Many HVAC systems have actual valves on the ducting so you can bias the air flow and compensate for the very problem you're having. The system on the house I used to live in had butterfly valves on every duct controlled by a small lever sticking out from the side of the duct.

When the lever is aligned with the duct, the valve is open. When it's across the duct, it's closed.

If your system doesn't have these, they can be installed easily and for a relatively small amount of money. DIY.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250