Bamm-Bamm Style RMH

I am about to start construction of my first RMH. It will be in my hybrid yurt/tepee (Yupee). The goal is to have a survivable, if not comfortable/luxourious, place to get through the 4b winter around here. Part of the floor in the yupee has been laid with shipping pallets that will have an additional layer of deconstructed pallet boards on top of them. About a quarter of the floor has been left bare for the core at the centre and a bed/bench mass forming an 'L' outward from there.

I have a "shippable" core built with Uncle Mud at last year's PTJ as well as a stack of regular fire bricks I have used to run a J-tube rocket stove the last couple of seasons. I'm considering use of either, but the loose bricks are full size so I'm thinking the shippable core with the splits will make for a more compact construction to save space in the yupee. The shipable core includes a 10" metal duct with Superwool insulation for the riser, and I have three 55 gallon drums to make the bell/manifold.

For the mass, I have a supply of concrete pavers, plain red clay bricks with three holes in each, and a whole lotta double sized red clay chimney bricks. I've been collecting 8" ducting from the Restore and will purchase whatever odd pieces are needed to complete the ducting run.

My plan is to construct the outer wall of the bench/bed area out of the three-hole bricks and use rebar anchored into the ground and bent twice to 'staple' the bricks in place. I would then stack the double red bricks just inside to fill the gaps left between the holed bricks, then fill the remainder of the cavity around the ducts with tamped earth. The table I found in the RMH Builder's Guide placed regular earth as a better conductor than pebbles and slightly worse than cob. I figure that I will be moving and/or reconfiguring this after a few years and the bricks/earth will be more convenient for this than cob.

My battery is running low, back with pictures and questions later...

My first quandry is the foundation. The 'Earth' exposed on the floor is a mix of the soil and underlying sand I dug from the higher side and moved to the side where the RMH is going a year ago. I'm thinking I should dig up the purest sand I can find and spread out an inch or two over the area to provide a bit of insulation and make the brick laying easier. Using just what I have on hand or natural materials I can come by locally, is there more/something else I could be doing before building this up?

I will be at this location most of the time to run the heater a few times a day, or even all day in the worst of weather, with a plentiful supply of well dried pine to fill the feed tube with.

After some prepping (the post marks the center of the yupee footprint)...

I've decided to lay out around 2" of sand under the whole area to provide some insulation to the ground and provide the easiest surface to build upon.

First I will assemble the firebox. I'm thinking that placing the core directly on the 2" of sand and leveling it there would be okay for insulation and support.

The shippable core will be surrounded by 4" of sand on each side, contained by 3.5" hollow clay bricks. There will be no mortor, the bricks will be held in place by rebar pounded into the ground. A few pieces of rebar will be longer and bent to an angled or opposite side's hole (working around barrel and feed box), 'stapling' the wall into vertical stability.

The mass will be contained by a similar wall, except with earth around the ducts instead of sand and the rebar in a pattern that will have small gaps between the bricks, both to make use of bricks I have with concrete stuck in the middle and to allow heat to radiate out easier. The rebar staples will be used  here also, although there is a need to be more creative because of the irregular bench shape.

I would love to see some drawings with dimensions and what is under the sand, maybe dome photos of the core to remind me how we did it.

Not sure what going up in the dome for pics would do, but here's some ground shots:

The riser is 36" tall. For future iterations of risers with shippable cores, I'd suggest leaving the 8" duct in the riser until it is at it's final destination, it doesn't ship well once that's removed. BTW, I don't remember how exactly we did that and will have to do it again to reconstruct the insulation layer, is that procedure outlined anywhere?

Coydon Wallham wrote:The riser is 36" tall. For future iterations of risers with shippable cores, I'd suggest leaving the 8" duct in the riser until it is at it's final destination, it doesn't ship well once that's removed. BTW, I don't remember how exactly we did that and will have to do it again to reconstruct the insulation layer, is that procedure outlined anywhere?

Plowing through my permies.com swag, I found this movie called "Free Heat" with some guy called Uncle Mud that talks all about how to make risers from superwool using two ducts...

Below are the schematics I've scribbled out while going along. The top down drawing is 3" per square, the side ones are 2". I've raised the courses of the brick walls by one level since this to provide more sand insulation between the undersoil and the firebox/mass and more air channels between them and the floor pallets.

The pictures show the advancing brickworks and the start of the floor surface that will cover the pallets. The flooring around the barrel/manifold will be moved out and rockwool placed under the brick retaining wall there.

Between the metal sheets and the fit of the firebrick in the shippable core, I have been thinking I would simply fill in around the firebox/manifold with sand and apply foil tape in a few places, that there would be no need for clay slip or mortar. It now occurs to me that the foil won't stick too well to the brick and areas around the barrel might get too hot for the tape. Particularly, where the manifold fits over the burn tunnel, advice is to cut the manifold a bit large and seal the gap after.

I did a bit of soil sampling around here earlier in the year, but it is clear I don't have time to develop the earthwork skills to work out a suitable material to trust with on this project if I'm going to finish it before the permafrost sets in. I have plenty of sifted sand to make a bucket or two of something to work with here. What kind of clay would be ideal to purchase for this application (to just seal gaps around the manifold and feed tube)?

I'm seeing various different recipes for cob/fireclay mortar/refractory mortar. Will all of these work here without precise tweaking? I think I'm just looking for something to make a sand/gas barrier that will last for a six month season and ideally be easy to remove after that.

If one were to stack a wall 8 bricks high on top of enough old school rock wool to support it 3/4" high, would that provide more or less insulation to the underlying surface than the same thickness of glass?  Asking for a friend...

I finally have a roof jack built for the exhaust that fits in the center ring (tono). For it I have a 28" long, 8"ID 10"OD insulated chimney pipe I recovered from the dump that I would like to put in it.

First concern, it is a Metalbestos. I'm getting odd search results with articles about products still allowed to contain asbestos, but nothing that contains a clear statement about this product. I recall finding such a statement when I first brought this home, but it looks like some good old media housecleaning has gone on since then to protect the industry image? I'm assuming it has asbestos in it, but from my work with remediation contractors recall such things are regarded as functionally safe as long as the material stays encapsulated. It looks in almost new condition to me, although one end has a bolted on cap and I can't really see much around there. Are such items known to fail without acute damage done to them?

Second, how warm is the outside of such double wall chimney likely to get? I am making my duct run on the short end to ensure good draw through the RMH, so expecting  higher than average exhaust temps. 300? 400? How much should I expect to see conduct through the insulation?

I have 3 layers of skirting to shed as much moisture away from the chimney before it enters the actual room below, but still need a solid seal at that point. What is the best material to have between the pipe and the wooden jack around it? Is there a high temperature silicon that could be applied there? Looks like regular stuff is good to 140*F. Are there ingredients to watch for/avoid that make a sealant better for this application while being as Wheaton Labs standards friendly as possible?

A reseller of Metalbestos has this on their site:

Class A pipe is often referred to as double wall, triple wall, or insulated chimney stove pipe. Selkirk Metalbestos pipe does not contain asbestos, Metalbestos is simply the model of pipe.

This makes me wonder is that just new products, and if they have been asbestos free all along why would you name your product after one of the most triggering examples of hazardous materials in contemporary culture...? (They have recently renamed the model "Metal Best Chimney".)

I am sure that Metalbestos has not used asbestos in a few decades at least, and anything you get that is not very old is going to be safe.

For a six month duration, you definitely do not want to use refractory cement. It is high quality and very permanent - cleaning it off of firebrick or red brick would be very difficult. It can be stronger than firebrick. Fireclay mortar would be the thing to use for this situation (and recommended by many for all uses.)  Fireclay mortar is strictly a gap filler and leveler, with about zero gluing ability. Do not use it for holding a structure together. Surround any such structure with something else like cob to ensure that it does not shift.

Glenn Herbert wrote:For a six month duration, you definitely do not want to use refractory cement. It is high quality and very permanent - cleaning it off of firebrick or red brick would be very difficult. It can be stronger than firebrick. Fireclay mortar would be the thing to use for this situation (and recommended by many for all uses.)  Fireclay mortar is strictly a gap filler and leveler, with about zero gluing ability. Do not use it for holding a structure together. Surround any such structure with something else like cob to ensure that it does not shift.

I talked with Thomas Robino and have some Lincoln Fireclay from Dragontech. He advised actually using Superwool to seal the gap under the clay while allowing for expansion. I have some rockwool from Freecycle to use there. I know it is more hazardous during installation so will wear a mask. I was thinking the top of the burn tunnel would get over 1400F and make superwool dangerous for deconstruction anyway, but on second thought probably not outside the bricks/at the base of the manifold? This seems like a common scenario, have the temperatures been measured in that area and recorded here?

Outside of a firebrick core the temperatures would never get above 1000F at the most, probably much less, unless there was a very thick layer of excellent insulation over it.

More focus on progress than updates lately as temps struggle to average above freezing. Progress was very slow working out the roof jack/skylight. It turned out looking a bit steampunk.  However, it incorporates a window I (un?)fortunately found at the Restore and has a plastic frame. I made the window mount to hinge out so any accumulating snow would dump down the side, The pic from the inside looks dark because of the light contrast, but it is decently bright inside when the sun is out. I've been working late into the night when the temps are holding out, so finished the topper in the dark one night.

I present Uff-dere, the Yupee topper:

The jack has the chimney cap, a sheet metal collar, the metal roof tabbed up to fit it, a half-assed chimney cricket over the join in the collar, and finally a flat wood surface under the metal with an opening slightly larger than the pipe. I bought a tube of plain silicone caulking with intent to climb up there and seal the gap between the pipe and the wood. Will the caulk by itself seal the 1/4" or less gap, or should I fill it with a substrate first?

Coydon Wallham wrote: Will the caulk by itself seal the 1/4" or less gap, or should I fill it with a substrate first?

My experience with anything over 1/8" as a gap tends to pull away or split the caulking, especially on a roof with often larger expansion/contractions.
Foam backer rod has worked well for me.

My thought was wood shims under the caulk, but in considering petroleum by-products, why wouldn't I just put some tape around it? I've had gaff tape on my acrylic motorcycle windshield to block headlight glare and it has held up to massive wind force and the elements for multiple seasons. Does standard duct tape hold well enough to stainless and smooth planed wood to last for a winter?

Fire Good!

Hey Coydon;
That stove will start working better after you get those pipes covered up.
Currently, you are radiating heat to the room too quickly, Very easy for your RMH to want to backdraft.
It looks good though, Congratulations!

EDIT)  I particularly like the empty Dragon Tech box in the one photo!

thomas rubino wrote:Hey Coydon;
That stove will start working better after you get those pipes covered up.
Currently, you are radiating heat to the room too quickly, Very easy for your RMH to want to backdraft.
It looks good though, Congratulations!

EDIT)  I particularly like the empty Dragon Tech box in the one photo!

What, that 'ol kindling box? I follow Abraham Washington's famed advice for posterity: Reduce, Reuse, Recycle.

I thought there might be a problem because I hadn't encapsulated much of the burn box in the insulating sand. With one piece of newspaper to prime the exhaust, a half dozen more in the firebox, and a handful of lumber scraps, she fired right up as well as any of the RMHs I've used at Wheaton labs. In fact, I think I had full pieces of cordwood roaring quicker than I had been able to even in the Fisher Price house. No smoke back yet. I bricked off the feed for a bit when it seemed to be getting too hot- I'm happy with keeping things in the 50s for now. It seemed to burn flawlessly for 6 or so hours as I worked on the mass until I went to bed. Oddly, I just went to relight it and the newspaper I had stuck in last night to prime had gone out right away and was still sitting there unburnt.

I have the barrel attached to the manifold with a lid clamp. It seems airtight on it's own, but it happens to be a few inches below the top of the feed tube (this is where a few extra inches on the riser would have made a difference, not something readily foreseeable). What is the strategy for insulating the bottom of the radiant chamber? Is insulation around the manifold needed to get the burn chamber temp up and add to the thermo siphon effect? I'm not sure if it would be better to fill everything up to the level of the feed tube with sand, or to keep the sand level around the barrel at the level of the clamp to radiate more immediate heat and leave the clamp more easily accessible?

I should finish the mass tonight. The ducts only measure 20' from manifold to exhaust. Since the CSA is 8" I'm guessing this is why it is drawing so well. There are in fact 11 elbows in the run, but 6 of them are less than 45*. The exhaust stack is reading ~80* at the bottom to 60* further up, but it feels hotter. I can touch it, but not for more than a few seconds. Does shiny ductwork not read properly on pointer thermometer guns? I noticed the few sections of duct that I have with a more mottled appearance read a few dozen degrees higher than the shiny ones.

I'll add here that I recognize how fortunate I am to have this going so well from the start. I doubt it would have gone this smooth without having the opportunity to work  in person with knowledgable folks (and real working RMHs) at the RMH Jamborie, a couple of PTJs, and a few months at Wheaton Labs' boot camp. Also help from people in threads like this, especially Mr. Rubino when working out a few key specifics. Oh yeah, that Mud guy is helpful too...

Coydon Wallham wrote:Does shiny ductwork not read properly on pointer thermometer guns? I noticed the few sections of duct that I have with a more mottled appearance read a few dozen degrees higher than the shiny ones.

Yep. Reflections work the same way at infrared as they do in visible. I often hit shiny galv or stainless flue pipe with a spot of flat black paint so that my temp gun gives me more accurate readings. Or you could wait for it to get dirty and oxidised. White or very light-coloured surfaces will read cooler than dark ones, as a general rule, and the same goes for shiny versus not shiny.

thomas rubino wrote:Hey Coydon;
That stove will start working better after you get those pipes covered up.
Currently, you are radiating heat to the room too quickly, Very easy for your RMH to want to backdraft.

As I've built up more mass around the ducts, the temps on my verticle exhaust seem to be going pretty high. I figured going in this might be the case with a short run, but thought it better to hedge my bets against a chronic cold plug problem in a tent-like environment.

Is it better to have more radiant surfaces to give off the heat quicker if the ducting run is short? I have yet to have one incident of backdraft so far. Everything seems to burn very cleanly. After a week of burning every day, at least six usually 12 or so hours constant, there is no ash build up to speak of. The only sign of any inefficiency is one point on the vertical exhaust where the tape had a ripple and some creosote looking stuff seeped out. This was the first or second day and nothing else like that has appeared. From the exterior all I see looks like steam coming out of the exhaust cap. Seems to me having more surfaces like exposed pipes to radiate heat out immediately would just prevent more heat from escaping the exhaust? I ask because my two 'T' cleanouts in the run are top-down. I was thinking of covering them with something like wool to keep them from being hot spots, but on second thought it occurs to me hot spots are nice when it is -10* out and it might help the system bleed off more heat before it escapes.

On a related note, have any guidance on installing a Dragon Breath Monitor? I'm thinking I would install it on the vertical exhaust where it is most convenient to read. Are there any other considerations to the location? Do I just drill a hole slightly smaller than the probe size and jam it in, does it need caulk or any kind of support?

Hi Coydon;
Just drill a slightly smaller hole in the vertical stack and poke it in.
The tin piece will keep it upright.
I do not, but you could use a bit of putty/calk to help hold it.

Now let's talk about covering your pipes.
Your run is short enough to allow exposed pipes, and you are gaining quick heat that way but not storing any for when the stove is out.
In your situation, you are extreme burning your RMH, just like I am, and you are going to have a higher stack temperature.
I think you should cover your pipes and see how well it does.

I have the mass built up to the minimum I was planning. I have a couple sheepskins for sitting surfaces and threw a futon mattress on for sleeping. The first couple nights I used my -40 Arctic sleeping bag on the futon. The bag itself didn't feel hot, but the futon under me was too hot to be comfortable. I switched to my 20 bag last night and was more comfortable, but the mattress still felt hot. I should be able to finish the floor tomorrow and will bring in cotton sheets and a wool blanket. We've also had temps oscillating around freezing, so will have to see how things are in frigid temps when they show up.

Draw has been flawless, no priming needed so far. It does take a painfully long time to get the firebox hot enough to cleanly burn some of the not-perfectly-dry wood I have. I've stapled up quilts and blankets around most of the yupee interior for insulation. There is a clear stratification layer where I can tell the section of uncovered wall starts. Need a couple more blankets to finish, then some sealing work around the doors.

So the mass is heating up well, but currently still need to keep the burn going or air temps plummet. I'm experimenting with throttling off the feed tube and/or using lower quality wood to keep a low level burn, but I'm thinking this could lead to creosote build up? Exhaust stack temps are around 300-350 when running full burn, and still reads around 150 when burning a couple branches with the bricks over 1/3 of the feed.

I've left the two vertical cleanouts uncovered, they make nice warming spots for beverages. The dirt with the top layer of bricks is still settling, I'll probably put a thin layer of more dirt over it later in the week and re-level those bricks. For the final top, I'm considering my first idea, a thin layer of dirt over all of the bricks and ceramic tiles set on top of that to spread the weight and limit the exposed dirt. I"m not sure how much it would take to get above the height of the pieces of crossing rebar, but it would increase the mass a decent amount. Would the tiles be prone to breaking?

A newer option would be to cover all of the brick gaps with clay mortar, now that I've had experience mixing some up to plug gaps in the manifold. Shouldn't be a problem having a heated space in the yupee now. Should make the top stable and dirt free, but the rebar would be sticking up exposed. I could mould in around the cleanouts this way, not sure what I would do with the tiles for this. Some wool batts under a ceramic tile?

BTW, please note the ultimate dump score, a panel from a Death Star corridor suitable for an exhaust guard...