Help finishing RMH

Hello all,
I have been researching rocket mass heaters a bit and decided to take a crack at one.  I built this cone completely from materials i had lying around.  
The feed tube and burn tunnel are built from 5 1/2" square tubing the riser is built from 5 1/4" round tube.  All of it is 3/8" thick.  
The feed tube is 8" long burn tunnel is 16" long and the riser is 32" high.   it all sits inside of an old propane tank.

My questions from here are.
1. Should I still insulate the heat riser?  I was hoping that being thick steel it would be sufficient.  I have heard people say the heat riser can reach around 2000*.  I don't really want to melt this thing.

2.  I plan on routing this through a heated bench in our basement.   What size exhaust pipe should I use to come out the side of the propane tank.   And what size should I use going through the bench?  I plan on making the bench 12' long with a simple down and back run in the bench and then tie into the existing exhaust pipe.

Here are a few pictures of what I have so far  

Where to start?  is there a top to the cut away bell/propane tank?

Is there a manifold at the bottom?

As it stands there, it will likely burn ok, but without the efficiency of a stove with insulated heat riser.

Also of course it could not be burned inside a building

Since metal conducts heat, the whole thing is trying to give off heat as soon as it is produced.

If there is a top not pictured and manifold at the bottom, the heat riser being uninsulated would cause the thermal movement of exhaust to stall at the top of the bell since efficient movement of the down draft between riser and bell requires a cooler temp outside the riser and hotter temp inside.

In a normal RMH the heat is supposed to build inside the exhaust stream from the firebox to the top of the heat riser, that is why clay bricks and clay are recommended for those parts, since they are both insulative and resistant to high temps. Those high temps and the extra combustion they generate cause the exhaust to be free of unburned hydrocarbons (smoke).

Hi bob, thanks for the reply.  
Yes it dose have a top, I just haven't welded it on yet. I wanted to figure out the issue of insulating the Riser before I welded it on the top. I haven't cut the hole in the bottom yet for the exhaust gases to come out because I wasn't sure what size I should go with at the bottom to go through my heated bench.

I don't really know what to say, you've obviously put a lot of work into this (and materials and energy) and it should last forever, which is no doubt what you were going for, but there is generally some clean out that needs to be done at intervals, and depending on what you use for insulation, there's a good chance there could be some flexing and cracking from thermal expansion/contraction, so the heat riser insulation will need to be repaired also .

Your exhaust can't be any smaller than your heat riser, but for such a small diameter riser you may not be able to push the exhaust very far as it is.

I started with a 6 in diameter, but after the first year I rebuilt everything into an 8in

It allows for a bigger fire box and more fuel and less fussing with it. If your greenhouse is small then you may get away with your current dimensions. otherwise you may want to consider a larger riser/firebox/ connecting tube. Also, be careful not to extend the sides of the firebox too high as that has a tendency to start to act like a stack and smoke back into the room. shorter firebox also allows you to clean ashes out of the tunnel up to the riser without as much trouble.

Insulation is really a major key in these, The next time I take mine apart I'll plan to rebuild/repair and add an extra inch or so of insulation to the riser.

I use a mix of clay, perlite, and high temp cement, an 8 inch stovepipe in the center that ends up burning out and whatever diameter the outside is according to how much material I have to work withas an outer shell for the mold.  Sometimes I just freehand the mix around the inner pipe

I finally found a cheaper source of bulk cement  so I can use more of it next time. That adds to durability but is not absolutely necessary. Ianto has built risers out of clay and sawdust that perform well under normal rocket stove operation, but are a bit fragile when you go to clean them.  (the sawdust burns away leaving a honeycomb riser that insulates very well.

As much as I admire your welding skill, if it were me, I would think about building a clay base using fire brick for the fire box and tunnel and then mold a riser. the manifold is incorporated in the base and goes directly into the bench as a continuous unit.

I have a feeling that if you get this to work, you're going to have problems with all that metal in the heart of the thing.

Clay is a wonderful experience to play with, very satisfying, and very forgiving, and for this application probably a better choice. I think that amount of metal is going to end up causing trouble.

If you want to keep your metal firebox etc, be sure to insulate everything, not just the riser.

If you do not insulate your burn tunnel and heat riser, you will be losing lots of heat just at the time you need it to make efficient combustion with no smoke or creosote. If you do insulate them, they will corrode from the heat plus atmosphere, within a couple of heating seasons more or less, and you will have to scrap and rebuild the core.

A 24' run with a 180 doubling back is pretty long for a 5 1/4" diameter system, risking not having enough draft to burn well without smokebacks, and possibly drawing so much of the relatively small amount of heat that there is not enough left to assist the draft with a warm chimney. Depending on the size of your greenhouse and the severity of your climate (and how far into the cold season you want to heat the greenhouse), this system may not be robust enough to give the heat you need, at least not without burning constantly, which is something the RMH design is intended to avoid.