It is a return vent
I am surprised this is not a more debated topic.
So many people have big tanks here on a second floor and never discuss this.[/ QUOTE]
I live in a condo that is almost 40 years old and I double checked with my management office on the weight load specs before installed my smaller Reefer 250. In my opinion people overlook this important point and have paid the price later on. I have seen pictures on R2R that show weight issues and their effects!