In most ducted heating and cooling systems, supply registers should remain open unless the system was specifically designed and balanced otherwise. Closing several increases duct pressure and can reduce airflow through the equipment instead of usefully sending all that air to other rooms.
Keep normal supply registers open
An adjustable register can support small comfort changes, but fully closing it changes system resistance. Start with registers open and correct balancing causes before using extreme closure.
Do not block return-air grilles
Return grilles let the system pull air back from the home. Furniture, rugs, or improvised filters in front of them reduce airflow and can increase noise and blower load.
An uncomfortable room needs diagnosis, not just closing other vents
Uneven temperature can come from duct leakage, insulation, solar exposure, or inadequate airflow. Closing registers in comfortable rooms may hide the cause while creating new issues.
Zoned systems are a designed exception
Motorized dampers and zone controls are designed to vary airflow in a coordinated way. A manually closed register is not equivalent to a zoned system; follow the equipment’s specific design and maintenance guidance.
Sources
- U.S. Department of Energy — Energy Saver — Residential energy efficiency, heating and cooling, and equipment use.