Grid and Register Mysteries | Subcontracting company

Residential grilles and dampers hold a variety of mysteries that seem to have eluded many who design, install, sell and service air distribution systems in our industry. Let’s take a look at several of these puzzles and how understanding them can help improve HVAC systems.

What names should I use?

Registers, vents, grilles, diffusers and other subcategories are descriptors used for residential applications.

For decades, NCI has taught simple terminology covering most basic residential needs. Registers cover supply duct outlets and direct air into the building. Grids cover return air ducts that draw air out of the building. When dealing with a mixed batch of grids and registers, the term grids can describe a combination of the two.

Supply dampers and airflow distribution

Supply records control, mix and distribute the air in a room. Besides the commonly used buffered dampers, there are many types of dampers that control the movement of air in a room and improve comfort.

When offering damper and grille upgrades to your customers, start your training at a manufacturer’s website. These websites usually contain photos of various registries. You can also find engineering data which describes the specifications of each model. I invite you to spend an hour studying different registry and grille data. Then, imagine the solutions that each can bring to your projects.

The solutions contained in the technical specifications generally include:

  • Flow ranges
  • Dimensions
  • face velocity
  • Discard
  • propagated
  • Degrees of deviation.

Studying this technical data will lead you to select custom dampers to solve the air distribution problems your customers want you to solve.

Return Grid Selection

The main function of the return grille is to cover the ends of the ducts where they connect to each room. But this is only the beginning.

When selecting return grilles, choose properly sized grilles to reduce noise and pressure drop. Select the appearance, style and color of the grids according to the desires of your customers.

You can also use return air filter grilles to help air filters with excessive static pressure drop and resistance to airflow. Installing filters in multiple return air filter grilles increases filter area and reduces return side static pressure.

It is essential to understand that return grilles rarely influence ambient air patterns, regardless of the face velocity of the return grille.

This illustration shows that the return inlet velocity only influences an area approximately 1.5 times the diameter from the grate face. The air velocity from a supply damper can often be felt much farther away, so it will influence the airflow of the room more.

What your customers see

Other than the thermostat, dampers and grilles are the only system components that your customers see on a regular basis. Their visible nature makes dampers and grilles an obvious candidate for upgrades when replacing equipment and upgrading duct systems.

Draw your customer’s attention to the existing grilles and note their condition. Many are visually downright gross. Replacing grates is a high margin job with higher appreciation value from customers. Take samples of premium grilles with you and offer replacements.

If grilles have been painted on the walls or ceiling, their removal may result in additional liability. The paint can stick to the grates and when removed will strip the paint and wallpaper from the wall or ceiling. This error is difficult to repair and often results in repainting the part. To avoid this problem, carefully cut the paint with a sharp knife around the grille before removing it. Also use replacement grilles with a wider frame to cover minor flaws.

Air mixing zones

When selecting replacement grilles and dampers with different air patterns, consider the principles of the air mixing zone. Good designers ensure that they take into account air mixing zones to avoid high velocity air blowing directly onto the occupants of the room.

The illustration on the left shows a room with floor supply registers and a ceiling return grille. A two-foot air mixing zone is designed inward from each wall and ceiling.

When selecting floor registers, choose a register offset of zero to fifteen degrees pointing toward the wall. This choice will mix the supply air with the room air and prevent curtains and wall hangings from blowing.

The same principle applies to ceiling and side wall registers. Since the air velocity zone of a return grille is so shallow, it rarely needs adjustable louvers.

Floor dampers Sound levels of the air mixing zone

Grid and Register engineering data also includes a sound rating for each grille at different airflows. It is important to note that noise levels increase as air velocity and airflow increases through a grille. This value can play a critical role in your selection depending on the type of part you are packaging.

The most commonly used unit of measurement for sound with registers and grid is Noise criteria, abbreviated as NC. The noise criteria is equal to decibels (dB) minus 10. The noise criteria in the field can be measured approximately with an inexpensive sound level meter or an app for your smartphone.

Standards are published for acceptable noise levels which depend on the use of the room. Some common NC ratings are:

  • Rooms – 20 to 25 NC
  • Family rooms – 25 to 30 NC
  • Kitchens – 30 to 35 NC
  • Apartments – 35 to 40 NC.

The pressure drop

The pressure drop is the resistance to airflow exerted by a grille, and it changes as the airflow changes. Ideally, each damper and grille in a system should have a similar resistance, otherwise system airflow will move to the grilles with the lowest pressure drop.

To compensate for registers that may require higher pressure to throw air into a larger or wider room, you can increase the size of the duct leading to this register to compensate for the increased resistance.

You can uncover many grille and damper mysteries in the technical specifications of the manufacturer. Interestingly, I called them hidden, but they’re in plain sight if you know where to look. They were hidden from me until a demanding request from a client pushed me to find a solution.

Please take an hour to visit your favorite grille manufacturer’s website and learn about a segment of your industry that you may or may not be familiar with.

Rob “Doc” Falke serves the industry as president of the National Comfort Institute, Inc., an HVAC-based training company and membership organization. If you are an HVAC professional interested in a free grille and damper selection process, contact Doc at or call 800-633-7058. Go to the NCI website at for free information, articles, downloads and current training opportunities

About Donald Martin

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