Naturally Aspirated, I do see what looks like a pressure relief valve on the gasser, that tells me it's pressurized and also if it were set up non pressurized it would not work, how would you keep the air that WILL develop in the upper heat loops out, and yes the circ may have been inadvertently left out but that is not the only problem with that drawing, it is also not what I would consider a primary secondary loop, I'm going to try and stop by Scotts on my way to Florida this weekend and perhaps I can enlist his help in posting a drawing of the way that I do it, some may think it is nuts but it has always worked for me, I'm certainly not saying that it's my way or the highway as I know there are many here smarter than I but I have spent many a night on a jobsite in my motor home thinking and discussing piping with some pretty darn intelligent folks over a few Buds, Sprinter, just because you made a mistake does not mean we don't love you, HUGS and keep posting, I have found several of your opinions to be helpful and we all make mistakes, GOD knows I have. Yoder, yes they do, you would not want it any other way, Scott, when I stop by remind me to show you how I shut the boiler off when it runs out of wood and why.