# How to Reduce Register and Grille Noise

Jan. 11, 2019
Let’s take a look at the top five sound-reducing options available to you.

Noisy registers and grilles are annoying to your customers. When they express this concern to you, what can you do to correct the situation? Let’s take a look at the top five sound-reducing options available to you.

How Loud is Too Loud?
A register or grille is too loud whenever your customer says it is. This is an opportunity to meet your customers desires and earn additional income.

Noise Criteria (NC) rates the sound levels of registers and grilles. This measurement is simply defined as the grille or registers sound, measured in decibels (or dB) minus 10 decibels.  Most residential grilles and registers are rated at 20-30 NC.

You can measure NC using an inexpensive sound meter costing less than \$100. To measure, turn on the system, measure its dB, then subtract 10 dB. Compare your result to acceptable grille noise levels between 20-30 NC.

Example: Your customer mentions an annoying sound in the entertainment room. It’s so loud, they must turn up the volume whenever the HVAC system comes on. With the room quiet, turn on the system. Using a sound meter, you measure register noise at 52 dB. Next, calculate the NC Rating (52 dB – 10 dB = 42 NC.)  42 NC is twice the noise level of a quiet register and 50% noisier than the highest acceptable NC rating.

Identify and Solve Noise Problems
Because many customers are used to their noisy systems, they benefit from you asking about the problem.

Ask questions like, “Which room is the noisiest,” and you’ll receive more noise reduction opportunities than ever before. Here are some common airflow noise causes and typical solutions you can offer.

Excessive Airflow
The most common cause of grille and register noise is excessive airflow. A study of grille and register specifications verifies that noise levels increase as more air moves through a grille or register.

The Solution –You have two options. If the airflow into the room is excessive, you can reduce the airflow through the grille or increase the size of the duct and register.

If the room has too much airflow compared to what is required, install a balancing damper at least five duct diameters before the register. Close the damper until the required amount of air is delivered into the room. Test to verify delivered airflow and noise reduction.

If the room has less airflow than needed and is noisy, usually this means the register or grille may be undersized. Increase the grille or duct size to match required room airflow. In this situation, you kill two birds with one stone: you increase airflow and decrease system noise.

Restrictive Registers
Supply registers must meet various airflow applications in the field. For example, some are designed to throw air longer distances than others. These registers have louvers close together to increase air speed. This also increases the air delivery noise and resistance to airflow.

The solution - If noisy, these registers should be replaced with ones sized for quieter operation. This may require replacing the sheet metal boot behind the register also.

You can also add another duct, boot, and register off the trunk duct to relieve airflow from the existing register.

When replacing registers, you’ll find them painted over. As a result, they may be stuck to the ceiling or wall. Be sure to use a razor knife to cut the paint at the register edge before removing it. If not, you may have to eat the cost of repainting the ceiling.

Restrictive Grilles
Return grilles and return ducts are frequently undersized. This causes high velocity noise at the grille and in the duct.

The solution - Because most are undersized, installing an additional return duct in another area of the building is usually your best option. This will also solve the problem of low system airflow by reducing total external static pressure.  Search for a duct path into another room or hallway. Be sure to approve the location with the owner first.

If access through the building prohibits installing an additional return duct, inspect to see if you can upsize the return grille near the equipment. Often return noise is caused by air-moving equipment located in a hallway, and where the return duct system is an open building cavity with a return grille screwed to it. If there is no other access, you may be able to cut in another grille in the back or side of the return cavity.

Some return grille noise is caused by the filters used. Replace the filter with a less restrictive one to do the trick. If all else fails, you can reduce noise by increasing the return grille’s open area. Use two needle nose pliers and adjust the louvers to a more open position. This reduces noise and increases airflow.

Face Dampers
Air volume dampers installed just behind the register or grille often cause extreme air noise.

If the grille is installed close to the fan, a damper may need to be closed to direct air farther from the fan.  This may cause the damper to scream.

The solution - Remove the face damper installed near the register or grille and replace it with an inline air volume damper in the duct. Install it far enough away to deaden any air noise through the grille.

Of course, there are many other causes to airflow noise such as a tight elbow concentrating the air velocity on one area of the register. A rattling noise can be caused by debris left in the duct, by a loose damper, or by flapping duct liner. Each of these have their own custom solution.

Now that you’re aware of grille noise causes and solutions, bring these opportunities and their solutions to your customers who need peace and quiet.

Rob “Doc” Falke serves the industry as president of National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free table showing Noise Criteria levels for different room types, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.