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.
Best Answer
These are just crude supply registers to provide heat/AC to the basement. They could have been installed with the regular installation or added at a later date. There should be register covers somewhere with a screw in the middle that when attached to the vent controls the opening of the vent. We had them all over our unfinished basement. When we had it finished. we utilized them with the drop ceiling.
You can replace them with straight ductwork but you'll need supply registers somewhere else.