Thank you so much for the welcome!
My greenhouse is 20x25. I do use it year round, however, it is mostly for seed starting. We don't have horrible winters here, but we do tend to have crazy freezes in the early part of the year. Last year I lost a bunch of my seedlings and I don't want that to happen again. I just need to keep it above freezing at those times. My plan was to build a 6" J tube and use the warming bench for my seeds that like to sit on a heating pad. LOL. I've honestly even thought about just doing a small wood burning stove surrounded by bricks, etc for thermal mass, no more than I'm actually gonna need to use it. However, I do have a large hoop house as well and will be building another and would love to plan for larger RMH systems for those.
I ordered the plans for the 6" system so that I could have a close look but I honestly think I'm even more confused now!?!? Maybe I need a nap!