking about the working pebble-style RMH. Paul says they have removed the outer wooden box, having decided it is unnecessary. They also plan to add an external air intake. They talk about the merits and demerits of doing this. Ernie advocates having stoves that draw fresh air into the room rather than directly into the stove, but Paul wants to be able to leave the stove running untended and is concerned that without an external air intake, there's a risk of smoke coming in. Ernie appeals to people to note the circumstances when smokeback occurs and publicize these problems. Paul says that sometimes a puff of smoke will come out of the feed tube but be immediately sucked back in. Ernie adds that he is very familiar with his own stove and often leaves it burning untended without smoke or fire worries.

They talk some more about air intake design and how often people try to use steel for this. This is problematic because steel overheats even when it is quite far from the fire (probably due to radiant heat).

The discussion veers into high temperatures (2500 to 3000 deg. F.) and how steel and other materials deal with them. Paul notes that stoves with steel in the heat riser often burn out (and mentions a video by web4deb). Ernie says that the heat-riser design involving the inner and outer tube with perlite/clay mix between was not intended for long-term intallations because the heat riser is quite fragile after the metal is gone. He recommends brick for better durability.

Paul brings the discussion to the topic of people who want to get into the shippable core business. It seems the people who have contacted Paul with this intention do not, generally (or perhaps universally), see eye-to-eye with Paul on the value of the work he (and Ernie and Erica) have done and will do to make such a business succeed. The upshot is, Paul and the Wisners will be freely sharing their information on shipable cores. Their monetizing strategy is to allow others to take the information, get a business going and then, if the product meets with their approval Paul will list the product or business on Permies and take an affiliate fee for any sales through that channel, with an additional $20 per core going to Ernie and Erica.

Erica arrived at this point. Turns out that she knows a bit about these rocket mass heater thingies too!

They talk about the criteria that will be used to decide whether a core gets their approval. Paul says it must require minimal DIY skills to install. Ernie says it must burn clean. Paul points out that right now the box-style RMH has a shipable core (by freight) and this could already be the makings of a business. Erica's main criterion is that anybody newly exposed to a RMH built with the shippable core experiences an OMG reaction. She doesn't want a mediocre product at a cheaper price.

They talk a bit about the appeal of RMHs including the upside down fire.

Paul brings up another criterion, which is that the core must include the manifold (the transition area between the barrel and the ducting). There's more discussion of Paul's DIY simplicity criterion. Ernie objects that he'd rather have people learn more DIY skills, though he sees no harm in making the shippable core simple. He says he's seen the general level of DIY skill go down over time. Erica notes that its much easier now to develop skills because information is out there that used to be closely guarded trade secrets.

Ernie says he and Erica are looking for many more installers. They have a lot of clients lined up who would like a RMH installed. Right now they have somebody within 4 states of any potential clients, but would like to get it to within a half hour drive. Paul suggested a thread at Permies to list all those who are willing to build someone a RMH and where they are. Each of the people listed could also have a thread of their own with a gallery of their work.

Ernie says he will want the installers to follow the Wisners' plans on their installations. He also wants them to publicize any problems they encounter so they can address them. Erica says free answers are available in the regular Permies forum within 48 hours and faster answers are available for a consultation fee through

Ernie sets Paul off by mentioning a particular incident and a short, rich (in f-bombs) discussion takes place about the appropriateness of customers wanting to know how you spend the money they give you.

Paul goes into a bit of detail about his preferred way to set up an affiliate program (with a 3rd party "money-fondler"). Erica brings up the low-toxicity of the refractory materials they are now using. They discuss embodied energy of the materials and Paul states that the savings of having a RMH easily outweigh this. He says he can't think of anything better to do for the environment than installing a RMH in a conventional home.

They talk a bit more about new versus existing homes. Paul said passive solar and wofati are definitely worthwhile for new construction. Erica recommends looking at pre-industrial homes for insights into how things can be done when considering new construction.

Paul starts wrapping the discussion up. Ernie messes up the tag-line just a bit.

Relevant Links

Rocket stove forum

Get a personal consultation with Paul Weaton, Ernie & Erica Wisner and others

Ernie & Erica Wisner's website

Youtube video of a rocket mass heater autopsy

Support the Empire

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To support production of these podcasts, make a donation here at Paul's Patreon page.

Posts: 3
Location: Alaska
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I live in Anchorage Alaska and have been watching this progress for a few years now. I have thought that this could be the answer to an interior Alaska air pollution problem mostly in Fairbanks due to lack of winter air movement patterns. I have been in Construction for over three decades. If there is anyone in this area that is building these I would like to meet them and help with installation. I would be happy to work at a substantially reduced rate. I am planing to make some outside in the spring on my back patio using info off the videos and the plans offered by Ernie and Erica. I will have to work with movable models. It just seems that no one is as interested in RMH as I am.(Here in Alaska) So by making a model to show might help with more interest and possibility of more people willing to try. I think people informed about RMHs would be happy to install one. The income stream is there just by looking at all the wood stove shops around town people would be happy to pay better than wood stove prices to save 80% on fire wood at 200 plus per cord over a ten year span could save 10-15 thousand at a 10-15 cords per year if you could get by with 2-4 due to increased efficiency. I have been asked to install a wood stove in the past three months and pointed them in this direction hoping they might opt for this type of heat source supplementing the original gas furnace. I believe in this and will continue to promote this as much as possible until I can install RMH on a regular basis. With all the knowledge that I have found here I would be happy to give a fee for every one made, to Ernie and Erica as to help with the betterment of everyone and to help with design improvements. I like the idea of the high temp unit.
Posts: 62
Location: Portugal
transportation gear earthworks solar rocket stoves
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I've been contemplating the RMH thing for a while, I like the idea of a "portable" design which works more on radiant heat than on a really big mass: I think it would fit into existing homes more easily.

While the large mass cob type heater is undoubtedly efficient, for many people it requires quite a re-work of their home, and they may not want that. However, a more radiant and less massy heater which can "drop in" in place of a regular wood stove might be more popular.

Interested about the comments about steel not coping well with the temperatures, though. Is that about thickness of the steel tube, or is any steel tube gonna fail prematurely?
Posts: 1481
Location: Vancouver Island
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Austin Shackles wrote:I've been contemplating the RMH thing for a while, I like the idea of a "portable" design which works more on radiant heat than on a really big mass: I think it would fit into existing homes more easily.

While the large mass cob type heater is undoubtedly efficient, for many people it requires quite a re-work of their home, and they may not want that. However, a more radiant and less massy heater which can "drop in" in place of a regular wood stove might be more popular.

Interested about the comments about steel not coping well with the temperatures, though. Is that about thickness of the steel tube, or is any steel tube gonna fail prematurely?

Portable or shippable core? There are Portable designs... but mostly they break down into manageable pieces and still weight rather a lot and take up a lot of room. They can be made to take up less room but then the foundation requires more attention because the weight ends up all in one spot.

Doing without the weight? Then why not just use a normal steel wood stove? These all meet insurance and can be permitted. I would suggest a pellet stove if it must fit into an assigned space. The pellets are fed at just the right rate for the needed heat output without a mass to keep things from getting too hot. Watching the progress on "the project", I saw they have used a number of pocket rockets. They are quick and cheap, but they put out too much heat and they may not last so long. I also saw that the users of these have already noticed that they need some mass to make things work better.

The threads on RMHs have the math explained as to why a small hot heater is not as good as a large and only warm heater. Basically, the heat goes right through the walls even with very good insulation because the heat differential is much higher from inside to outside.

I personally think making the RMH so it fits what people want at the expense of having perform as expected is a bad choice. The RMH has a reputation that will be destroyed by making a variant that is no better than the average $300 wood stove and requires some know how to make it run on top of that. The MASS is what makes it special, take away that so it will fit someones idea of interior design and you have lost everything. Function determines form, not the other way around.

Steel not coping? At high temperature, steel melts, with oxygen it also burns. Thicker will last longer, but still "wear out" at the higher temperatures in a RMH. The RMH burns much faster than the average wood stove. It is meant to burn very fast for a short time and then run of stored heat for a long time (sometimes days). Before one starts thinking about a mass heater (RMH or masonry), they have to set aside everything they know about wood burning heat no matter how expert they may be. These are very different than most north americans are used to. So different that the builders of masonry mass heaters have the user sign that they have read and understand the operating instructions before they call the install complete.

Another thing not mentioned, but your post sort of points this way (the stuff about home owners expectations), is that the idea behind the RMH is to heat people not homes. Way too many people measure comfort by looking at the air temperature and expecting 22C. Heating air is bad! trying to stay warm by heating air is bad. It is generally not healthy. Open a door and all your heat goes out with the air... so what do people do? they seal their home. There are standards for air replacement in a home but these are minimums for survival. The needed volume of air replacement to thrive is 4 to 8 times as much as "required" (check out what labs that experiment on animals ... healthy ones, use for air replacement). A high mass heating system is generally not bothered by drafts or other forms of air replacement.
Posts: 31
Location: Northglenn, Colorado
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OK, I have 2 cents to throw in on the shippable core. I don't know what the design should be but I have some ideas on distribution. The Open Source Ecology guys have a different attitude towards these things that I think could be adapted. They provide the plans and steps to build it for free with the hopes that regional small business will be created with compatible parts, or any standard fabrication shop can reproduce the parts. I realize this doesn't address the issue of Ernie and Erica getting paid. I would suggest given Paul's success with KickStarter that there is ransom created, with the amount of money it would take to repay Ernie, Erica and Paul for their hard work, with the giveaways being something like hard copies of the design or a DVD showing how to build the core. If the KickStarter is funded then the design is released under something like an open source license allowing regional fabricators to build them and make them available to be picked up or delivered locally thus reducing shipping costs and sacrifices that need to made to reduce weight. I'd call it something like Micro-distribution.

I know there are many arguments against and reasons people wouldn't want to do it this way but I though I'd at least raise the idea.

Incidentally I'm a professional CAD designer specializing in HVAC and piping systems and would be interested in helping create professional design documents, maybe for free if in the end the ideals of the project align with my personal philosophies; e.g. the plans are released under an open source license, or for a reasonable fee if not.

Thanks to all for their hard and pioneering work on this.

Posts: 24
Location: Dawson City, Yukon
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I just wanted to thank Erica for the connection to the past. The whole point about gonig back to looking at the pre-industrial history of like climate structures and pillaging ideas.

As well, I was blessed to receive the WBS 2.0 for Christmas, they were AWESOME! Well worth the money. The discussion in this podcast certainly did what was mentioned in the videos, expand on what the dvds talked about.

I have built two pocket rockets previously (success), many small rocket stoves (successes), a couple of cooking rocket stoves (awesoem successes) and have experimented with a small scale rocket inspired lime kiln (dismall failure but learned alot for LK1.1) and a couple of other things and found that I learned a good deal from the videos. Anyways, Paul, Ernie and Erica you all rock.
Posts: 162
Location: USDA 5b - Central IL
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As soon as I heard about RMH's I thought of the Kilcher family -"Alaska the Last Frontier" on Discovery.

I am mixed on the shippable core, I like the idea of open source but at the same time I think that Ernie and Erica should get paid for all their work/research...I have bought plans from them and will continue to buy plans from them but there are those out there that won't.
The knights of nee want a shrubbery. And a tiny ad:
Heat your home with the twigs that naturally fall of the trees in your yard
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