RMH for dummies in process question

Hi,
  Got my bottom layer leveled and sanded.  Doing a dry build for the next layer before clay slipping it in.
Two pressing questions:
 1)  When do I transition from full brick to split brick in the burn tower?
 
*** 2) for 6" system, the CSA is about 30.  I was following plans for 5.25 x 5.75  (30.1875 area), BUT This gives me a 3/4" gap in the burn tunnel (AFTER I slip in a split brick; too narrow to add a full) , which I could cob in...?  IF I change the dimensions to 5.5 x 5.5 (30.25 area),
   I get enough space to put in a full brick which makes for a smoother tunnel (less drag!), is the 30.25 close enough for my CSA???  (This is the photo, where I increased the gap, so I could fit a second half brick-except I would you  a whole).

Thank you!!

IF I change the length of the burn tunnel- that would solve my problem!! It is currently 23.5", so if I change that to 23, I should be  okay?!

You're talking about 1/2" to 3/4" on a RMH?
No question that would absolutely destroy the efficiency of the heater and it'll NEVER work right... will never warm a house, will never cook a meal.... you get the point....






On second thought- why are you even asking a question about that small of a dimension? Just adjust and move on. That's the beauty of DIY. You can do anything you want.

Hi Stacie;  #1)  You normally transition at the height of the feed tube so apx 16"or so  #2) I've not built a 6" system but I've always heard 5.5 x 5.5 as standard.  #3) Shortening a burn tunnel is always better than lengthening it.
About your riser,  what kind of and how thick is your insulating blanket? Is it Kao wool ?  ceramic fiber ?  How much do you have ? There may be another option for your riser.  

The main reason for full firebrick use would be structural strength, and past the feed tube and beginning of the burn tunnel, there should be no major stresses on the core. On the other hand, the less mass in the core, the quicker it will come up to full operating temperature. So I would advise to move to thinner firebrick in the hot faces as soon as you can, with the caveat that you need to have it well supported and all structural loads taken by external masonry or similar material. Add the best insulation you can around your abrasion-resistant hot face materials.