can horizontal transition area be to large??

Hi All; I'm building my first rmh and i'm at the transition area from the barrel to the horizontal pipe. everything i've read says don't make it to small, but most pictures i've seen show a relativly small shaped cob area leading directly to the horizontal pipe. I have been using brick to shape my transition and will coat it with cob. I seem to have an area apx. 2x2x2 to use. I am about to start cobbing the brick in and plan on using sheet metal to shape the (roof) over top, my horizontal pipe enters a foot or so from the bottom of the barrel. I have a nice depresion below that for ash collection. I am just wondering if this is to large an area as i can easly shorten it up now.

Thomas, this should make a bell of some kind. It should be all right. Carefull with the metal, as it expands and contracts at a diferent rate than cob, so you could have leaks. I'd advise you to use pavers or something like this, suported by a brick pile in the middle of the area.

T.R. : In a Word NO ! This is the area to go Big ! It is one of the most common reasons for failure of your R.M.H. to be happy ! This is where you want to lavish your greatest
care to make all all your transition angles as gentle and sweeping as you possibly can !

You should have a clean out in this area, and this should allow you the access through out this part of your build to make those large sweeping turns as you go from the fat
Doughnut shape of the space between the inside walls of your barrel, and the outside walls of your Heat Riser, passing vertically down into the transition area where you make
your graceful and large transition curves horizontally through your R.M.H. Base and into your Horizontal exhaust pipe !

This is a good place to think of the flow of exhaust gases as like a stream of water, you want your Flow to be as smooth and unimpeded as possible, with no eddies or snags.
This would be a good place to use hardware cloth to define and allow you to shape the walls of the transition area and end up with interior walls as smooth as sheetrock!

For the Good of the Craft ! Think like fire, Flow like gas, Don't be the Marshmallow ! As always your comments and questions are Solicited and are Welcome ! Big Al

Hi Guys; Thanks for the advice! I will switch from using sheet metal to hardware cloth, and posably using some large shale pieces that won't need a support pile to form the top. One of the nice benefits of progress is that my sceptical freinds and neibors are starting to see & understand what i have been talking endlessly about for the last few months!

that looks great, i built my first recently and also didn't know the dimensions for the manifold but i did it very similar in size to yours and it's working great, i have it running all day .
here are pics from that

https://www.dropbox.com/sh/w145d2m7ami0zpq/7J1luXC2n0

Thanks for this topic as I was afraid of making mistakes there as well.
I also have doubts when estimating the half moon zone's size down the barrel for the smoke to enter the transition area.

I am not sure what is a hardware cloth... Is it this iron stuff to make fences?
Do you mean to do something like ferocement?

I was thinking about leeks as well if using metal sheet.
But for supporting the dome during the making, is there a solution with something that can me removed or destroyed?
(including destroyed by fire )

Xisca : Yes, it is difficult to make a hole in the side of the barrel at its bottom too large, be generous, Hardware cloth is a large mesh sized metal screening material usually used for
reinforcement where we want to screen out very small things, here in the states we often place a section of Hardware cloth on the outside of a door 1/2 way up a fine meshed
screen door so that the second layer of hardware cloth will stop the dogs from tearing up the screen when they want to come back inside !

We talked in the past about doing the horizontal piping out of 1/2 round clay tiles, if you carefully shaped the bottom half of your transitional areas out of fine sand and clay slip it
would be nice to have the hardware cloth to hold onto the sand, clay slip mixture, In some places they use a larger size of wire mesh for support in applying stucco, if you have
someone who can do the bottom half of your Transitional area with or without mesh so that that part had very smooth walls, and then packed in sand, and then finished the top 1/2
with more fine finishing plaster, and it was allowed to dry slowly, with or without a coarse mesh to hold onto the finish plaster- you would have the right shape. If the whole section
was shaped like a cone or large funnel with the large end at the barrel, and then tapering down to the small end that received the end of the Horizontal stovepipe You could wait until
every thing was dry, then carefully clean out the sand by hand and have a good transitional area that also contains an ash pit, and a clean out !

If this was not clear to you, let me know and I will try to get some pictures to explain better ! For the Craft Big AL !