Chimney Design

To avoid piercing the roof, I plan  to vent the RMH out the opaque north wall of my greenhouse into a chimney that rises from its own foundation.

My tentative design calls for stacked 55 gallon drums, with 8" pipe run up through the middle.
The space between the inner and outer skins would be filled with soil.

That might be overbuilding it  so rather than barrels I might use smaller containers or progressively smaller containers for the outside skin.

Maybe I could just build a pallet lumber box  to hold the soil?
There should be very little heat left by the time the exhaust reaches the chimney, would 3 plus inches of soil be enough to protect the wood?

Tell me what you think.

This is the spot where the chimney should rise from.

Hi William,   The first time I read your post I didn't understand what you were trying to do and so I just hoped someone else would respond and make it all clear to me. Not sure if I've got it any better now but I'll give it a shot.
So I'm assuming the soil is meant to be an insulative layer between the vertical exhaust pipe and combustibles close to it?
If so, I'm thinking that the soil is going to act like a mass and suck out heat from the chimney and rob it of draft.
If your not planning on using commercial insulated pipe for your final exhaust, it still definitely needs to be insulated. Condensation and poor draft would be forever biting you in the butt otherwise.  

An inexpensive alternative, I know some people who have used one size larger pipe over their existing single walled pipe and just filling the gap with perlite or vermiculite.
I went with rock wool wrapped around mine with an outer coating that protects it. Certainly not as pretty or certifiable to probably any codes, but it works and feel it is safe for my use.

Let me know if this is on the right track or if I'm out in left field with what your asking.  

Thanks for you thoughtful reply!
Yes, you are exactly correct, I was trying to insulate the chimney from the tarp wall.
I also wanted it to stand on its own,  so a column of earth seemed ideal.

So, air gap,  rockwool, or perlite?
The air gap has an advantage of self protection against water infiltration.
Anything else would still need the outside layer anyway.
I'll have to price things out.
I know rockwool can recover from  being wet, so maybe a wrap of wire and aluminum foil would be enough.
What did you use?


Are high temperature insulation materials actually needed?
The chimney exit temps are supposed to be fairly low,  right?
I'm thinking fiberglass instead of rockwool .
But that stuff is nasty anyway.

I've been working on the bell,  crafting walls of steel studs and sheet metal that will be filled with tamped soil.
Could this same construction, with the walls left empty make for an adequately insulated chimney?
I  have the materials already, and it would be easy to add perlite or rockwool latter on,  if it's needed.

The only thing I can really recommend as a homemade insulated pipe is the perlite/vermiculite between the pipes idea. See here for an example: Advice on a RMH build in Hokkaido Japan
Definitely, I would say no to using fiberglass insulation.
My 1.5" thick rockwool wrapped pipe with a layer of bubble wrap insulation certainly works, but I know my stove exhaust temps never get so hot that it would cause a problem for only me using it.
I certainly don't want to encourage it though as in my early use of it, I had a spot where the rock wool didn't meet perfectly and melted a bit of the bubble wrap.

If your going to use tamped soil as a mass, be sure to put in as much rock or other dense materials to eliminate as many air pockets as possible which will only act to insulate and resist being heated up which is exactly the opposite of what you want to do.

Steel studs and sheet metal as your outside layer of your chimney should work too as an alternative to just using a larger pipe as an outer form. I don't think the air gap would be enough insulation on its own though.

My first concern was how much the soil was eventually going to weigh and whether or not it would be stable stacked in a column like that. Dry, crumbly earth is actually not too bad as an insulator, I recall from a book on passively heated greenhouses. Damp earth is a good conductor, and I imagine tamped earth somewhere in between. What I used for the exterior piping on mine is some 5" gas appliance exhaust. It is double layered, with maybe a .25" air gap between the two layers. It's not truly insulated, but the extra layer keeps the cold air off of it, and I've had no problems. If you are unsure of your draft, insulated would be best as Gerry said. I would not have a problem using fiberglass if you already have some to get rid of, but I wouldn't buy some for it. I would use perlite or vermiculite from a garden supply shop. And I would keep the insulation dry, whatever it is. If it traps water, it can rust through the pipes.

Alright, the fiberglass is off the table.
I have a line on some used double walled pipe which should be fine if its the right size.
It might be 10", which would be a shame.

If I go with a perlite insulation, what thickness would be good?


Regarding the tamped soil, I thinking wetting it will help collapse any air bubbles, correct me if I'm wrong.
I also have a reciprocating saw I plan on using to vibrate the forms,to collapse air bubbles.

Stones or brick are preferable from a density viewpoint, but dirt/mud can fill tiny crevices, so I intend to use both.
I think sand would be a great additive, but buying sand by the bag hurts my soul.
I have plans/parts to build a 55 gallon tumbler which would be perfect for grinding bottle glass and ceramic toilets into a grog for these applications, but I won't let myself be diverted with that project.

While we are discussing chimneys, I'm wondering about including a bypass at the top of the bell.
Normally its a no brainer , but my bell will be roughly 3.5290 m2 ISA, as compare to the recommended 8.85 m2 recommended for an 8" batch box.
I intend to tap it to heat as well as the mass, but I'm thinking the relative low ISA might make a bypass overkill.

Hi William;   I don't understand? Why would 10" double wall be bad?
Admittedly I've never seen 10" double wall but I'm sure its out there.
Enlarging your already enlarged 8" system to 10" chimney would be a good thing.

Oh!
I thought the rule of  "maintain system size throughout" applied to the chimney as well?
If it is 10" ID, not a problem then?

Hi William;
Not a problem to enlarge when you go vertical.
Your 8" is going to be a big beast !  

thomas rubino wrote:Hi William;
Not a problem to enlarge when you go vertical.
Your 8" is going to be a big beast !  

Wonderful!
Yes 8" is huge for the space, but they say that bigger rockets are less finicky, plus I want to burn pallet slats.
One slat is 40", cut in half 20" , needing at least 24" in the firebox.
Crappy wood, but easy to aquire, process and stack, dry and in bulk.
I will be buying a cordless circular saw just to make harvest easier.
I'm also casting the firebox, and an easily  reproducible  8" batch box seems ideal for more applications, like a bank of bread ovens, heating my enclosed front porch, powering foundry, still or forge...
I figure I can always throttle back.


What do you think about the need for a bypass, given the undersized bell?


Just curious, would a 15" diameter vertical chimney be too much?
I have lots of water heater tanks...

William Bronson wrote:Oh!
I thought the rule of  "maintain system size throughout" applied to the chimney as well?
If it is 10" ID, not a problem then?

I don't expect it would be a problem. I read from some of the gurus that the chimney can sometimes be smaller and work ok because the gasses have cooled and contracted enough they have less volume. My RMH doesn't really have a uniform csa, but the chimney is the smallest diameter of the whole system and it has an excellent draw. About the 15," I don't know. I've never heard of it being done, but my instincts tell me a smoke shelf like a traditional fireplace has might make such a large chimney work well.

I googled smoke shelf, and I kinda savvy it.
Better to stick with chimneys that don't need added features.

Picked up 10 feet of 8" ID double wall pipe for $20 at my favorite surplus store.
Totally worth it.
They even had  a 5 foot length of 10" ID but that was too short anyway.
Couldn't find a tee there but I did get a 90° elbow for the transition from the bottom of bell out the side of the cabinet.
I might build a sheet metal box instead of buying a $13 tee for the base of the chimney.

This is what I've gotten done on the exhaust so far.

So here is the elbow that will channel the exhaust from the bottom of the bell,  into the cabinet beneath and out the back.
I'm considering enclosing it so I can pack it in dirt, the idea being to seal any leaks in the joints.