A lot of air ducts systems contain dampers within the duct work. The purpose of the dampers is to control air flow, which in turn affects the temperature in the room. Rooms that either face the sun, or contain a lot of windows may need more AC in the summer, but less heat in the winter. A room with minimal to no sun exposure may need more heat, but less AC. This is where the dampers do their work.
Dampers are usually found where the main trunk line connects to the supply duct. (duct work that supplies air to various areas of your home). You will see a lever that can be turned to adjust the position of the damper. You’ll want to turn this to a fully open position. If your dampers need to be adjusted seasonally, you may find it helpful to use a marker and mark the duct work with what position the lever is in and mark its position optimal flow. It is also a good idea to write which area of your home this particular duct services.
Once the damper is open, go to the room it services (be sure the register or vent is fully open) and check the air flow. If you feel it is sufficient, leave the damper in this position. If it is a particularly sunny or warm space, you might want to adjust the damper. Repeat this process throughout your home until you have achieved your desired air flow.
Also, remember not to close off the dampers on your vent covers as they are the breathing point for your system. Restricting air to and from the furnace could put a heavy work load and increase pressure on your HVAC system!