I have the auto air eliminator, as well as the hand vent (by the galvanized bucket) the hand valve is actually the highest in the system. I fill the system from the bottom and up, with the hand valve open, before any pressure is registering on the gauges. I hope this is getting out any bulk amounts of air. I have let it fill a couple times, and run a little then vent down at the high points again. I think the tank is suppose to have water and not air right? The port for the exp tank is "half a pipe" lower than either of the air bleeds, so I'm hoping most the air is going out before it goes into the tank. The closed system holds the volume of the boiler, plus the 1 1/4 pipe, and the ~400 feet of 1" copper heat exchangers in the tank, not sure what the volume is but I have noticed that I'm building pressure from where I start at ambient temp around 65 or 70 at 8psi, and it likes to be around 18 when up to temp the highest I've seen is 172 on the loop so far. The 1500 gallons in the big tank are just thermal battery, just being heated up, and then cooling off with house loads. I call it the "Stupid water" lol. I started heating the big tank of water this past weekend, and got the top up to 130, the bottom is about 110 right now. A 20 degree delta top to bottom. The circ pump is low and pushes directly into the bottom of the boiler, and the boiler feeds right out to the highest line. Then past the Pop safety valve (30psi) into the air eliminator, past the TEE for the tank, then finally to the bypass or recirculation loop, which goes down the wall past the two pressure gauges and the throttling valve. I use the throttling valve to put a little more pressure going towards the heat exchangers, so it is forced out to the tank except enough for the 3 way thermic valve to sense boiler water temp. Then turns back through a strainer and into the inlet of the pump. The pics didn't show the pipe arrangement the best, so I'll probably draw a diagram and post it up here.