Split barrel bench vs parallel 8" round duct

Here's the situation. I want to build an 8" system with an L shaped bench and I could do split barrel, but I'm having trouble making the air sealing work in my head with the split barrel.

Here is the configuration: L shape, side A on the left is 9 feet long, side B on the right is 10 feet long.  The chimney is located about 3 feet from the end on side A.

I could place the combustion unit at the end of side A next to the chimney and use a serpentine pipe setup to go down to the end of side B and back. Or I could put it at the end of side B and use a split barrel setup, OR two parallel pipes to move the exhaust to the chimney on side A.  I would kinda like to do the latter since it places the radiant barrel more toward the center of the space.

I've done the math on the ISA, I'm in a pretty good place in any configuration, probably going to have fairly warm exhaust.

The idea I have right now is to sit the barrels on a floor of Hardie board and then use cob and/or silicone to seal them. I can also weld them end to end, at least for structural strength.

Somebody please poke holes in my ideas. Am I worrying too much about air sealing (and condensation) in the split barrels?

Hello Solomon,    
First off, are you planning on an 8" batch box or J tube?
As far as leaks, there have been many people who have made a bell from half barrels with great success with no leaks.
If your cuts are not matching up quite right, no worries, just cover with some hardware cloth then cover with a few layers of cob.
Expansion joints (especially near the manifold/core area) may also require some cf blanket to be able to flex a bit with dissimilar materials (steel meeting cob)
Cob alone may continually crack in these places otherwise and could potentially leak after time.
Some folks after getting their RMH's all roughed in will burn a smokey fire which will definitely show you where any leaks could be and need fixing.
Usually a wet finger to smear the leaky area is all that's needed. The finish coat will then super seal it.

To help put you at ease a bit further, once draft is established (most are right from the start, especially if they are still warm), the RMH operates under negative pressure, which means it will be sucking air through any cracks that may still be there.

A recent build where the fellow used the half barrel system can be found here: RMH build
There are other threads that have talked about them as well if you do a search.

Peters ISA calculations are only relating to batch boxes. Matt Walker has also said that for the most part the ISA numbers can be ignored if you use a bypass. This helps to make the bench/bell much more bigger than the limits of the numbers if you wish.

Hi Solomon;    I second all Gerry suggested and I would like to tell you that Matt Walker's Stove chat live is in about an hour!
Here is the link  https://www.youtube.com/user/broaudio?app=desktop

Gerry Parent wrote:are you planning on an 8" batch box or J tube?

J tube, ceramic fiber board core.


Matt Walker has also said that for the most part the ISA numbers can be ignored if you use a bypass. Can't really use a bypass when there is nowhere to bypass to. This plan would place the core well away from the chimney. I'm planning on using compressed air to induce air flow up the chimney, if necessary. I'll be having about 20 feet of insulated vertical chimney, so I'm optimistic that will work excellently.

I was originally planning on building a system much more conventionally with a single serpentine run of 8" round duct. But if I can put the core on the other end and use barrels or parallel ducts it might be better to reach my goals in heating my house.

If you want to put the combustion core on the far end of the bench from the chimney, I would go for a half-barrel rather than a parallel duct cavity. The half barrels will have practically zero friction to inhibit draft, and it is  general wisdom that two parallel duct paths will never actually balance flow; one will take more of the flow than the other even if they appear identical, and an L-shaped bench will not have identical parallel channels.

How much of the chimney will be inside the house envelope? If most of it will be, you may be fine in most situations for natural draft.

From the very bottom to the insulated section is about 8 feet, then 9 feet insulated, then probably another 10 from the attic up. So, from the RMH to the air is about 27 feet, 17 inside the envelope.

Equal flow is not important. Mathematically it will be equal headloss between the channels. So it will be balanced enough. Plus, parallel ducts means half the headloss overall.

Unequal flow means that one duct will be hotter than the other, and you will have unequal temperatures on the bench. Probably not a huge difference at the lengths you are talking about. I think the more important factor will be the reduced drag in the half barrels.

Yes, though barrels have their own problems, heat will be concentrated at the crown where the hottest gases are, and also where the thinnest cob is.

This is true, which is why I will not be using barrels in the bench extension to my bell, but a rectilinear masonry box with a slab top.

I have even observed that the cob layer around the square brick bell does not get near as warm at the corners as on the flat faces. I have a 4 to 6" radius on the 6" thick cob layer, and if redoing it would make it a 10 or 12" radius so the surface would be more evenly warm.

I'm not terribly concerned about even heating of the bench. I'm going to stack up cob on it. I'm looking for thermal mass. Probably going to make it twin bed width so people can sleep on it. If it ends up being too tall to sit on, I'll brick up a step in front so people's feet aren't dangling.