When it comes to selecting registers or grilles, there are several characteristics that must be weighed in order to make a good decision. Study the manufacturer’s engineering data carefully to be sure you make the right choice. The primary engineering factors include pressure drop, throw, noise levels and air mixture. Social factors including cost and appearance also play a role in the selection process. Include each of these factors in your grille and register selection process.
Engineering Criteria
Pressure drop is determined by how much resistance the grille gives to the air flowing through it. The more restrictive, the higher the pressure-drop. Excessive drop may affect the performance of the entire system. Typical Δp is .02-in. to .03-in.
Throw, which is the distance from the register that the air can be felt, is related to pressure drop. The greater the pressure drop, the more throw is generated. Throw must be matched to the area that must be served by a grille. Draft is also a result of throw. Excessive drafts are objectionable in heating mode, but should also be avoided in cooling mode. Typical throw is ± 10-ft.
Noise levels are rated as Noise Criteria (NC) in most catalogs. Different room occupancies call for various noise levels. Executive offices should have a NC of approximately 20, but restaurants can have NC’s over 40.
Air mixing is essential in obtaining comfort. It provides ventilation and stops air stratification. Air mixing blends air that enters a room through infiltration, and evens out temperatures on surfaces that affect comfort.
Heating or cooling mode will demand different properties from the grille and registers. Normally, grilles are selected and sized for cooling mode, because airflow is greater and throw is more critical. It’s rare that grilles will be changed from season to season so don’t base your decision on the current season of the year. If you’re thinking heating airflow, will the grille do the right job in cooling mode as well?
Social Factors
Appearance and cost are too often sacrificed for the sake of an eight-dollar register. These are the part of the system that is actually visible to customers, and is an opportunity that is often missed. More expensive registers also perform better and last longer. Recommend register and grille replacements on your next project.
What do you call them? Normally, the supply outlets are referred to as registers because they give direction and control to the airflow. Return inlets are referred to as grilles because they are simply a covering to the duct. In the industry the descriptors ‘grille’ and ‘register’ for supply or return devices are often used interchangeably.
When ordering from a distributor, the register or grille dimensions can really mess up an order. Is the largest dimension listed first, or the smaller dimension listed first? You have to take in to account all the variables when making this decision. What is the throw, pressure drop, sound, appearance, and cost of the grille or register? It varies depending on the manufacturer and the type of grille or register.
The Counter Guy
When it comes down to it, your success often depends on the guy or gal who takes your order at the distributors. A good counter guy who knows the product you need, as well as the differences between each manufacturer’s specs, can successfully guide you through the selection process. These people are key to our success far more that we ever give them credit.
Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested in an NCI Grille Selection Procedure, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website for free information, technical articles and downloads.
