Comparison of Grilles and Slot Diffusers in Ventilation Systems

This page outlines the various types of grilles and slot diffusers used in ventilation systems, their characteristics, and suitable applications. Proper selection and specification of these components are crucial to achieving effective air distribution, aesthetic appeal, and system efficiency in HVAC designs.

Ceiling Concealed unit

The front mouth is for Air Supply.
On the side, the following can be seen:

Drain piping connections
Liquid state refrigerant piping connections
Gas state refrigerant piping connections
Electrical connections in an enclosed box

The return air is either at the back or underneath the unit. There must be a clear, direct route to the return air (i.e., not placed right up against a wall).
Permanent access for servicing and maintenance is required for all of the above. The minimum size is a 500 x 500 mm access panel, strategically placed in a corner to allow engineers to investigate and replace parts in the future. — The front mouth is for Air Supply.

On the side, the following can be seen:

Drain piping connections

Liquid state refrigerant piping connections

Gas state refrigerant piping connections

Electrical connections in an enclosed box

The return air is either at the back or underneath the unit. There must be a clear, direct route to the return air (i.e., not placed right up against a wall).

Permanent access for servicing and maintenance is required for all of the above. The minimum size is a 500 x 500 mm access panel, strategically placed in a corner to allow engineers to investigate and replace parts in the future.

Back View
At the back is the return air mouth which is just as big as the supply mouth
The filter fits inside the supply area mouth — **Back View**
At the back is the return air mouth which is just as big as the supply mouth The filter fits inside the supply area mouth

Underside view
Most ceiling concealed units come with an interchangeable plate so that the return air can be located in either position as shown — **Underside view**
Most ceiling concealed units come with an interchangeable plate so that the return air can be located in either position as shown

Floor standing/vertically hung Concealed unit

Side view
The green is the supply mouth
The brown is the return air mouth
The orange is the drain connections
The blue is the refrigerant pipes — **Side view**
The green is the supply mouth
The brown is the return air mouth
The orange is the drain connections
The blue is the refrigerant pipes

**Example of Supply and Return directly on to unit**

Considerations When Designing and Specifying Grilles and Diffusers

Aesthetics

Aesthetic preferences vary, but the placement and dimensions of diffusers must align with the supply or return air chambers of the fan coil unit. Every concealed unit requires airflow for both supply and return, which are fed from different areas.

Visibility behind the grilles, including unfinished areas or ducts, is often unsightly. Therefore, the 45-degree angle blades provide the additional benefit of obscuring this view.

Air Flow Velocity

Each unit produces different air volumes, with velocity determined by the volume divided by the free area of the grilles. Maintaining a velocity between 2-3 m/s ensures effectiveness without causing nuisance noise. Additionally, losses due to external static pressure in duct runs must be considered.

Positions and Dimensions

Accurate airflow calculations, rooted in fluid dynamics, are essential for achieving the desired environmental comfort. The size and number of diffusers are crucial for delivering the required amount of airflow. Positioning the diffusers in the room can often be more challenging than anticipated, as it requires careful consideration of both space and optimal airflow distribution.

The ductwork between the grilles and the units should be designed to satisfy the customer aesthetically and to minimize the external static pressure that the airflow must overcome. Therefore, the unit’s position must be carefully considered. Additionally, due to its spatial geometry, finding a suitable position to hang the unit can be challenging.

Accessibility for Maintenance

Concealed units must be accessible for future servicing and maintenance, particularly for electrical parts and filters. A minimum access panel size of 500x500mm, located under the right-hand corner of the unit, is recommended.

Controls

Ducted/concealed units come with a hard-wired, wall-mounted Bluetooth controller that can generally be used with a smartphone app. Additionally, a WiFi module is available for control via the Onecta app for an additional cost. The Madoka controller is a hard-wired remote controller with touch screen operation designed for end-user control, while the Bluetooth app is intended for installer use.

The wiring for this controller is completed during the first fix appointment. The controller functions as a thermostat for the unit, as the sensor on the unit itself is often not relevant because it is outside the area it serves. It is possible to install a concealed thermostat sensor in the room for an additional cost. The controller is available in different colors.

Types of Grilles

1. Supply Air Grilles

Description: Designed to supply air into a room.

Features: Adjustable vanes for directing airflow; typically have greater free area for higher airflow.
Suitable Situations: Offices, classrooms, and residential spaces where airflow direction needs management.

2. Return Air Grilles

Description: Used for returning air back to the HVAC system.

Features: Often have a fixed blade design; lower pressure drop compared to supply grilles.
Suitable Situations: Commercial buildings and residential areas requiring efficient air recirculation.

Some grilles can be fitted with a removable core, allowing access to the necessary parts of the unit through the grille itself.

3. Eggcrate Grilles

Description: Feature a grid-like structure with high free area.

Features: High air volume capacity; minimal resistance to airflow.
Suitable Situations: High-capacity return air applications in data centers, gyms, and large open areas.

Types of Slot Diffusers

1. Linear Slot Diffusers

Description: Long, narrow diffusers that blend into the ceiling or wall design.

Features: Sleek appearance; even air distribution; adjustable vanes for direction control.
Suitable Situations: Modern office spaces, retail stores, and high-end residential applications where aesthetics are important.

2. Plenum Slot Diffusers

Description: Incorporate a plenum box for better air distribution.

Features: Uniform airflow; handle higher air volumes than standard linear slot diffusers.
Suitable Situations: Large commercial buildings, auditoriums, and conference rooms needing consistent airflow over a larger space.

Comparison Summary

Feature	Supply Air Grilles	Return Air Grilles	Eggcrate Grilles	Linear Slot Diffusers	Plenum Slot Diffusers
Airflow Direction	Adjustable	Fixed	Fixed	Adjustable	Adjustable
Air Volume Capacity	Moderate to High	High	Very High	Moderate to High	High
Aesthetics	Moderate	Moderate	Low	High	High
Best Use Case	Offices, Classrooms	Commercial, Residential	Data Centers, Gyms	Offices, Retail, Residential	Commercial, Auditoriums
Installation Location	Ceilings, Walls	Ceilings, Walls	Ceilings, Walls	Ceilings, Walls	Ceilings, Walls

Suitability for Specific Situations

Offices:
Linear slot diffusers for aesthetic appeal and even air distribution; supply and return air grilles for specific airflow needs.
Residential Spaces:
Supply air grilles for adjustable airflow; return air grilles for efficient air return.
Commercial Buildings:
Plenum slot diffusers for high air volume and uniform distribution; eggcrate grilles for high-capacity return air.
Large Open Areas (Gyms, Auditoriums):
Eggcrate grilles for high airflow; plenum slot diffusers for consistent air distribution over large spaces.
Data Centers:
Eggcrate grilles for their high air volume capacity and minimal resistance to airflow.

Grille Blade Orientation: Straight vs. 45-Degree Blades

Straight Blade Grilles

Blade Orientation: Vertical or horizontal without tilt.
Airflow Direction: Direct, unobstructed.
Aesthetic: Simple, clean look.

Suitability:

General use with straightforward airflow patterns.
Uniform air distribution without specific directional needs.

45-Degree Blade Grilles

Blade Orientation: Blades tilted at 45 degrees.
Airflow Direction: Redirects airflow at an angle.
Aesthetic: Distinct look, reduces direct sight lines into ducts.

Suitability:

Spaces requiring directed airflow, such as avoiding drafts or targeting specific zones.
Applications needing noise reduction and enhanced occupant comfort.

Conclusion

The choice of grilles and slot diffusers in HVAC systems depends on the specific needs of the space. Straight blade grilles are suitable for general use and uniform air distribution, while 45-degree blade grilles are ideal for applications requiring directional control and noise reduction. Understanding these differences helps HVAC designers and engineers select the most appropriate components for each situation. 45-degree blade grilles provide the additional benefit of obscuring the view behind the grilles.