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AIR DISTRIBUTION GUIDE
What Is the Difference Between a Grille, Diffuser and Air Valve?
A grille is a louvred terminal for supply or extract air; a diffuser distributes supply air in a controlled ceiling pattern; and an air valve or anemostat distributes air around a circular or square terminal. Selection depends on airflow, throw, velocity, noise and architectural position.
Direct answer
A grille is a louvred terminal for supply or extract air; a diffuser distributes supply air in a controlled ceiling pattern; and an air valve or anemostat distributes air around a circular or square terminal. Selection depends on airflow, throw, velocity, noise and architectural position.
Technical review: MDES Engineering Technical TeamUpdated: 2026-08-05
Grille
Fixed or adjustable blades control direction. Different blade and damper options serve supply, extract, transfer and outdoor-air applications.
Diffuser
A diffuser spreads supply air over a broad ceiling area. Discharge pattern, throw and ceiling effect determine comfort performance.
Air valve/anemostat
Circular or square terminals distribute air around their perimeter and adjustable designs can assist flow balancing. They are common in small and medium spaces.
Quick selection summary
| Product | Typical duty |
|---|---|
| Grille | Supply, extract or transfer |
| Diffuser | Broad ceiling distribution |
| Air valve | Radial adjustable distribution |
Key points
- Do not select from duct size alone.
- Check flow and face velocity together.
- Review throw and sound level.
- Damper and plenum are part of the system.
Frequently asked technical questions
Can a diffuser be used for extract air?
Some models can; verify the product data and pressure drop.
Is a larger grille quieter?
At equal flow it may reduce face velocity, but blades, dampers and duct conditions also affect noise.
Why is a plenum box used?
It improves distribution to the terminal, simplifies connection and may incorporate a damper or insulation.
Which information is required for a ventilation equipment quotation?
Specify the product type, dimensions, airflow, pressure or acoustic criteria, material, finish colour, installation method, quantity, project standard and delivery location. Final selection must be verified against project calculations and operating conditions.
Technical references
- Project airflow, velocity, pressure-drop and acoustic calculations
- Manufacturer data sheets and application details
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