We always appreciate our cozy heat when the temperatures plummet and our air conditioning when it’s scorching hot out. But one thing most homeowners don’t love about the HVAC systems is how loud they can be. Building engineer Ross Trethewey breaks down the common causes and some simple solutions.
Common Causes
There are a few common causes of HVAC system noise. First, the proximity to the furnace or air handler itself will impact noise. The closer a register is to the motor or inner workings of the system, the louder it will be.
Second, you need to consider air velocity, which can create the whistling sound you hear from registers or vibrations in the ductwork. It gets worse when the vents are restricted by the homeowner closing the damper in the room, as the same volume of air is trying to escape from a smaller opening, often creating the whistle.
Solving a Noisy Fan
The closer you are to the unit’s fan, the louder the perceived noise will be. Adding flexible, insulated ductwork between the fan and the vents will cut down on that noise. Also, making sure the ductwork is extended rather than compressed will help absorb more noise and vibration.
Adding a bend anywhere there’s a straight run of ductwork can significantly reduce noise as well. This doesn’t have to be a hard bend—even a curve will help reduce noise.
Fixing Air Velocity
Air velocity is one of the most widespread issues in an HVAC system, as you can experience the noise regardless of how far you are from the main unit.
One solution is to adjust the balance damper, which will restrict flow further back into the system, reducing perceived noise at the vent. However, this can create velocity issues elsewhere as it redirects the same volume of air to another vent.
Another solution is to install less restrictive vents. Replacing bulky wood vent covers with thinner metal covers will help. Also, installing larger vent covers with more surface area gives the moving air more room to escape, creating less turbulence and whistling.
Resources
Ross used a sound level meter that measured the sound pressure in decibels.
Noisy Fan Solution
A fan is going to sound louder the closer it is to the supply and return vents. Ross advises adding insulated flexible ducts in between the fan and vent. This will create distance from the vent and the curve of the flex duct will help absorb sound.
Air Velocity Solutions
When airflow is restricted, air speeds up to squeeze through smaller openings. The faster the air moves, the more turbulence and noise it creates.
Ross explains one solution is adjusting the balance damper. By adjusting airflow at the register, the noise will be heard louder at the final exit point. If you can access the balancing damper, the turbulence and noise will happen further upstream in the ductwork, away from the exit point.
If accessing the damper level is an issue, Ross suggests looking at the register. Wooden registers are usually bulkier and have more airflow resistance than metal registers. More restriction increases the velocity and noise of the air coming through.
Replacing a register with a larger one could also help. Ross explains how switching from a 4×10 register to a 6×10 register increases the area for airflow. The diameter size will stay the same, but a larger boot will need to be installed.
