Hy vent installed in tee on return?

99JT

I'm in the process of installing a hydronic unit heater in my garage which is being supplied by the boiler in my house which has a taco air separator. 

Question is would their be any benefit to adding a tee with a hy vent in the return line which is by far the highest point in the system

STEVEusaPA

Might be the highest part of the system, but on the return, probably suck in air. If you have a micro absorber air elimination anywhere on that loop, pumping away, it will take care of any air problem after initial purging.

hot_rod

On the unit heater, should be a supply and return connection. Whichever is the highest one. If connections are at the same level on the unit, either will work

As long as you have 5 psi or more at that vent, you should not pull air in.

Assuming near boiler piping is correct, pumping away from the expansion tank

A manual vent would work, you should only need to purge it once on start up if you have a good air purger, shown by Steve, at the boiler