Why is there a whooshing noise from my vents?

Whooshing or rushing air noise from vents almost always means the air velocity is too high for the duct or register size. This happens when a room's duct was undersized during installation, when a register was partially closed reducing area, or when the HVAC system was replaced with a higher-capacity unit without resizing the ductwork. Standard duct design practice limits branch duct velocity to 700 FPM and trunk ducts to 900 FPM. Use this calculator to check your velocity — if it exceeds these limits, the duct or register needs to be upsized.

What air velocity is too high for residential HVAC ducts?

Standard duct design practice sets these maximum velocities for residential systems: branch ducts (to registers) 700 FPM, trunk ducts 900 FPM, supply plenums 1200 FPM, return ducts 600 FPM. Above these limits you get noise, increased static pressure, and wasted fan energy. At 1000 FPM in a bedroom branch duct, occupants hear a constant rush of air from the register. The fix is upsizing the duct or adding a second branch to split the airflow.

💨 Air Velocity

Air Velocity Calculator

Calculate air velocity in ducts from CFM and duct size — or find the duct size needed for a target velocity. Instantly see if your velocity causes noise or comfort problems. Used by HVAC contractors to diagnose loud vents and right-size ductwork.

Unit System:

⚙ Solve For

💨 Inputs

Airflow *

CFM

Duct Diameter *

Target Velocity

FPM

Duct Run Type

💨 Air Velocity Results

Parameter	Value	Status

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High duct velocity: the hidden cause of HVAC noise and comfort problems

If you hear a constant whooshing from your vents whenever the furnace or AC runs, the duct delivering air to that room is too small for the airflow being pushed through it. This is one of the most common HVAC problems in Canadian homes — especially after a furnace or heat pump replacement where the new system has higher airflow than the old one, but the ductwork was never resized. The fix is straightforward: identify which duct is over-velocity, and either upsize it or add a parallel branch to split the airflow.

Standard velocity limits and why they exist

Standard duct design practice sets velocity limits based on acoustics research: branch ducts to bedroom registers at 700 FPM or less, trunk ducts at 900 FPM, returns at 600 FPM. At 700 FPM, a 6-inch round duct carries 137 CFM. If 200 CFM is forced through the same duct, velocity reaches 1,023 FPM and occupants hear it clearly. This also increases static pressure across that branch by 214%, starving the room of airflow in high-friction conditions.

Heat pump ductwork: a growing problem in Canada

Cold-climate heat pumps operating at low outdoor temperatures need higher airflow than gas furnaces for the same heating output — often 25-40% more CFM. A duct system designed for a 60,000 BTU/hr furnace at 1,000 CFM may need to handle 1,350 CFM for an equivalent heat pump. Every duct in the system is now over-designed airflow and velocity will exceed limits in multiple branches. Before replacing a furnace with a heat pump, check velocity in every branch with this calculator using the new system's airflow. Use the duct sizing calculator to determine which branches need upsizing.

Frequently Asked Questions

My vents are loud — how do I know if it's the duct or the register?+

Hold your hand 6 inches from the register. If you feel strong turbulent air with a rushing sound, the duct feeding that register is over-velocity. If the air feels smooth but you still hear noise, the register itself is too small — the free area (open area of the register face) is creating the velocity. Register free area is typically 50-70% of face area. Enter the register's free area dimensions in this calculator to check register velocity separately. Both problems produce the same symptom but have different fixes: duct over-velocity requires a larger duct or second branch; register noise requires a larger register or diffuser.

Can high duct velocity damage my HVAC system?+

High velocity itself doesn't directly damage the equipment, but it causes high static pressure which forces the fan to work against more resistance than it was designed for. Variable-speed ECM motors compensate by running faster and drawing more amperage — increasing energy use and heat generation. PSC motors (in older systems) cannot compensate and simply move less air, reducing capacity and potentially overheating the heat exchanger. Long-term high static operation on a gas furnace risks cracking the heat exchanger within 5-8 years instead of 15-20. Check static pressure with the static pressure calculator if you suspect this is happening.