Newby here from Northern Michigan's first RMH, an outdoor tent heater: A work in progress...

Ken May : Welcome to Permies.com, our sister site,Richsoil.com, and a Big Welcome To the Rocket, Wood stove, and Cob Forums! With 28,000 + Fellow Members
Worldwide you should be able to come here 24 / 7 and find a kindred spirit who wants to talk about what you want to talk about !

Ernie and Erica are our Forum Leaders for the Rocket And wood stoves forums and we all like to say good things about these good people !

You can goto Richsoil.com Click on Rocket Mass Heaters and watch that hole series of videos, each on is a pretty good primer for the next one. The one for taking

the paint off of your barrel and treating your First10' of pipe to lock the Galvanizing to the stove pipe will answer your question !

Always use the best insulation and as much of it as you can afford. while Rocket Mass Heaters Made 20 + years ago had less insulation, with todays designs we see
as much as a 50% improvement in operating temperatures, and even more efficiencies and cleaner burns with even less wood !

I do like your Experimental Heat Riser ! We do run into an immediate problem with trying to build prototypes to a smaller scale ,While a 8'' system has a Cross-
Sectional Area of 49 sq in, it has a perimeter of 25 '', a 6'' system has a Cross-sectional Area of 27sq. in., its perimeter is 19'', your 4'' heat Riser made into a 4''
system has a Cross-Sectional Area of 12.5 sq. in and a perimeter of 12.5''.

When we go down in size by 1/2 we reduce the area by 1/4, when we are measuring the flow through these pipe sizes, the larger % of Interior surface area
(perimeter)to the Cross-Sectional area -means that with the smaller pipe we are seeing much higher % of flow losses due to the laminar effect and friction losses.

Generally, we recommend that for a first time build that a 6'' system is as small as anyone should go and is a good size to 'Learn on' !

A 4'' design is considered an advanced build, and gives you a very tiny Thermal Mass Bench for the amount of work you put into it, especially when compared to a 6''
system !

Think like fire! Flow like gas! Don't be a Marshmallow! As Always, your questions and comments are solicited and Welcome ! Big AL

10' x 20' (presumably uninsulated) is a pretty good-sized space, and too big for a 4" system in cold weather even if you get it working well. If you are making tests for a system to go in your church, you need to be working with an 8" system, as you will most likely need something of that size for a space like that. Don't expect that you can just scale up a 6" system that you get working right; not everything will scale exactly in proportion, not to mention the possible better combustion in a larger system offset by the different cross section to length factors in the heat exchange mass. Working with the same size components as your final will be much more useful, and you may be able to directly reuse some components.

Thanks for helping me pry my head outta my ass. Greatly appreciated!!

K, the 4" riser has nothing whatsoever to do with the build, just for cooking. And while my church is 1,500 sq ft im only heating 140 sf of it, a 10x14' room behind the pastors quarters. I was going to try a 6" pipe for the mass heater. I have much reading to do. Appreciate the help!!😊
Heres another rookie mistake. (attached). Looks like i need to repair where i smashed it with a hammer. Ill slit the bucket first then tape it next time. This is just my front deck cooking stove.

For clarification, the build is eventually going in this room that has a chimney, which is an addition built onto the pastor's quarters of my church. I've heard there may be problems with using a regular chimney with a RMH? I need to research this. The 3rd photo is of the exterior.

There is no inherent problem with using a regular chimney. As long as it is in good condition and appropriately sized, it should be fine.