To start, I'd like to thank all for the comments, and support.
I've not fired this up yet because its going to smoke like crazy the first time i do, so I want to wait until night time to do it. The shop, which was a stand alone two bay garage, is insulated beautifully. Air gaps around the bay doors, but if it's a problem when the real cold weather hits, I have plastic I can cover them over with.
I wish I could run the pipes underground, but getting them into the shop would be challenging. I think. For now I'm going to see about wrapping them in kaowool and then seeing if I can waterproof it somehow. 50 degree weather for the next two days so i need to jump on that. Didnt think about aircrete over the pipes. Have to give that some though.
I wanted to do 6" pipe, but had the 4" on hand. Suck part is, two weeks after I bricked in the 4", my brother in law told me he has a ton of 6" I could have had. If 4" isnt moving the heat fast enough, I'll get some 6" from him and replace the 4" or maybe run another set of 4". Have to see what would be easier.
Right now the roof are just a couple of road signs I picked up from work years ago. Not sure what the reflective part is made of, but I bet it smokes bad once it heats up. Going to do aircrete interlocking slabs at some point for the roof, but I haven't done it before so it'll be a bit before I get that going.
Dont know if I can link on here, but here's a
video of what aircrete is if you havent seen it before. Should have made the whole thing out of this stuff.
https://youtu.be/zTvy5Ut8eps
Updated pic from this morning. Added support wire cos' I almost lost it in high winds the other day. Buckets with bricks are holding it for now, I'll anchor it properly at some point in the near future. Also had 4" of water in the barrel. Going to have to seal everything up. Maybe use high heat silicone, or that refractory putty I have.