Getting ready to redo duct insulation. On supply ducts off the trunk there are some sort of valve-like devices teed into the duct and oriented to point down. Same diameter as the duct. Butterfly doors with some sort of doohickey centered in between. The current insulation has a gap such that either side of this valve and the valve itself is not insulated, and moisture tends to condense on the bare metal surfaces. I am tempted to replace them with straight duct. But I don't want to break some function. House dates back to the 1960s. It does look as though they may have been retrofit into the existing ductwork. I have not been able to find something similar on the internet.
I expect that it is some sort of damper, but it is not inside the duct. This is throwing me. If only I knew the name to search on, I'd find it right off.:) Here is a picture.