A while back my friend Coenraad of House Alive! natural builders got me in touch with Max Edleson of Firespeaking.com because I was playing with the idea to build some sort of cob masonry heater. After talking with Max and sharing what I was looking to do, he directed me to a set of plans for his “Cabin Stove”, a mini masonry heater, on his website. I shared this with my friend and collaborator Weston last year and he set out to build one. It performed so amazingly well that we decided to follow his lead and make one in our place this past summer. Also, another neighbor made a third stove just recently so, as Max put it in a recent email, we are on the cutting edge of mini-masonry heater development. That sounds nice, doesn’t it?
With this article I hope to share our combined experiences including performance, mistakes, and ideas for improvements. Firespeaking.com has great pics along with step-by-step directions so it’s best to check out their site to get the full idea if you’re interested in building one of these little marvels. I should also mention, as people read and learn from this article, that we live without electricity and fossil fuels so our needs and wants differ some from regular folks.
Masonry heaters in general weave exhaust channels through masses of stone or brick to maximize the absorption of heat before it goes out of the chimney and into the open air. They are touted for much greater efficiency, cleaner burns, and lovely radiant heat. They are also often huge (weighing several tons), expensive, and require a professional to install. The mini-masonry heater keeps all the good and eliminates the bad.
These heaters are made of firebrick and have a central burn chamber. Once lit, the smoke and heat travel in a short loop around a channel of brick and exit to regular 6-inch stovepipe. Our top is covered with a large steel cooktop. For a door, Weston fabricated one out of steel for his and ours.
Weston bought standard firebricks (full size – about 4.5 inches x 4.5 inches x 9 inches and decided not to use refractory (high heat) mortar based on the recommendation of an old mason where he bought the bricks. While he saved on the cost of the mortar because the refractory stuff sells for about $125 bag, his mortar was a lot harder to use and several of his joints have popped leaving small gaps between bricks. After waiting a month for the mortar to cure (you want it to fully dry out so there’s no water left to expand and pop seams) he started it up. It was and is amazing! With very little wood it heats up fast and radiates a crazy amount of heat. Most days he will start it with some kindling and add one split log and that’s it for hours. Now, his space is small – maybe 350 square feet – but he spends most of his winter barefoot and in shorts inside his house often with windows open. With such amazing results we planned ours for an August build so it would have plenty of time to cure and be ready for the cold weather.
As luck would have it (bad luck, perhaps…read on) We found a couple hundred used fire bricks from a friend’s dad who is a former potter and kiln builder. These bricks are the same size as the new store bought ones but are much more dense. He described them as having better thermal mass qualities – more capacity to store heat and release it slowly over time. He described the new bricks as better insulators with more air gaps to prevent heat transfer. This all made great sense to me for what we were looking to achieve so we happily took the bricks. We also bought the refractory mortar (called “Heat Stop 50”) after Weston’s experience and Max’s advice. It was really easy to work.
I began the whole process by reinforcing our subfloor by pouring two two-foot square and 6 to 8-inch deep concrete pads reinforced with rebar under my house. Into these pads I set a concrete block and let that set for several days. I added a couple more blocks atop them to approach the floor joists. I set a 6x6 (I think, if I remember correctly) from the two piers to span a few joists. I pounded a couple wedges above them to tighten them up and left it all for a month or so while I gathered other supplies.
Next, I went topside and built a base of firebricks on top of my existing regular brick hearth base. This served two purposes: to build up the finished height a bit to our desired level and to have a safe and durable barrier between the regular bricks and the great heat that comes with firing. Then, I started building, one course at a time. Max’s plans are easy to follow and progress was “quick”: all told I spent 16 hours laying the 7 levels of bricks. My salvaged bricks were a little wonky and I definitely should have spent more time using my neighbor’s grinder to smooth them out and take off old mortar. However, I assumed I would be covering it all with a layer of earth plaster which would hide the bricks, their irregularities, and any popped seams (more on that experience later).
According to Max, the recommended method of building the base, “…would either be to have actually cut out the floor, reframe any structural members, and build a masonry foundation up from the ground; or to do as you have done, re-enforce the wood floor and then put a layer of a proven insulation below the first course of bricks. The main materials available on the market are something called Foam Glass and/or Calcium Silicate. More homemade solutions could be clay-perlite, cement perlite, or pre-fired clay-sawdust mixes although there is little or no specific information on performance of these materials and recommended mixes or thicknesses. One additional idea is to build the first couple of courses so that you have bricks on edge (called shiners) and then ones bridging these so that you effectively have air flow underneath the heater. Whenever you generate convection, the potential for heat build-up is vastly decreased.” I’ll be going under our house with a heat probe to measure the actual temperatures generated there to ensure ours is safe.
From here, I waited. Life happens and Weston couldn’t get around to making the top or the doors until October. During this wait I decided to put a skin of earth plaster atop the bricks. I assumed the skin would just be a little more thermal mass and look great when covered with a finish plaster. When we got the cooktop (made from 3/8-inch steel cut from a very large I-beam) I made a janky door and we started it up. We noticed several discouraging things right away. First, it didn’t heat like Weston’s at all. Second, the top arced greatly with the heat, so much so that we could see the flames licking the steel through one-inch gaps on the edges. That didn’t seem good! The cooktop is supposed to rest on one-inch fire rope. Weston used the same metal for his and found that his arced a great deal in the middle leaving large gaps in the front and back when it is really burning hot. He decided to flip our top over thinking that the arcing was a result of the great heat in the center of the cooktop right above the burn chamber and that the natural curve of this particular steel would work against itself thus making a flattened top when in use. Turns out this was way off and that the metal arced on its edges, with its natural curve, kind of like wings lifting off the bricks. What can I say? We’re the people without the answers and experimentation is what we’re up to around here anyway. Next time we’ll do better and in fact, our neighbor is already taking advantage of that nugget of knowledge with his stove. We’ve since mitigated this by adding more fire rope on the affected areas.
More troubling was the difference in performance. We figured there were three variables. First, and most ominous, was the type of bricks. Second was the cob skin. Third was our janky door without proper breathers and seal. Before we even got the door I removed some of the plaster and was startled by the incredible difference in heat transfer from the “naked” and exposed bricks to the plastered bricks. On the plastered spots I could keep my hand there all day. On the naked spots it was far too hot to leave my hand for more than a second or two. OK, the plaster came off! That was a good thing as we gained a lot more heat. I was still really nervous about the bricks and thinking I’d be rebuilding our stove next summer. Ugh!
Once we got the beautiful door we tried again. With the ability to regulate the burn we were able to get a lot more heat with much less wood. While it still didn’t behave like Weston’s it was performing well. Different, but good. We’ve learned that it works best when we start a really hot initial fire and then just add a log here and there as needed into the hot embers. In our climate with our easy winters full of sun we generally only need a fire in the morning and the late afternoon (which we also use to cook our food and heat bath/dish water) and never stack it up for the night as we find it’s just never needed. As Max has pointed out to me, masonry heaters are meant to have vigorous combustion to maintain a clean burn with very low pollution. The bricks we used seem to require a lot more heat to get through them and out into our home. But, once it’s moving, the heat keeps coming for a long time. Overall we are very satisfied with the performance. However, we’ll see how Eric’s (our neighbor) performs in his home, which is more similar in size to ours, later this season and reserve judgment until then.
We love the cook top! It’s huge and can hold several large pots at once. We just butchered our three pigs and spent four days making pots and pots of bone broth and lard atop the ample surface. It also has a range of temperatures from really hot in a foot square center area all the way to just warming on the edges. Wonderful functionality!
We’ll be adding an oven to the bottom of the burn chamber as soon as we can. This will be a simple box of steel with an open front into which we can slide a casserole dish. The over feature serves a few functions as well. For starters, it makes the stove legal in our area. There is a little caveat that if a stove is in one’s kitchen and has an oven it need not meet EPA standards and so on. While we believe our stoves are super-efficient and burn real clean there are no standards on which to measure them and fit them into the box of regulations. Second, the oven will greatly enhance our cooking options and make the cold months even better. Last, the oven will bring the fire closer to the top making small fires more effective for cooking as the flames more easily touch the steel and heat it quickly. I actually made the burn chamber floor with full size bricks although Max’s plans call for using half-width bricks. I figured it would be stronger and last longer (after all, the bottom gets the brunt of the bangs and bumps from logs and such) and bring the fire closer even without the oven addition.
Max’s design also incorporates a water heating loop to hook up to a thermosiphon, a feature we decided against in our model. Last year, in our little old woodstove, we rigged up a thermosiphon for hot water and while it worked beautifully, we found we didn’t use the 10 gallons it heated very often and that it was easier to simply heat a large pot of water on the cooktop for showers and dishwater. That is even easier to do with our cabin stove because the cooktop is huge and hot.
We also left out the bi-pass starting valve that Max includes because Weston never used his as his draft was great enough to pull the smoke out through the whole J-loop right away from a fresh fire. Ours performs the same.
We took off Weston’s top and cleaned his stove out before this cold season and were happy to see no creosote build up. There was only a very fine coating of ash all around the channel and into his stovepipe. This he vacuumed out with a shop vac in a jiffy.
Both Weston and I also changed the top a bit by making the cooktop go all the wall across the entire top of the bricks and not making a little brick chimney like Max's models (see his pic above). Max mentioned that this was a weak point in the bricks so we just avoided that issue.
Although I’ve never met Max in person, he is a super guy who has been a great help and inspiration as we’ve gone through this process. It’s been fun to share our experience and pics with him as we go and we look forward to assessing Eric’s heater soon. If you decide to do make your own heater, let Max and I know how it goes. Also, buy his great book, Build Your Own Barrel Oven. It's a treat!
Purchase Build Your Own Barrel Oven and learn more about DIY masonry heating at Firespeaking.com.
(Top) Illustration by Max Edleson
(Bottom) Photo by Kyle Chandler-Isacksen
Kyle and his wife, Katy, are directors and founders of the Be the Change Project, an urban homestead and education center dedicated to service and simplicity and rooted in Integral Nonviolence in Reno, NV. Email him to take his great natural building workshop in June of 2015.