Air Vent at boiler. Necessary with Air Sep?

That will depend on the type of compression tank you're using.  (expansion tank). If you have a tank that has some type of membrane that separates the air from the water side of the system, then of course you should have an automatic air vent somewhere.   

If however you have the older technology steel compression tank where the air and the water are in contact with each other, then you may want to make sure that the tank is equipped with the AirTrol® tank fitting, and use the top of the air scoop to vent air into the compression tank. 

Can you explain the reason you feel you need the air separator? Are you experiencing air problems?

Depends on the circulator location.  If you have the air sep on the supply (HOT) side of the boiler piping and the circ is pumping away from the air sep, AND you attach the Ex-60 expansion tank and boiler fill valve at that same point, you will actually eliminate the air problem.  It all has something to do with the Physics of  Water and Boyle's Law of dissolved gasses.  Long story short… Hot water and low pressure will release dissolved air.  Put a vent there and all the air in the system will get absorbed in the water and released at the air sep.  (ask me how I know this)

thats a lot of heat.
how big and where is this home?

17 feet above the circulator pump that may be about 3 feet above the boiler pressure gauge indicates that you need about 8.9 PSI of static pressure to get the system filled. add a little extra for a buffer and you can operate at 12 PSI for that system. So 15 PSI at the boiler gauge is more than adequate without going overboard. Set the air charge in the expansion tank to 15 PSI when the expansion tank os not connected to the boiler water pressure. That would mean with the boiler pressure below 12 PSI if you want to check it without disconnecting it. Then set your auto fill pressure to 14.5 PSI to 15 PSI so not to add more water to the 15 PSI expansion tank at cold startup. You should be good to go with pumping away from the expansion tank!

That looks great! I am wondering how all the rest of the radiators are connected? Obvioulsy you have more than one radiator or Fin Tubbed.

Are they all in one series loop of baseboard

or are they all convectors with fin tubes inside a cabinet?

Then as you may already know, Purging air from the system is not going to work. You must vent the air at each radiator to get the system started. so the highlighted valves and fittings are probably a waste of $$$

Purging only works where there are no Tee fittings involved and since your have a parallel piped system with 2 tee fittings for every radiator, purging is not an option, Venting is the only way to get the system filled with water.

Once the system is operating, then you can use the pumping away design to remove any air that may have found its way into high spots. That will happen over a short time (about a week or less) depending on the amount of air stuck in the system. Pumping away uses Physics of water (Boyle's Law) to remove air on a molecular level and vent it on a molecular level at the air vent just above the expansion tank.

The capped tapping on the Weil McLain boiler is designed to be uses with the old style expansion tank that do not have a membrane between the air and the water. There is really no benefit to using that as a vent when you have the MicroBubble style air separator like a Spirovent or a Caleffi Discal. Those vents are like air vents on steroids. The other vents are just something that will eventually leak and need replacing.

Here is something I just told another member about the difference between the two different types of expansion tanks:

Like.

This will remove air from the system as the system operates with the help of the laws of Physics. But each radiator convector must have at least some water in it so that water can flow and can absorb the air into the water and carry it to the hottest, lowest pressure location where there is an air separator.

I like your diagram

@AlwaysLearning2024 for the most part once you fill the system with water, and that water tests fine, that is the last time you need to do anything with the water. That water should stay in there forever (unless you need to remove it for a repair) because it will become inert in a very short time. That means that all the things that are dissolved in the water like dissolved air or oxygen, will rust a little bit of the metal, then it is done. No more oxygen will be left in the water to make more rust. So that closed system's water will get a gray coloring and will no longer have harmful agents dissolved to make problems.

If for some reason your system develops a leak and the auto water feed keeps adding fresh water to replace the leaking water, then you will find that the new fresh water has new dissolved oxygen and other agents that will corrode away the inners of the cast iron and steel parts. Keeping the system free from leaks and keeping the existing water in there will reduce or eliminate the need for adding chemicals or other treatments.

The drain valve at the bottom of the system is just fine for adding chemicals and water treatments if needed.