I was looking for Siegenthaler's stuff in my (admittedly disorganized) files, gave up and grabbed the first fin-tube spec I dug up ioff the web. (If it's cherry picking, it wasn't intentional, and like I said, I don't trust Slant-Fin's calc for 120F performance based on physical intuition.)

I have no experience with fin tube at 120F but have dialed back 2 systems previously run at 180F down to 140F (~20F delta on the loops, for condensing temp returns), and they've run through design-day conditions without a hitch. I suspect that the lower you go in temp the more sensitive fin tube is to ANY amount of air obstruction, since the convective forces eventually get to be pretty weak in so small a vertical dimension, and even if it matches the model in a lab at 100F, it probably doesn't have sufficient consistency in the real world to be able to design for it. I thought I'd remembered Siegenthaler's model only got really steep below 100F ( http://www.pmengineer.com/PME/2001/03/Files/Images/11127.jpg ), but figure the practical limits would be higher than that.