the think is the AC is cooling air from the room (recycling it ) and not taking it from outside, which is why you are getting that vacuum effect, you are better off just not exhausting your miners to outside if they put out less heat than your temperature outside the barn.
ding ding ding!
AC doesn't/shouldn't draw air from outside the room it's trying to cool. Just think about it. Bringing in hot air from outside and trying to cool it is far less efficient than just trying to cool the air that's already inside, along with the fact that it would be creating positive pressure which would force cooled air outside. Instead, it recycles the air inside.
By having your miners blowing air outside of the room, it's creating negative pressure inside the building. Less air crossing the heatsinks, less air to absorb heat, they'll hang out and get toasty.
At least, that's how I understand it...