Thanks for that very useful link. I've been looking for a comprehensive guide like that for some time now.
I see your point about the soffits, but I think I'm SOL there. The soffits that feed that portion of the roof are indoors. Seriously, whoever built those two rooms didn't care that much about thinking things through.
These two pictures are looking towards the master bedroom (the one with the issue) from the sun room, which is where the roof ended and the soffits are located.
Since we are never in the sun room, it gets really hot. If the soffits get any air sucked in, it will be hot.
But you got me thinking... maybe my solution is to try to cool the sun room so that it will provide the chambers in the roof with cooler air through the soffits. Maybe I just need to keep it at around 80 or so degrees. Not exactly arctic temperatures, but cooler than whatever it has in those chambers...
I am not sure you want to feed your cooled air here, you still have to replace it some where else.
I would be more inclined to remove the plywood soffit in that room and build an insulated box from side to side and cut a good size vent into which ever side would be the coolest.
Or install vents in the outside wall of this room and help the heat in this room with free air flow.
Ah yes, that vent pipe is harmless, I believe it's plugged on the other side. It used to be for the old boiler, but they installed a newer one and with it a different exhaust. I'll double check on it, though, thanks for pointing that out!