I am not entirely sure what this big duct work is for.
Its not the exhaust, because im sure the exhaust is the smaller duct on the gar left side (exiting form the enter image description heretop). The big duct as a lever, which i know is to open/close the valve inside the duct.
The question is, what is this duct for?
And should i be keeping the valve opened or closed?
This is the valve lever:
Best Answer
It's a "damper". It can be manually set (open or closed) depending on quantity of air needed.
During the design of the HVAC system, duct sizes are determined based on size and location of rooms they serve...kind of a best guess. Once the system (ducts) are installed, the quantity of air can be adjusted by opening or closing these dampers.
Often we put these dampers on "fresh air" intake ducts. However, they're still used to adjust the amount of outside air. (We usually start at 10% of the total air required.) In residential construction we usually don't have fresh air intake ducts, because of air leakage around doors and windows.
This damper appears to be on the "return air" duct. Keeping the damper "full open", uses the maximum amount of return air.