| Parameter | Value | Note |
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Why register sizing matters for comfort and noise
The supply register is the final link between your duct system and the room. Get it right and the room is comfortable and quiet. Get it wrong and you have either a hissing register that keeps occupants awake, or a register that delivers air straight down rather than throwing it across the room — creating cold floors and hot ceilings. Register sizing is about three things: face velocity for noise, free area for pressure drop, and throw for air distribution.
Face velocity limits by room type
Bedrooms and living areas where people spend quiet time need lower face velocities: 300-500 FPM at the register face. Corridors and utility spaces can tolerate 500-700 FPM. Return grilles are particularly sensitive — return air velocity above 400-500 FPM creates a noticeable suction noise at the grille. Most noise complaints traced to registers are fixed by simply upsizing the register face or adding a second register to split the airflow. Free area is typically 55-70% of face area — always use the manufacturer's published value, not an estimate.
Throw distance and air distribution
Throw is how far the supply air jet reaches before it decays to 50 FPM — the terminal velocity below which the jet no longer effectively mixes room air. For ceiling registers, throw should reach at least 75% of the distance to the far wall. Under-throw means cold spots near the floor in winter. Over-throw (too much velocity) means the air jet hits the opposite wall and creates a draft. Select registers with adjustable blade deflection so throw angle can be tuned after installation.
Frequently Asked Questions
ASHRAE recommends supply register face velocities of 300-500 FPM for bedrooms and other noise-sensitive spaces. At 300 FPM the register is virtually silent. At 500 FPM there is a slight air movement sound that most people find acceptable. Above 600 FPM in a bedroom, most occupants will notice the sound, especially at night. The face velocity is calculated as CFM divided by free area (in ft²). Free area is the register face area multiplied by the free area percentage — typically 55-65% for 2-way supply registers. Always verify with the manufacturer's specification sheet.
Return grilles should be sized for 300-500 FPM face velocity to minimize suction noise and pressure loss. For a 1,200 CFM system with one central return, the required free area is 1200 / 400 FPM = 3.0 ft². A 20x25 inch grille with 70% free area has 2.43 ft² free area — slightly too small. A 24x24 inch grille at 70% free area = 2.80 ft² — better. Two 16x20 returns each handling 600 CFM are often quieter than one large return. Canadian homes are frequently under-returned — a single 14x20 return serving a 1,400 CFM system is a very common cause of high static pressure and poor system performance.