Air Distribution Design: How to Size Grilles, Diffusers and Ducts
Air distribution design turns a room's load into airflow, then sizes the terminals, ducts and dampers that carry it. A step-by-step sequence for MEP consultants and contractors.
Air distribution design is the process of turning a room's cooling or heating load into a supply airflow, splitting that airflow across diffusers and grilles, and sizing the ducts that feed them so that velocity, pressure loss, throw and noise all stay within limits. Goodwind manufactures the terminals, dampers and duct accessories used at each step, linked below. For the central equipment that feeds the network, see our HVAC equipment size calculation guide.
The Air Distribution Design Sequence at a Glance
| Step | What you decide | Main input |
|---|---|---|
| 1. Room airflow | Supply airflow per zone | Sensible load and supply temperature difference |
| 2. Supply terminals | Diffuser type, size and count | Airflow per terminal, throw, noise target |
| 3. Return and exhaust grilles | Grille type and size | Return airflow and free area |
| 4. Ducts and branches | Duct, plenum box and flexible run sizes | Airflow and design friction rate |
| 5. Dampers and access | Balancing, fire, reverse-flow and service points | Layout, fire code, maintenance needs |
| 6. Fan pressure and balancing | Static pressure to overcome, site balancing | Pressure loss on the critical path |
Step 1: Convert the Room Load Into Airflow
Supply airflow follows from the room's sensible cooling load and the temperature difference between the room and the supply air. In SI units, airflow in L/s is roughly the sensible load in watts divided by 1.2 times the temperature difference in kelvin. In imperial units, CFM equals the sensible load in BTU/h divided by 1.08 times the temperature difference in °F. The load calculation supplies the load, and the supply temperature difference is set by the project's design basis.
As an example, a zone with a 6 kW sensible load supplied 10 K below room temperature needs about 500 L/s, roughly 1,050 CFM. This covers the sensible load only. Latent load affects the coil selection and the supply air condition, which in turn sets the temperature difference used here.
Step 2: Size and Select the Supply Terminals
Split the Airflow Across Terminals
Divide the zone airflow by the number of terminals the layout calls for. The layout comes first: terminals are spaced so that each covers a roughly even share of the floor area, and the airflow per terminal follows from that count.
Read the Data Sheet for Neck Velocity, Throw and Noise
For each size and airflow, the manufacturer's performance data gives neck velocity, pressure drop, throw and a noise level, usually as an NC value. Neck velocity is the airflow divided by the neck area. A smaller neck raises velocity, which adds noise and pressure drop and lengthens the throw. A larger neck does the opposite. Pick the size that meets the room's noise target and still reaches where the air needs to go.
Check the Throw Against the Room
Throw is the distance from the terminal to the point where air speed has dropped to a stated terminal velocity. Data sheets commonly quote it at 0.25, 0.5 and 0.75 m/s, about 50, 100 and 150 fpm. The throw should reach toward the nearest wall or the stream from the next diffuser without dropping air onto occupants. A common comfort target for the occupied zone is about 0.25 m/s (50 fpm) or lower, though the project's comfort criteria govern.
Match the Terminal to the Space
| Space or condition | Terminal | Model |
|---|---|---|
| Standard ceiling grid, directional throw | Square diffuser | GT-SD |
| Flat lattice face that drops into a ceiling grid | Grid diffuser | GT-GD |
| Flush ceiling with no visible pattern | Plaque diffuser, round plaque diffuser | See guide, GT-RPD |
| High air change rate or heavy internal heat gain | Swirl diffuser | GT-SWD |
| Low ceiling, gentle air spread | Pan diffuser | See guide |
| Round duct connection | Circular diffuser | See guide |
| Linear, architectural look | Slot diffuser, flowbar, trimless slot diffuser | G-SLT, GT-H-SLT, GT-T-SLT |
| Large, high spaces such as stadiums and airports | Jet diffuser, single skin spot nozzle, double skin eyeball nozzle | GT-JN, GT-SSSJN, GT-EB |
Step 3: Size Return and Exhaust Grilles by Free Area
Return and exhaust grilles have no throw to achieve, so the limits are noise and pressure drop. Velocity is the airflow divided by the free area, not the nominal face area. For example, 500 L/s (0.5 m³/s) through a grille with 0.25 m² of free area runs at 2 m/s.
Free area differs by type. An egg crate grille has high free area, while decorative, double louver and non-vision faces pass less air than an egg crate grille of the same face size. Use the free area on the data sheet, and size the grille from that figure rather than from the face dimensions.
| Situation | Grille | Model |
|---|---|---|
| High free area for return or exhaust openings | Egg crate grille | ECG-D001 |
| Architectural perforated face | Perforated grille | PF-BG |
| Round duct or circular opening | Round grille | GT-RG |
| Linear look, straight run | Slot or linear bar grille | G-LBG |
| Curved wall or ceiling | Linear curved grille | GT-LCG |
| Ornamental pattern | Decorative grille | G-DG |
| Reduced sightline | Double louver grille | G-DD |
| Fully blocked sightline | Non-vision grille | G-NVG |
For openings to the outside, see our louver guide, where weather performance matters alongside free area.
Step 4: Size the Ducts, Plenum Boxes and Flexible Runs
Duct Area From Airflow and Velocity
Duct area is the airflow divided by the design velocity. For illustration, 500 L/s at 5 m/s needs 0.1 m², which is about a 360 mm round duct. The velocity itself comes from the sizing method and the project's noise limits. Our duct sizing definition covers the basics, and three methods are in common use:
- Equal friction sizes every section for the same pressure loss per metre. A common starting point is 0.8 to 1.0 Pa per metre, about 0.1 in. w.g. per 100 ft, adjusted to the project's noise and space limits.
- Static regain sizes the duct after each branch so the static pressure regained from the drop in velocity offsets the friction in the next section. It is typically used on larger, higher-velocity mains.
- Velocity reduction steps the design velocity down by selected amounts along the run.
Plenum Boxes and Flexible Duct at the Terminal
A plenum box sits between the branch duct and the diffuser, and its neck should match the diffuser. Flexible duct is for the final connection only. Keep it short, fully extended and supported, because sag and tight bends add pressure loss. Specs for both are in our ductwork accessories guide.
Step 5: Add Dampers and Access Where the System Needs Them
- For balancing, use volume control dampers at branches and butterfly collar dampers or aluminium collar dampers at round takeoffs.
- To stop reverse flow through an idle fan, use a backdraft damper.
- For process and other heavy-duty airflow, use an industrial control damper.
- Where ducts cross fire-rated barriers, see where fire and smoke dampers are required.
- Place access doors beside dampers, coils and filters so they can be reached after the ceiling is finished.
Step 6: Check the Critical Path and Balance on Site
The critical path is the run with the highest total pressure loss. Add the duct friction, fitting losses and the terminal pressure drop at design airflow along that path to find the static pressure the fan has to overcome. That figure feeds the equipment selection, covered in our equipment selection criteria. After installation, balancing sets each branch and terminal to its design airflow, and commissioning confirms the system performs as designed.
Putting It Into a Specification
The formulas and examples above are general design practice, not Goodwind product ratings. Final sizes follow the project's design standard and the data sheet of the product selected.
For MEP Consultants at the Design Stage
Put Airflow, Neck Size and Noise Target on the Schedule
List the design airflow, neck size and NC target for each terminal rather than only a product name. A schedule that carries those figures lets the contractor check any substitute against the same criteria.
State Free Area for Returns
For return and exhaust grilles, give a minimum free area or a maximum face velocity, not just the face size. Two grilles with the same face size can pass very different airflows.
For Contractors at Procurement Stage
Check Submittals Against the Schedule
Compare the supplier's data sheet airflow, throw and NC values with the schedule before ordering, and flag any difference to the consultant.
Confirm the Whole Connection Chain
The damper, plenum box, flexible duct and terminal neck at each outlet need to match each other, not just the drawing individually. Confirm model codes with Goodwind before finalizing an order.
FAQ
What does air distribution design include?
It covers converting the room load to airflow, selecting and sizing supply terminals and return grilles, sizing ducts and branches, adding dampers and access points, and checking fan pressure and site balancing.
How do you size a diffuser for a room?
Divide the zone airflow by the number of diffusers, then choose the size from the data sheet that gives the required throw within the room's noise target at that airflow.
How do you calculate CFM for a room?
CFM equals the sensible load in BTU/h divided by 1.08 times the temperature difference in °F. In SI units, L/s is roughly the load in watts divided by 1.2 times the temperature difference in kelvin.
What is neck velocity and why does it matter?
Neck velocity is the airflow divided by the area of the diffuser's neck. It affects noise, pressure drop and throw, so it is one of the first values to check on a data sheet.
Which duct sizing method should I use?
Equal friction is a common starting point for low-velocity comfort systems. Static regain and velocity reduction suit specific layouts, and the project's design standard decides.
Planning a terminal layout or a full air distribution package? Contact Goodwind for technical input, or see our equipment size calculation guide for the central plant side.