My install is about done (with a bit of elec to go). I am now preparing to fill er up. I have:
Weil CGa boiler
Taco 4900 Series Air Seperator on supply side
Taco 1/8" Hy-Vent on boiler
Boiler drain valve
Watts ET Expansion Tank
Taco Cartridge Style Pressure Reducing Valve (3350-T2)
My stem is an old converted steam/gravity system and I have no idea how much water it takes.
? What is the best way to fill the system? Just turn on the water and let the 3350 fill until ~12psi is reached?
If the reduction/autofill valve does not also contain a backflow preventor, you need to add one as you potentially have a means to pollute your drinking water.
Getting all of the air out of the system may be a pain depending on how it is setup. The circulator won't pump against blocked air, and you don't want to run it dry.
If there is a vent at the highest radiator, open that and slowly add water to the system. Have someone monitor that vent. When you get some water, shut it and let the system pressurize to the nominal 12# or so. This has worked for me on my own system, but see what the pros have to say. It will take awhile for the system to purge all of the air both trapped in the system and that disolved in the fresh water you used to fill the boiler and lines. An air scoop like a Spirovent is also a useful addition. It essentially 'combs' microbubbles out of the water stream with a SS brush. The Taco device may operate similarly, and is probably also a good choice.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013