Supply and Return Ducts

HVAC ductwork has two halves:

• Supply ducts carry conditioned (heated or cooled) air from the furnace/air handler to registers in each room. The vents you feel air blowing out of.
• Return ducts pull air from rooms back to the HVAC system to be re-conditioned. The big grilles (usually in hallways or at the bottom of walls) that suck in air.

Why both matter

HVAC systems are essentially closed loops. For every cubic foot of supply air pushed into a room, a cubic foot needs to return to the system. Inadequate returns are one of the most common Colorado home HVAC problems — many older homes have just one tiny central return that can’t handle the air a modern blower wants to move.

Signs of return-air problems

• Pressure differentials between rooms (doors that slam shut or won’t latch)
• Whistling at door undercuts
• One room always hot or cold no matter what
• High static pressure measured at the air handler

Adding return-air pathways (transfer grilles, jumper ducts, or new dedicated returns) often delivers more comfort improvement than upgrading the furnace.

Related: Ductwork, Plenum, CFM, Static Pressure.