Our Philosophy

Festina lente
-make haste...slowly

Saturday, December 14, 2013

Say Hello To My Little Dragon - Rocket Mass Heater in a Wall Tent

This is the combustion unit in all its temporary glory - standard fire brick J-tube built on a thick 16"X 24" landscape paver base (3 of them laid side-by-side to make a 4' x 2' platform) raised up to create an air space on old clay bricks salvaged from our house in Macon, wide side vertical (~4"), one under each paver joint, both sides, all set on a rigid reflective insulation panel in contact with the wood floor (the thin white strip at the bottom).  For those unfamiliar with rocket mass heaters (RMH) the flue is INSIDE the steel barrel; in this case they call it a heat riser, and ours is an 8" x 8" square clay flue pipe surrounded by a sheet metal jacket with perlite infill between the two for insulation, capped with a little mud to hold the perlite in.  The internal heat riser creates a heat pump that actively moves combustion gases through the unit.  As the gases fall down the perimeter of the barrel, they give up a substantial portion of their heat via radiation to the room before collecting in the bottom of the barrel and exiting through the horizontal flue, actively pumped to the exterior.  The galvalum can closer to you in the photo is the jacket for the feed tube, similarly insulated with perlite, with a tight-fitting lid to kill the fire when we're done with a burn.  The whole shootin' match is enshrouded in concrete we poured into a form (we were in a bit of a hurry or we would have cobbed it - more on that in another post).  The combustion unit weighs approximately half a ton, is supported by extra reinforcement under the tent deck, and cost roughly $200.  She's not going to win any beauty contests, but when you feel the heat it produces on next to no wood, an undeniable fondness for her begins to creep in.

Wet, wet, wettest year I've ever lived through, and abandoned for most of 4 months, there was (and still is) plenty of rehab to do on the old tent and deck floor.  On the left side you can see the mildew and floor wear we're dealing with; to the right where we're headed.  We're painting over the mildewed canvas with an oil-based primer to lock out the fungal unpleasantness (any better ideas?), and you can see where I've sanded and oiled the pine floor.
Not without its own shortcomings, one of the major advantages of a RMH in a tent is that a vertical flue jack through the tent roof isn't necessary, since the flue can exit the tent horizontally.  This nasty mess was caused by leaves and moisture falling in around the flue pipe and getting stuck between the canvas and poly rain fly.  Like any good composting process, it's completely digested the canvas at the pressure point against the internal framework.  Without a vertical flue stack this weakness is never introduced and the integrity of the rain fly remains uncompromised.   This mess has since been stitched, painted, and fully covered from above.

Turn around, view to the south during renovation.  The floor is done now and the painting getting there.  We will also be adding more of the 4' x 8' rigid reflective insulating panels, like we used under the combustion unit, around the walls to help hold heat in, and probably painting over them to match the wall.  Since this photo a screen door has been stripped down, fitted with 4 mil clear plastic sheeting for "panes," and hung in place of the old zippered flaps on a 2x6 frame anchored to the deck.  Makes coming and going so much easier; I wish we'd done that at the beginning.  The RMH combustion unit is built roughly on the old wear shadow spot (lower right of the photo) where the box stove lived last winter. 

This is where the real magic of a RMH comes into play.  With an active heat pumping action and a horizontal flue, we can run a long length of flue pipe through a thermal mass to absorb the remaining heat and store it to dissipate into the room over time.  Unwilling to cart 2 tons of our sandy, clayey, gravely subsoil in by bucket to fill this bench until the trouble-shooting is complete, this is about all you get for now.  There is about a quarter ton of thermal mass under the children's bed to help hold a little extra heat and the flue pipe in place now, and we will be doing our side under the bed soon since it will remain in that form once we build the wood cabin on this platform in the spring.  We're filling the bench only up to near the top of the flue pipe with the subsoil and then topping it off with pea gravel for better heat movement up through the mass and into the bed, and a cleaner finish.  The near section will change forms in the next phase, so we may not be filling it at all.  This bench is 16' long, and the back 10' will be our bedroom in the cabin; the front 6', plus the bend and the combustion unit will, once approved by the county, be cobbed into a permanent couch the shape of a dragon, complete with scales made from old favorite broken pottery we've been carting around with us, emerging from the fairy tale section of the floor-to-ceiling bookcase that will become the wall between the living and sleeping areas.  Safety-wise, the floor of this mass bench is raised up on a 2x6 riser to create an air space underneath, though closed off to keep any wandering mice out.  Once you get around the 90 degree elbow, the flue pipe isn't very hot, so I'm not worried about the mass bench causing any problems with the wood floor, especially with the air space separating the two.  More reinforcement is being added underneath the mass bench as it gains weight.  More details in the future.

Did I mention the trouble-shooting?  RMHs are not for laissez faire fire tenders.  They require attention, and at least a rudimentary knowledge of fire physics and fluid dynamics.  And it helps if you're a bit of a pyro, too.  The 90-degree elbow in this shot has been replaced by an 8-8-8 T-section and cap to provide cleaning access to both the short stretch from the combustion unit and to run my 20' flue brush through the long length of the mass bench and out the far end.  Apparently these dragons like to have their intestines cleaned regularly.  At the far end, the outdoor bit, I replaced the original 90-degree elbow there with a T-section and cap as well.  Besides flue brush access, when the weather is wonky I can go out and light a primer fire in the T-section under the outdoor vertical stack, or slip a battery-powered fan in, to help encourage the draw to move in the right direction until the combustion unit warms up.  Smoke-back into the tent, bad; smoke out the far end vertical flue, good.  Once the system is hot it rockets beautifully and cleanly, with only an occasional tiny belch that usually gets sucked back down the feed tube.  The exit gases move so slowly and are at such a cool temperature when they exit that you can lick the flue pipe (if you really want to) - it's hardly ever above 100 degrees F, even when the rocket is roaring.

So this is the idea for this particular design.  Beds ride on top of thermal mass bench to help keep sweeties warm at night.  Even without the thermal mass in the bench the extra warmth is noticeable.  I built simple plywood platforms, left over from our bath house project, to hold the beds up level with the bench, and, since there's no box spring, give us a bunch of storage space underneath.

Now the kids' bed is added, although I think that's a big pink bunny benefitting from the design, and not Ella.  Theirs is the warmest spot, near the combustion unit, which doles out a considerable amount of heat through the night, too.

You don't burn this kind of stove through the night, though.  You burn it hot and fast, for a few hours before bed, and in the case of a tent, when you wake up in the morning.  Then you cap the feed tube, kill the fire, and bask in the stove's warmth between burns.  This is one of the shortcomings of a RMH in a tent: there's not much insulation in the shell of the structure to hold onto the heat you produce.  We actually use a propane space heater during the night so we can get some good sleep, not be up all night tending a wood stove, but we certainly benefit from the slow-release warmth of a system like this under and around our beds.  And we really built it for the future cabin, when it will function more like it's supposed to.  Most RMH users claim to only burn once a day, for 2-3 hours before bed, and get plenty of heat through the night and the next day from that one burn, sometimes burning only every other day! The savings in firewood (money or processing time) is immense.  Even in a tent, with almost no insulation, we never burn more than 2 cubic feet of firewood a day, and generally less than one.  Last winter we burned ~600 cubic feet in our box stove (our stove season is about 5 months).  Yes, there is a little propane cost to be considered, but only so long as our house is a tent.  The fuel you see in the photo above is kiln-dry hardwood off-cuts I get for free by the trunk-load from a local cabinet shop.
Hard to see through the lunch break, but I've borrowed a trick from Ernie and Erica Wisner's playbook for our stove.  When buying the steel drum for our heat exchanger I got one with a removable lid and lid clamp, so I could open it from the top for inspection and cleaning.  A little stove insulating cord in the lip of the lid keeps it air tight.  Here you see Ella demonstrating the proper use of the stove once it's been shut down after a burn.  The little black and silver fan at the back of the stove top, driven only by a heat differential between its solid base resting on the stove and cooler aerial radiator, is a must-have for tent dwelling.  They are a a bit pricey, but so worth it.  We got ours from Lehman's.

I can't really explain the sense of self-satisfaction I got, and continue to get, from building an inexpensive wood stove from scratch that actually works and keeps my family warm.  I didn't come to the project with any particular skill in this department, and no more experience than one (long) winter with ANY sort of wood stove.  Rocket mass heaters burn very little wood - generally less than a quarter of what a typical box stove burns, and some claim as little as a tenth.  The fuel feeds into the unit vertically, (the fire rocketing sideways through the burn chamber and up the heat riser) and has to be processed down to fairly slim pieces (mine doesn't want to burn anything larger than 3" across, and even that seems to be a chore), but overall the labor input seems very minor compared to the endless splitting of heavy, chunky wood of last winter.  And the fuel requirements lend themselves very well to a coppicing woodlot scheme, which produces thin round fuel every few years on rotation, and which I'm currently developing on our land.  The stove does look a bit like a burn barrel in your living room, but can be dressed up in any number of ways.  Like I mentioned above, we plan to do a cob dragon sculpture around ours, which should hide a lot of the industrial look, but you could stack brick or stone around the parts you don't like for sure.  Just keep in mind that you're adding thermal mass, for long term heat storage and release, and covering the quick radiant warmth you get from a lot of exposed steel.  It wouldn't work very well in a tent in other words.  But coupled with a well-insulated house, or a thermally-massive house, like adobe or cob, I can see a stove like this being a major boon to the owner-builder's comfort and work load.  Weird and quirky, yes, but inexpensive and efficient, too, and utterly fantastic.

Special thanks to Andrew for his help with the materials gathering, layout, and pointers.
Tripp out.


  1. I'll post some more detailed shots of the RMH build in a subsequent post, especially as we get into the permitting process with the county building department. This post was more about installing one in a tent and how to hopefully make that work. Hope you enjoyed it!

  2. Brilliant.
    Wondered how you were getting on in the tent.
    Looks like you are back home.
    best Phil
    (Scottish Border UK)

    1. Back home and loving it...thanks for stopping by.

  3. Tripp
    Keep that stove burning bright!
    Let us know how you are doing.
    Does the 'arctic break-out' reach that far?

    1. Last night was the coldest temp I've seen since we left eastern Washington, and probably the coldest I've seen in the north Georgia mountains since I started coming up here! We abandoned camp for a few days and took refuge in the family cabin a little north of our place. Even right on the trout stream under the back deck it was +3F this morning. I imagine it was 0 or worse up the hill. Good news is, we should be right back to our normal dry and sunny 40s/20s and rainy 50s/30s rotation by tomorrow. If you'll send me your address I'll send your Scottish border weather back to you. No charge;)

      Thanks for checking on us!

  4. Hi Tripp
    Hope that all worked out.
    We continue mild enough after a lot of strong wind, rain (flooding in parts of UK with tidal surges).
    We are on the other side of that wavy jet stream - these positions seem to get stuck for longer these days. A guy up near your Great Lakes tells me that they have been in lock-down since the middle of December and all of them including the cat suffering from 'cabin fever'.
    best to you all

  5. That’s a very great project! RMHs are known to be one of the cleanest and most efficient ways to heat a home. Of course, there are disadvantages, like having to tend the fire regularly and having to put clean-outs along the exhaust line for access cleaning. Still, when all is said and done, I think RMHs are better and more environment-friendly.

    Monica Ryan

  6. These are very useful pieces of information that will be of great use to me in a good position future Pretty. I came across your blog and wanted to say I really enjoyed reading your blog. Either way I will subscribe to their feed, and I hope to publish soon. Thanks for all the hard work. Appreciate the brushes, which are well done. I really appreciate your professional approach.


  7. Would you be interested in helping me build one out of cob? This is something I am interested in doing but have never done before.

  8. This is an interesting article. I have never seen a mass heater like this before. It is amazing what new technology they are coming out with these days. for Drum heater. Really your informative blog helpful for Drum heater

  9. Did you actually get a permit for this? How did that process go? I live in Georgia too but probably in a less rural area than you (Winder).