In the last post I mentioned the three Es. I really need to write more about these soon, but that is something of a conceptual post. Mind you I have nothing against conceptual writings. I expect to engaging in a fair amount of that here with this blog project. In fact, as an artist my personal experience has been that the written word is probably the best avenue to try and convey complex concepts. Maybe it’s just me, but conceptual art trying to communicate its ideas through visuals alone often seems lacking, usually requiring a written statement to go with it that actually explains the concepts.
(Please note some of the links in this blog post are affiliate links. What this means is that should you click through them and make a qualifying purchase I will receive a commission which I’d certainly appreciate since it helps support this blog project. However, this shouldn’t increase your cost any, and certainly don’t ever feel like I’m pressuring you to buy things through the links I offer or anywhere else. I’m all about being frugal first!)
Anyway, conceptual essays are all good, but I wanted to really kick this blog off with some concrete stuff, a major construction project I’m experimenting with here on my homestead. So I’m picking one that demonstrates well how the three Es interrelate, a rocket mass heater.
As a reminder the three Es are the economy, energy, and the environment. While I don’t know that it’s a natural law that they must relate in this way, here is the quick and simple explanation of how it tends to work in the real world. As you increase economic activity you increase energy use and cause more damage to the environment due to more extraction of it’s resources and discarding of waste.
Right now I’m seeing rocket mass heaters as the most exciting thing out there for the potential to engage in environmentally beneficial activities that make a real difference. For any of you aging hippies out there who were into the appropriate technology movement this is something that would have been in your tool kit, except they didn’t exist in the 60’s. This is a new development! It seems shocking to me considering how long humans have been working with fire that there could be something radically new for simply burning sticks and logs, but here it is.
What makes it so exciting you may ask? Well for starters as I noted in the title to this post a rocket mass heater is 50 to 90 percent more efficient that a regular wood burning stove. Let me state that another way that might have more impact. If you were heating your home with 10 cords of wood in a winter a rocket mass heater might reduce that all the way down to just ONE cord! That would be 90% more efficient. You might not get that good, but you could reasonably expect to cut your wood use in half. These numbers aren’t from some laboratory where highly trained people are burning everything in the perfect conditions to get the optimum results. These are the reductions people are reporting from their own real world experience.
It gets better though. Not only are these heaters more efficient but they burn way cleaner. If you have a wood burner, or have neighbors who use one you know their potential to be spewing out lots of smoke into the air. With a rocket mass heater there might be a tiny bit of smoke in the first few minutes of a burn as it gets warmed up, and perhaps a bit at the end as it cools too much. Otherwise, it is burning so hot it burns the smoke! A smokey fire is a sign of an inefficient fire, one that’s wasting fuel. Depending on conditions you may see gasses venting out from the chimney of a rocket mass heater, but what you are seeing is basically just water vapor and carbon dioxide. If you look carefully you won’t see the yellow or blueish tinge to the cloud that you see with actual smoke. When I look at mine I generally see nothing at all. I understand one of the best designed rocket mass heaters was tested at its peak performance and found to actually be burning cleaner than a natural gas furnace!
In terms of emissions it also needs to be noted that with a regular wood stove you generally need to keep it burning all the time if you want to keep it heating your space. A rocket mass heater is different. This is where the mass part comes in. You do a shorter, much hotter burn that heats up the mass which then continues to release the heat over a long period after the fire is out. So no only do you have less emission while it’s burning, you have NO emissions when it’s coasting on the heat of the thermal mass!
I don’t know if there are any hard statistics on this, but I feel like it’s a much safer wood heater than traditional stoves. The first reason is simply, as noted above, you aren’t burning it as long. In particular there is no need to try filling a fire box and getting it to do a slow burn through the night to maintain some heating capacity. Another major safety feature for me is that with a properly designed and built rocket mass heater there shouldn’t be much risk of a chimney fire. As I understand it chimney fires result from creosote building up in the pipes and eventually igniting. These heaters however burn so hot the creosote is ignited back in the burn chamber or heat riser section. (Mind you I still plan to regularly inspect and/or clean my chimney.) Again creosote build up is a sign of inefficiency. That’s fuel which should be burned not sent up a chimney! I should also note that these zones of the heater that burn at these high temperatures, sometimes over 2000 degrees F, are buried deep in the mass. The outer surface temperatures won’t be that hot. The barrel you see in the photo does get hot, like a regular wood stove does, so the normal cautions must be followed with that. However, the heat exchange bench, which is the part you see on the right side of my photo, gets wonderfully warm, but not too hot. Many people design this to function as a place to sit or even lay out on. I have certainly done so on mine, and oh it is blissfully nice to have that warmth conducting straight into a cold body. (For any gardeners out there, it seems to make a nice warm bench for germinating seeds too.)
I first learned about these from a little book titled “Rocket Mass Heaters – Supereffient Woodstoves” by Ianto Evans. As I understand it, Ianto is the one who originated these back in the late 70’s to late 80’s. I found the information in the book intriguing, but at the same time was left feeling like it was too daunting a task to make one for my home. Maybe I’d play around with one “sometime” for an outbuilding. That sometime never quite happened. In preparation for this post I went back and started rereading Ianto’s book. With a bit more knowledge and experience behind me now I’m finding useful things in it that didn’t really impact me initially. I think I’m now going to have to go ahead and finish rereading it! I have the second version of the book. There is now an even more updated 3rd version which is what I’ll link to.
So rocket mass heaters just lingered around in the back of my head for a few years as something I’d like to be able to do, but didn’t seem feasible for my home which is actually a small mobile home. Then a couple years ago I read that there was a new rocket mass heater book out that was way better and up to date with developments in the heater that have proven themselves over time. This one was, “The Rocket Mass Heater Builder’s Guide” by Erica and Ernie Wisner. I got it, read it, and found it much more useful. It’s just what the title says, a builder’s guide based on proven designs worked out in the years of experimenting by the true pioneers of these heaters. If you read about someone building a rocket mass heater “by the book”, this is the book they are talking about! It is without a doubt the book to have if you are going to build one.
After reading Erica and Ernie’s book I started seriously considering trying to build one in my little trailer home. Still I wasn’t sure. The biggest stumbling block for me was the fact that I don’t have a solid floor to build on, such as a concrete slab. These things weight a lot and I was worried about collapsing the floor, or how to handle cutting out part of my floor and building up a solid base.
The final pieces I needed to move forward were in a 4 DVD set “Better Wood Heat – DIY Rocket Mass Heaters” produced by Paul Wheaton. Of particular interest to me was the pebble style rocket mass heater he designed and built in conjunction with Erica and Ernie Wisner in his doublewide manufactured home. Here was someone who has actually built one in a mobile home on the existing floor after adding some extra bracing underneath. I definitely wanted to know about that! One year he even carefully measured the amount of wood needed to heat his place through a Montana winter and found it came out to .66 cords, so basically 2/3 of a cord!
I found this DVD set to be highly useful. It features many of the true innovators who have done the real work of developing this heating system Ianto first originated. Erica and Ernie’s book was indispensable, but being able to combine that with seeing videos of multiple heaters being built really helped clarify things and make it seem doable. The fact that it’s actually Erica and Ernie directing most of the construction work in the videos is a great bonus since I know the information will mesh with their book.
If any of this sounds at all intriguing you are probably wondering just how much one costs. If you are able to find someone in your area to build one for you then you have a rare option, and I expect it would be fairly expensive. Odds are you will need to build it yourself. They were designed to be owner built, and why I mentioned earlier that they fit in the appropriate technology tool kit. I’ve read claims that a rocket mass heater can be built for as little as $100. Personally I find that figure quite dubious. I think you’d have to get mighty lucky in scoring free salvage materials to make one that cheaply. I suspect $500 is a more reasonable low end price. Despite the fact that I’m generally a frugal guy, which will no doubt become clear as this blog progresses, mine has been on the higher end of costs. I suspect by the time I’m fully done with it I’ll have spent between $1500 to $2000. They really don’t need to be this expensive though.
This feels like it’s becoming quite the long post. I still wanted to share a bit of the process of my build as well as my efficiency results so far. Perhaps I should save all that for a future post… but I’m not going to. I’m excited about these as want to share it now. So let’s jump into a bunch of photos with a bit of explanations. Let me make this clear though, what I am showing you is NOT intended to be a guide for building your own. I am leaving out a bunch of what I did including critical safety features you aren’t seeing but would be needed to prevent this from being what Erica and Ernie call “Flaming Freakshows of Death”.
Figure 1 shows my the space I’m working in, my living room after I removed my old wood stove. I am going to use the existing chimney so it’s location played a big factor in where everything else needed to be built. My inherent problem with this space is that it’s just not large enough for the size the heat exchange bench could be to efficiently draw off as much heat as is possible from the exhaust. I can’t really get around this short of building a significant addition onto the home.
In figure 2 I have built the elevated base of my heater. Since I am both building on a wood floor and doing a pebble style I need to allow for air flow underneath. You might be able to see a series of holes drilled into the plywood base. In a pebble style rocket mass heater there needs to be air flow moving through the mass of pea gravel that is storing the heat otherwise the pebbles will act more as insulation preventing the heat from transferring through well. I should probably note here that the very first thing I did was a completely nasty job of wiggling under the trailer to add extra bracing to support the floor. To my delight though I discovered that one of the two main steel I-beams that form the undercarriage of the mobile home lands dead center down the long line of the heat exchange bench where the bulk of the weight will be. That made things much easier!
With the next image, figure 3, you can see some of the layers that form the critical insulation beneath the heater core. The core is the part of the rocket mass heater that gets super hot. This insulation helps it get super hot, but is also needed to prevent my floor from burning! What you are mostly seeing here are ceramic fiber boards, a nice but more expensive option for this insulation layer.
Next in figure 4 you can see what is called the J-tube. This is the engine of a rocket mass heater. I made mine from two kinds of firebrick, a heavy durable dense brick for the lower sections, and a lightweight insulation type for the heat riser section on the right hand side. The short side of the J shape is the feed tube where I’ll be inserting the firewood. The flames will then burn horizontally until they reach the heat riser which will act like a chimney providing a powerful upward draw.
In figure 5 I’ve made a big leap forward constructing what is known as the manifold as well as adding the barrel and duct work that runs through the bench and out to the chimney. When the super hot air coming out of the heat riser hits the top of the barrel it begins to rapids shed heat, falling as it cools. This action again helps provide a strong directional draft which will be used to push the still hot air horizontally through the ducts in the bench. The final rise of the chimney again provides draft assistance to pull what should be much cooler exhaust gasses up and out.
Figure 6 shows the bench partially filled with pea gravel and stones which are the mass part of my rocket mass heater. These function like a temperature battery, taking the heat from the duct work and slowly releasing it over time after the fire is out.
Figure 7 shows you my rocket mass heater as it stands today. I haven’t done any of the trim and finish work yet because I wanted to see just how this worked before hand, not to mention the fact that I ran out of time and really needed to get back to making my artwork or all my galleries would start to get upset with me. The plan is to tile the exterior sides of the box and do either granite or marble slabs for the top.
Figure 8 is a shot of the fire in the feed tube. Alas you can’t tell in the photo but the flames are going horizontally along the burn chamber toward the heat riser. It’s pretty neat to see fire burning sideways!
So I’ve been using my rocket mass heater as my primary heat source for roughly the past 3 months as I type this. (Dec. – Feb.) What are my personal real world results? Do they compare with the claims? I’m inclined to accept the notion that you could reduce your wood use by up to 90%. Figure 9 shows the average amount of wood I use in a day to heat my 600sq/ft home. This is a bit less than half of what I used to use, so I’d say I’m at a 50% to 55% reduction. I was using between 2 to 3 cords of wood a year. If I extrapolate out based on what I’ve used so far this winter I would guess that I’m running between a cord to 1.5 cords to heat my home through a Michigan winter.
I also need to factor in that this heater has reduced my propane use by another 80 to 90 percent. With my traditional wood stove if I was away for the day the regular propane fueled furnace would kick in when the interior temperature dropped below 58 degrees F. It would also almost always run some during the night after my wood stove cooled down, again when the thermostat got below 58. Since the moment I started using my rocket mass heater in late November it has been a rare occurrence for the furnace, which is still set at 58 degrees F, to turn on. As a crude measure my 500 gallon propane tank was 40% full when I first fired up the rocket heater. Now it’s at about 35%, so that would mean I’ve used roughly 25 gallons of propane to supplement the firewood so far this winter.
When I consider the reduction in supplemental propane use, the fact that I know my rocket mass heater is smaller than it could be for full efficiency, and add in that I made a pebble style which is inherently less efficient than a traditional cob style, plus throw in that my old wood stove was a soap stone model which was better than the average wood stove, I can see where 90% reductions are feasible for others to get.
While I’m quite happy with these results and wouldn’t be upset if they never got better, I think I can improve them. I’m still considering this a work in progress and before next winter rolls around I hope to try making a change or two to boost the performance even more. My ideal goal would be to get my firewood needs down to half a cord! I’m not sure if I can get there or not given my specific limitations, but I’m going to try and get it at least under a cord.
One other thing I should mention is that rocket mass heaters like to use smaller diameter pieces of wood. You can use larger pieces though they don’t tend to burn as hot as smaller ones will. Using smaller diameter pieces could be seen as a negative since it involves more splitting of the logs, though since you are splitting fewer cords in total it’s likely still less work. However, the upside is that what is generally seen by others as too small to bother with for firewood is great for a rocket mass heater. Tree trimmings and pruning can work wonderfully. It’s conceivable a chainsaw wouldn’t even be needed to cut the winter supply, though in actual practice it would still be very handy.
In this blog from time to time I may point out trends or business opportunities I see just waiting for someone to seize and run with them. Rocket mass heaters are one such instance. It’s kind of shocking to realize how few people have been involved in developing these, seemingly with very limited resources from what I can tell. Yet they’ve gotten them to the current stage where they’re ready to go prime time. I have to imagine there are fabulous opportunities here for entrepreneurs to really develop the business and marketing to make this at least as mainstream as wood stoves. Why shouldn’t there be one or more companies in every major city that focus on supplying or servicing rocket mass heaters? I’m happy being a studio artist so I’m not going to do this, but I throw this thought out there for your consideration.
To sum up and tie this back into the three Es framework, I’ve found that a rocket mass heater greatly reduces energy needs, which reduces my personal economic activity (i.e. saves me money), while also reducing emissions, waste, and strain on the environment. The fuel source, trees, are renewable and function within the existing carbon cycle rather that introducing more fossil carbon into our atmosphere. By using the practices of coppicing or pollarding I don’t even need to kill the trees. (There will probably be a future post on this sometime.)
If the planned improvements to my heater work well enough I may be able to all but eliminate my home heating costs on a sustainable basis! Even if they don’t work I’ve slashed these costs down considerably while making things better for the environment, and improving my quality of life. Can a rocket mass heater work for you as well?
Update as of December 20, 2021. I finally decided I wasn’t going to do any major changes to the RMH and actually set about doing the finish work. I thought I should share this so you all don’t think these have to look ugly and unfinished!
I had been considering removing the pebbles and going ahead to try a solid cob mass which would make for a more efficient heater. However, I just don’t dare put that much weight on my trailer floor, even if this is all centered over the main steel beam underneath. So I’m going with my original plan of doing tile on the sides.
I rummaged around at a couple salvage building supply places and found some nice stone tiles for a buck a piece that would work for me. The nice thing with this project is that it needs relatively few tiles compared to doing a whole floor. Hence, I could utilize the left overs from someone else’s job. I’ve also used some salvage wood trim I got from a buddy who was remodeling his house to go around the upper vent openings.
In this next photo you can see that I’ve gotten the tiles set and grouted in place. I’ll note this was a pain in the butt to do on the back section by the wall. I could have left a bit more room to access that space for this job, but in the long run I wanted to preserve as much open floor space in the rest of the room as possible. So it was tough reaching back there to set the tiles, and they aren’t perfect, but then again you really can’t ever see that side well because it is right up near the back wall.
For the top surface I was hoping to be able to get some sort of stone slabs. I’d kept my eyes open for a few years when at the salvage yards. I saw many things that would work, but they would have been awkward for me to transport home and cut to fit. Then I ran across a pile of salvage slate stair steps. They were perfect for me! Easy for one person to handle. The long sides were all neatly cut and parallel, setting perfectly next to each other. I just had to trim the short sides to fit. I found a diamond saw on a 4.5 inch grinder worked surprisingly well for the job!
Right around the barrel there were some weird shapes, but there cob came to the rescue. I just mixed up more of the cob that I used to set the barrel originally and sculpted it in to fill the odd spaces.
To finish this all off I made a clay based paint utilizing the same fire clay I used for the cob so the colors would match. This was then painted on the wood trim in many thin layers. I’m quite happy with how that worked out. Then I needed something to trim out the inner liner of the vent ports. I didn’t want anything too thick or it would reduce the opening size. Being a metalsmith working most frequently in copper I decided to splurge and buy some copper for this. I did go thin, using 12 ounce copper, but then folded over the outside edge so it would look a bit thicker, and also deal with any sharp edges.
So here are the final photos of the finished rocket mass heater!
Overall I’m very pleased with this project. I still wish I could have made a full size cob style rocket mass heater, but this is what will work in my space so I’ll have to save that project for another space and another time. In the years that I’ve been using this to heat my home now I seem to be consistently using about 1.25 to 1.33 cords of wood and virtually zero supplemental propane via my regular furnace. (When I’m away from home for days I do still need to use the furnace a bit to keep the place from freezing.)
So far I’ve also been able to get all my firewood from my property or wood scavenged here and there. This might not last as I am using lots of ash trees that were killed by the emerald ash borer. However, as things seem to be going I’ve got several more years before I run out of my current supply of firewood from my tiny homestead. If I had a few more acres I’m pretty sure I would have an indefinite supply that kept replenishing itself.
My hope is that all this will help to inspire some of you to delve into the world of rocket mass heaters! They are fabulous things. Sitting on the warm bench on a cold day is just divine.
For convenience here again are the links to the books and DVDs I found useful. Some of these are affiliate links where I earn a commission on qualifying sales.
“The Rocket Mass Heater Builder’s Guide” by Erica and Ernie Wisner
“Rocket Mass Heaters – Supereffient Woodstoves” by Ianto Evans and Leslie Jackson
“Better Wood Heat – DIY Rocket Mass Heaters” produced by Paul Wheaton I should note that if you want to learn more about rocket mass heaters this link brings you to a page with a short video and lots of other information.
Since I first wrote this blog entry I’ve added other blog articles about or related to rocket mass heaters that might be of interest should these appeal to you. Here are direct links to them:
Being a Chimney Sweep with a Rocket Mass Heater
An Incredible Tool for Splitting Kindling
So how much is in a cord of wood?
Studio Snippet
After my last post I thought of something that might be of interest to those of you reading this blog which I’ll start implementing now. I’m calling it a Studio Snippet. In this I’ll end each post with a little snap shot into what I’m working on in my art studio at the moment. Since this post has gotten so long I’m going to keep this one very brief.
Today I’ve been cleaning up and polishing the silver rims on a new batch of vessels. I want to get these done before I head off to teach a couple workshops so I can bring them with me as examples. My fingers are a bit sore from the hours of sanding, but I did get this stage done this evening in time to finish writing this post. My next step will be to apply the gold leaf to the interiors.
I’m happy to have a site where I can again allow comments. (I had to shut them off on my main website because the spam was simply uncontrollable!) So please I encourage you to share thoughts of your own. My general rule about comments though is just to play nice. Differing views are fine, but I’m not interested in engaging in or moderating verbal fights. If I feel things get out of hand, by whatever criteria I decide, I’ll just start blocking or deleting things.
I wanted to sign up for your blog posts.
Thanks
Hi Linda. Thanks. I believe to sign up for notifications of new blog posts all you have to do is check the “notify me of new posts by email” box below the comment box. You might have to make a comment of some sort, also check the notify by email box and click on the “post comment” button.
Very informative, David!
Thanks Cara! Welcome to the blog. This heater is what I imagine I was yammering on about at one of the LaFontsee Gallery receptions. 🙂
This is right up my beliefs. Thanks for putting into words my thoughts.
My pleasure. Thanks for the comment, Larry.
Hi David. This is very interesting. Looks to be energy efficient to quite a degree. Up where I live, in Québec, things are very ‘regulated’ , by ‘Insurance Co. law’. I don’t think this technology would be accepted (unfortunately) . We do have what is called ‘slow combustion’ stoves which burn the smoke as well, I used to own one in my previous house, and when it burned, you could hardly see any smoke coming out the chimney. But, that being said, I really like the Rocket-mass-heater. Wish I could see one in operation…
Greetings Claude. While I admit I don’t know anything about regulations in Quebec I don’t know that I’d write off your ability to do this just yet. I know there are people in Canada who have made rocket mass heaters. It might well fit in under existing laws. In Erica and Ernie’s book, “The Rocket Mass Heater Builder’s Guide”, there is a section where they talk about building codes in general. These, of course, vary in specifics from place to place, but the gist is that they are something of a mix between regular wood burning appliances, which already have codes, and what are often known as masonry stoves, which are also generally covered in codes. This is probably closer to a masonry stove as far as code is concerned. From your comment it sounds like emissions are the issue up there, which is certainly understandable. A rocket mass heater should be able to pass any emissions standards. There may even be someone with one near you. I was heartened to find as I was gathering materials to build mine that the various suppliers of odd stuff like firebricks knew about these heaters since other people in my area had been looking for the materials before me! Anyway, it’s worth researching a bit more as these are just so much more efficient!
Very interesting David! Do update us on your attempts to improve the efficiency of your heater!
Hey Kristy, I certainly will post the updates as things go better… or worse. One never knows how it will work out, but this blog isn’t just about what works. It’s also about sharing what hasn’t worked so well. 😉
You did a beautiful job on this! Want to follow your blog so I’m posting here and checking the boxes for getting email updates. Thanks!
Thanks Dianne! Hopefully I’ll actually get my butt back in gear and start writing more blog entries too.
Thanks so much David for sharing your process and results so articulately. An inspirational gift.
You are most welcome.
Certainly one of the nicer, more Wife friendly RMH I’ve seen. Very nice. One thought though. Have you considered some expanded decorative metal to surround/hide the barrel. I know from my experience, with my wife, a 55 gallon drum in the living room is a non-starter.
Thanks CM. One could spend a lot of money and have something else manufactured to replace the 55 gallon drum, but the catch is that to function best in dispersing the radiant heat it provides you’d still want a large black cylinder shape, rather like a 55 gallon drum! Making something to replace it would also cost a ton more. That said if it was being custom made a bit of extra effort could be put into making it way easier to open up for the annual clean out. Also if one removed the characteristic ridges of the drum, or at least did significant altering of where they are located our brains could probably see the shape as something else that doesn’t scream “recycled product of industrial waste”. As a metalsmith it’s not impossible that some day I might tackle the task of designing and making something to replace the drum or perhaps crafting some decorative copper stripes to alter it’s appearance some. The drum doesn’t really bother me personally, but there is no doubt that when I look at the RMH with my critical artistic eye the drum is the big aesthetic fail point begging to be improved!
Hi David, Wonderful post – thank you for sharing. Interesting to see the ‘pebble style’ heater insides.
Thanks Nancy. In hindsight now I wish I had taken more photos along the way as I built this, but glad I took enough to give people a sense of what the innards are like.
Great article! I would like to have seen pictures of the clean out ports to know hiw the are pieced into the main exhaust pipe. To know, I lived in Korea back in the early 1980s. Those folks used undal charcoal stoves to cook on and ran the stove pipe through the rough concrete floors. The exhast pipes would leak through the cracks in the floor and people died in their sleep. It was called undal poisoning.
Thanks for sharing your awesome project.
Thanks Brian. In hindsight I wish I’d taken more photos of the progress of building this. The clean out ports are basically just duct work T’s with end caps on the clean out to be removed when cleaning. One does want to make sure they are put back on securely! I’ve also made sure to have a carbon monoxide detector near the RMH to alert me if there are leaks. Thus far it hasn’t been an issue at all! I did also seal up all the joints on the ducting too though when I made this.
Great job on this. It’s a beautiful design. What thickness plywood did you use for the bottom of the box? What thickness are the boards underneath the plywood bottom to raise it up off the floor, 1 inch?
I’m looking into building a pebble style rocket mass heater from the “free heat” 3d plans but they built straight on concrete without a bottom to the wooden box. I have a wood floor in a stick built house with a crawl space underneath so I’m trying to figure out what to do for the bottom of the box to protect the wood floor.
Greetings Rex,
Thanks. I’m pretty happy with how this turned out too.
I used 3/4 inch plywood on the base. Underneath I used 2/4’s double thick so it would make a 3″ air gap underneath. What I used as my guide to help me do the pebble style was the 4 DVD set Better Wood Heat. https://permies.com/wiki/63855f175/Wood-Heat-DIY-Rocket-Mass
I think it was in the 2nd DVD they do a pebble style in a double-wide mobile home. The pebble style is a bit different in that you do need to get air flow moving through the mass of pebbles. That 3″ space underneath is where I have the lower air intake vents. I also managed to center my mass bench over the steel I-beam of my mobile home, plus I added bracing under that I-beam to help support the weight.
It was a daunting task for me to tackle this build, esp. having never done one before, however, I don’t regret it at all! It’s been wonderful to heat my place with a RMH!
Thanks for the detailed response!
I’ve watched the pebble style builds from the Better Wood Heat series and that’s what initially gave me confidence I’d be able to build a RMH at my place on wood flooring.
The plans I have from the 2021 pebble style build use 1 inch holes around the sides of the pebble mass for air flow. I may add some on the bottom as well like you did since I’ll be creating a bottom for it.
They made a “five minute riser” using “super wool” for the 2021 build but I think I’ll probably build the riser with firebrick.
Roughly how much pea gravel did you use for your build?
It still seems like a daunting task for me and I have plans with measurements which should make some things a lot easier to figure out. But it should definitely be worth it in the winter and it does seem like a wonderful way to heat your house.
Next step for me is to start building the wooden frame and figure out what I can source locally and what I’ll have to order online for the other materials that are needed.
Nice that you’ve seen the builds in the Better Wood Heat series too. I wonder if I could have used more vent holes all around the box myself. I don’t know if that would have improved the performance. What I have is on the low end of RMH performance, but still WAY better than my old soapstone wood stove!
I made my riser from the soft insulated firebricks, though I’ve read now that this is not generally recommended due to their cost and them breaking down fast. I will say what I got was expensive, but I don’t see any breakdown at all in the firebricks of the heat riser after 4 or 5 years. Though I did also get ones rated for higher temperatures. I think mine were rated for 2600 degrees F if I remember right, maybe a bit higher.
As to how much pea gravel I used, I don’t really remember. I believe I calculated at least roughly how many cubic yards of space my box was and bought based on that. I bought the pea gravel from a local landscape supply company which sold it by the yard. Or did they sell it by the ton and I had to guestimate how many yards that was? I just don’t remember. I do know I also got what they called pond stones, rocks that were say 3 to 6 inches in size, which I also added into the mass mix. Basically I would built it up in layers, putting down some pea gravel then by hand fitting in the larger rocks, then filling the space around the rocks with the gravel and so on. In the end I had purchased more gravel and rocks than I needed for the RMH, but I’ve used these elsewhere around my homestead.
What I’ve seen is that if you can find landscape suppliers that sell the hard firebricks you’ll likely get them cheaper than if you get them from foundry suppiers, which is where I got some of the ceramic board. I actually got my hard firebricks from a local brick supplier for the building trade. They didn’t have soft insulated ones though so I got them from the foundry supply. At the foundry supply the hard firebricks were almost twice the cost though! If you are lucky one of the big box home improvement stores might carry firebricks. Right now I’ve visiting in Mesa, AZ and the Home Depot here has them in stock. I wish the one at home in MI carried them! It would make things so much easier.
That’s good to know that your riser out of insulated firebricks has held up well for that long. I find the riser material to be one of the more difficult choices to make for the RMH. Seems like many were initially excited about “super wool” for a riser which isn’t hazardous to the lungs like the ceramic fiber wool but then it was pointed out that once it gets above 1470 F some of the fibers will crystalize and become airborne inside the riser as christobalite. But as long as one uses a respirator when dismantling, it shouldn’t be an issue.
I should need about a cubic yard of pea gravel based on calculations. I already have some pond stones at my place but will probably get more to make sure I have enough.
The closest Home Depot to me is able to ship to store half-thickness firebricks for about 4-5$ per brick but doesn’t have any in stock. The plans call for orange firebrick for the insulating base below the core and white firebrick for the wood feed and burn tunnel with sand surrounding everything to insulate things further. If I understand correctly, the orange firebrick is considered regular firebrick which is lighter and the white firebrick is the insulated firebricks. I will have to see if any landscape suppliers in my area have firebrick. The ceramic boards you used for the insulating base seem like a nice lighter option, but I’m sure a bit more expensive.
Interesting to hear that about the super wool. I hadn’t heard of that issue, though I haven’t really been following all the latest developments since I finished mine. Though I did buy a box of super wool which I have stashed away for future use. I’m imagining that at some point I will need to rebuild the J-Tube, or at least part of it since that first top firebrick in the feed tube is known to crack over time from all the heat/cool stress it gets. I used pearlite for the insulation around my J-tube, but might go with the super wool should I need to rebuild it, or if I build another one. I don’t know if that will be better or not.
That price for half bricks from your Home Depot seems high to me. I believe I’ve seen those before sold in multi packs for relining wood stoves or something. Seems like it’s marked up to a premium. If you can find some place that has them by the pallet that’s where you tend to get a better price. It’s been a while I but I think I was paying about $2 a brick, maybe less, for full size hard firebricks. Though as I recall the half bricks were about the same cost as full ones.
Regarding firebrick color I don’t think that has much bearing on whether they are hard or soft. My guess is that the color is more a factor of whatever the source clay material was that was used to make the bricks. That said, when I saw the hard firebricks in stock at the Arizona Home Depot they were orange. I think I’ve only seen the soft insulating kind in white.
I’d make sure you have good insulation underneath the burn chamber so you don’t risk burning the wood. That burn chamber will get really hot for a long time! The ceramic boards were fairly pricey but I don’t regret it.
It would be nice to make a RMH straight on a solid non burning surface rather than a wood floor, but sometimes you just have to work with what you got!
Inspirational, thanks so much.
Keen to try one of these heaters in my workshop studio. I would never be allowed to install in the main house. I would never be able to get that 55g drum past my wife’s approval (the asthmatics department).
Sorry asthetics
Thanks Bruce.
A workshop would be a great place to initially try one out. I would have done that myself except my workshop doesn’t have an easy way to take a chimney out because I built it with a living roof.
The asthetics of the 55 gallon barrel does bother many, and I will admit my artistic eye says that is the visual part of mine that feels inconsistant with the look of the rest. I have no problem with it personally given how beautifully it heats my home, however I have contemplated ways I could add decorative copper elements to visually tie it in with the rest. I probably should do that at some point if for no other reason than to demonstrate the ability to transform the look.