Grow Room Ducting Guide Australia
Grow Room Ducting Guide Australia
Correct duct sizing can make the difference between an inline fan delivering close to its intended airflow and a ventilation system that is noisy, restrictive and difficult to control.
This guide explains grow-room duct diameter, air velocity, pressure loss, flexible versus rigid ducting, bends, reducers, Y-pieces and fan installation effects.
Duct diameter should be selected around the airflow the room needs to move — not simply copied from the connection size on the fan.
Why Grow Room Ducting Matters
The fan is only one part of the ventilation system.
Air must move through:
Each component creates resistance.
As system resistance rises, most fans deliver less airflow.
Aim for:
- adequate diameter;
- short practical runs;
- smooth airflow paths;
- fully extended flexible duct;
- few bends;
- gradual transitions;
- and an unrestricted final discharge.
How to Choose Grow Room Duct Diameter
The first consideration is the airflow the duct needs to carry.
For round duct:
Air velocity is:
For the same airflow, reducing duct diameter increases air velocity.
Higher velocity generally means:
- greater straight-duct friction;
- greater fitting pressure losses;
- more aerodynamic noise;
- and more pressure for the fan to overcome.
Example: 500 m³/h Through Common Duct Sizes
| Duct Diameter | Approx. Air Velocity | Approx. Velocity Pressure | Practical Observation |
|---|---|---|---|
| 100 mm | 17.7 m/s | 188 Pa | Very high velocity for 500 m³/h |
| 125 mm | 11.3 m/s | 77 Pa | High velocity |
| 150 mm | 7.9 m/s | 37 Pa | Much more practical |
| 160 mm | 6.9 m/s | 29 Pa | Lower resistance potential |
| 200 mm | 4.4 m/s | 12 Pa | Lower velocity and noise potential |
| 250 mm | 2.8 m/s | 5 Pa | Low velocity |
| 315 mm | 1.8 m/s | 2 Pa | Very low velocity at this airflow |
The duct still has to:
- fit the installation;
- connect sensibly to the fan and filter;
- maintain useful air distribution;
- and remain commercially practical.
Round Duct Pressure Loss Calculator
Estimate velocity, straight-duct friction and fitting pressure loss for a round grow-room duct.
This is a planning calculator using Darcy-Weisbach and an iterative Colebrook friction-factor approximation. Actual manufactured duct and fitting losses can differ. Use published manufacturer or engineering fitting data where available.
How Duct Pressure Loss Is Calculated
Straight duct pressure loss can be represented using the Darcy-Weisbach relationship:
Where:
- ΔP = pressure loss;
- f = friction factor;
- L = duct length;
- D = duct diameter;
- ρ = air density;
- V = air velocity.
Fittings are commonly represented using:
This explains why the loss created by a bend cannot accurately be represented by one fixed percentage at every airflow and duct diameter.
Flexible Duct vs Rigid Duct
Advantages:
- easy to install;
- easy to route;
- inexpensive;
- useful around grow tents and movable equipment.
Disadvantages:
- rough internal surface;
- more resistance than smooth rigid duct;
- can sag;
- can kink;
- and becomes dramatically more restrictive when compressed.
Advantages:
- smoother internal surface;
- lower friction potential;
- maintains its cross-sectional shape;
- better suited to long permanent runs.
Disadvantages:
- less flexible;
- requires more deliberate fitting and support;
- can be less convenient around temporary installations.
Stretch Flexible Duct Fully
One of the easiest ways to create unnecessary ventilation resistance is to install flexible duct in a compressed condition.
The internal helix becomes more pronounced and greatly increases turbulence and pressure loss.
ASHRAE Flexible-Duct Example
ASHRAE provides a useful engineering example using approximately:
- 250 mm duct;
- 1.8 m installed length;
- 470 L/s airflow;
- and approximately 9.6 m/s duct velocity.
| Duct Condition | Total Pressure Resistance | Increase vs Rigid Spiral |
|---|---|---|
| Rigid spiral metal | 7.7 Pa | Baseline |
| Flexible — fully stretched | 11.4 Pa | 48% higher |
| Flexible — 4% compressed | 19.5 Pa | 153% higher |
| Flexible — 15% compressed | 40.0 Pa | 419% higher |
| Flexible — 30% compressed | 68.9 Pa | 795% higher |
The practical lesson is simple: cut flexible duct to an appropriate length and install it fully extended. Do not leave several metres of unused duct concertinaed behind the fan.
How Much Airflow Does a 90-Degree Bend Reduce?
There is no single correct percentage for every 90-degree bend.
Loss depends on:
- duct velocity;
- diameter;
- bend radius;
- elbow construction;
- surface roughness;
- and airflow entering the bend.
Avoid Tight Bends Directly at the Fan
Fan performance data is generally established under controlled test conditions with relatively uniform airflow entering the fan.
A tight elbow positioned immediately against the inlet can feed the impeller with non-uniform or swirling air.
This creates what fan engineers call system effect.
System effect can also increase:
- noise;
- vibration;
- power requirement;
- and mechanical stress.
AMCA guidance strongly recommends keeping inlet elbows approximately three duct diameters away from axial or centrifugal fan inlets where practical. This is engineering guidance rather than a mandatory grow-tent installation rule, but it illustrates why a sharp elbow directly against a fan can reduce performance.
Fan Outlet Conditions Matter Too
Air leaving a fan may not immediately have a uniform velocity profile.
Putting a tight bend or major obstruction directly against the discharge can create additional system effect before the airflow has had an opportunity to develop.
Reducers and Duct Transitions
Reducers allow components with different connection diameters to be joined.
They are useful, but a reducer does not make the smaller downstream duct behave like the larger duct.
The long 150 mm section still determines much of the downstream velocity and friction.
Use Gradual Transitions Where Practical
An abrupt area change generally creates more turbulence and pressure loss than a properly designed gradual transition.
Y-Pieces and Multiple Duct Branches
A Y-piece allows one duct to divide into two branches or two ducts to combine into a common main.
The airflow does not automatically divide equally.
A shorter, larger or less restrictive branch will generally take a greater share of the total airflow.
Two Equal Branches Still Need Enough Main-Duct Area
For comparison:
| Configuration | Approx. Cross-Sectional Area |
|---|---|
| One 150 mm duct | 177 cm² |
| Two 150 mm ducts combined | 353 cm² |
| One 200 mm duct | 314 cm² |
| One 250 mm duct | 491 cm² |
This shows why combining two sizeable branches into a smaller main duct can increase velocity significantly.
If equal branch airflow matters, design the branches with similar resistance or provide a means of balancing and measuring them.
Multiple Fans on One Duct System
Do not simply add the maximum ratings of two fans and assume that is the combined installed airflow.
The result depends on:
- whether the fans are in series or parallel;
- shared duct resistance;
- branch geometry;
- backflow paths;
- and each fan's performance curve.
Seal Duct Joints
Duct leakage can alter airflow and filtration performance.
The effect depends on where the leak occurs.
A leak before the fan can pull unintended air into the duct system.
If the leak is downstream of a carbon filter but upstream of the fan, untreated air may bypass the filter.
A leak after the fan can release air before it reaches the intended discharge location.
Use appropriate clamps, mechanical connections and sealing methods so the air travels through the intended path.
Support Flexible Duct Properly
Long unsupported flexible duct can sag between support points.
Sagging:
- increases effective duct length;
- distorts the internal cross-section;
- creates additional turbulence;
- and can collect condensation in some installations.
Acoustic Ducting and Ventilation Noise
Acoustic ducting can help reduce sound transmitted along the ventilation path, but it should not be used as a substitute for correct duct sizing.
A noisy system may be caused by:
- excessive duct velocity;
- fan speed;
- turbulent bends;
- restrictive outlets;
- structure-borne vibration;
- or ductborne fan noise.
Grow Room Duct Design Sequence
A better design process is:
Common Grow Room Ducting Mistakes
| Mistake | Why It Causes Problems | Better Approach |
|---|---|---|
| Choosing duct solely from fan flange size | The flange may not be ideal for a long high-airflow run | Check duct velocity and resistance at required airflow |
| Leaving flexible duct compressed | Compression can dramatically increase pressure loss | Cut to length and fully extend it |
| Using several tight 90° bends | Adds turbulence and fitting losses | Use fewer, smoother changes of direction |
| Tight elbow directly on fan inlet | Can create non-uniform fan inlet flow and system effect | Provide straight approach duct where practical |
| Immediately reducing a large fan to small duct | The smaller run creates high velocity and resistance | Maintain adequate main-duct diameter |
| Assuming Y branches split equally | Air divides according to branch resistance | Balance branches where equal airflow matters |
| Crushing duct behind equipment | Reduces area and increases restriction | Maintain full duct shape |
| Ignoring outlet restriction | The final grille or opening can become the bottleneck | Size the discharge for the design airflow |
| Ignoring duct leaks | Air may bypass filtration or intended discharge | Clamp and seal connections |
| Reducing noise by choking airflow | Creates additional resistance and turbulence | Reduce fan speed or improve duct/acoustic design |
Ducting Troubleshooting
The fan sounds powerful but airflow is weak
Check for compressed flexible duct, small duct diameter, blocked intake openings, carbon-filter restriction, tight bends and a restrictive final outlet. Fan sound is not a reliable measurement of airflow.
Opening the tent door dramatically increases airflow
The intake path is likely restrictive. The exhaust fan is receiving easier replacement air through the open door. Review passive intake free area, screens and intake ducting.
The duct is much louder after adding a reducer
The smaller downstream diameter may have increased air velocity and turbulence. Check the required airflow against the velocity in the smaller duct.
One branch of a Y-piece moves much more air than the other
The two branches likely have different resistance. Compare diameter, length, bends, filters and outlet restrictions. Airflow does not automatically split equally.
Can I simply use a bigger fan to overcome bad ducting?
A stronger fan may overcome some resistance, but correcting an undersized or poorly routed duct system can reduce noise, pressure requirement and power consumption while improving delivered airflow.
Continue Designing Your Ventilation System
Calculate the delivered airflow your grow tent or room actually requires.
Understand carbon-filter ratings, pressure drop and fan matching.
Learn how replacement air, passive intake and negative pressure affect exhaust performance.
Shop Grow Room Ducting at Green Genius
Green Genius stocks flexible ducting, acoustic ducting, reducers, Y-pieces and other ventilation equipment for grow tents and indoor growing spaces.
Grow Room Ducting FAQs
What size ducting should I use for a grow room?
Choose the duct size from the airflow the room needs to carry. Larger diameter duct generally reduces velocity and pressure loss for the same airflow. The fan flange size is a useful starting point but should not be the only consideration.
Is rigid duct better than flexible duct?
Smooth rigid duct generally provides lower resistance and better shape control, making it attractive for longer permanent runs. Flexible duct is convenient for grow tents and short connections but should be installed fully extended and without kinks or compression.
How much airflow do I lose through a 90-degree bend?
There is no universal percentage. Loss depends on the duct velocity, bend radius and geometry. Broad smooth bends generally create less loss than tight elbows or kinked flexible duct.
Can I reduce a 200 mm fan to 150 mm duct?
Physically, a suitable reducer can connect the two sizes. Whether it is a good design depends on the required airflow, length of the 150 mm section and the resulting pressure loss and noise.
Should flexible duct be stretched tight?
It should be installed fully extended without unnecessary compression. Compressed flexible duct can create dramatically greater pressure resistance than the same duct when fully stretched.
Does a Y-piece split airflow equally?
Not necessarily. Airflow divides according to the resistance of each branch. Equal airflow requires sufficiently similar branches or deliberate balancing.