Range Hood Duct Size and Pa Guide
Range Hood Duct Size Guidance for Best Performance
To help your range hood perform at its best, we recommend using the original duct size wherever possible. The duct size, duct route and wall opening all affect how smoothly air can leave the property.
For many high-extraction ROBAM-style cooker hoods, the original duct size is around 18.5cm. If your existing wall opening is between 15cm and 18.5cm, the supplied adaptor rings or reducer accessories may usually help with installation.
If the wall opening is smaller than 15cm, we strongly recommend asking qualified personnel to enlarge the wall opening before installation.
Quick Recommendation
| Existing wall opening | Recommendation |
|---|---|
| 18.5cm | Best option. Closest to the original duct size and recommended for best performance. |
| 15cm to 18.5cm | Generally workable using suitable adaptor rings or reducer accessories. |
| 15cm | Usually the smallest recommended practical outlet size for this type of high-extraction hood. |
| 10cm | Too small. The wall opening should be enlarged by qualified personnel before installation. |
Why Original Duct Size Is Recommended
A powerful range hood is designed to move a large amount of air. The closer the ducting is to the original outlet size, the more smoothly the air can leave the room.
| Benefit | Explanation |
|---|---|
| Stronger extraction | Air can leave the room more smoothly. |
| Better smoke removal | Fewer restrictions help the hood clear cooking fumes more effectively. |
| Lower operating noise | Air does not need to rush through a very narrow opening. |
| Better motor efficiency | The motor works under more suitable conditions. |
| Better long-term performance | Reduced unnecessary pressure on the system. |
This does not mean the product is weak. In fact, powerful hoods benefit even more from correct ducting, because they are designed to move air efficiently through a suitable outlet.
Estimated Pressure Guide: 18.5cm vs 15cm vs 10cm
The table below gives a broad estimate for moving around 1200m³/h through different round duct sizes, including common bend situations. Most installations will have at least one bend unless the hood vents directly through the wall behind it.
| Duct / wall opening | No bend | 1 normal bend | 2 normal bends | 3 normal bends |
|---|---|---|---|---|
| 18.5cm original duct | ~92 Pa | ~161 Pa | ~230 Pa | ~299 Pa |
| 15cm reduced duct | ~213 Pa | ~373 Pa | ~533 Pa | ~693 Pa |
| 10cm small opening | ~1081 Pa | ~1892 Pa | ~2703 Pa | ~3514 Pa |
| Opening size | General interpretation |
|---|---|
| 18.5cm | Best option for airflow and efficiency. |
| 15cm | Generally workable, especially with suitable reducer or accessory rings. |
| 10cm | Not recommended. The wall opening should be enlarged by qualified personnel. |
Bends and Duct Route
Most installations will have at least one bend, and this is normal. However, softer and wider-radius bends are always preferred. Sharp bends, crushed flexible ducting or long duct routes can create unnecessary resistance.
| Duct route condition | Guidance |
|---|---|
| Short and straight ducting | Best performance. |
| One smooth bend | Common and generally acceptable. |
| Sharp 90° bend | Avoid where possible. |
| Multiple bends | Use a stronger model and larger duct where possible. |
| Ducting upstairs or over a long distance | A professional or higher-static-pressure model is recommended. |
| Flexible ducting | Avoid crushing, sagging or tight folding. |
General rule: use the shortest practical duct route, the largest suitable duct size, and the smoothest bends possible.
Entry-Level vs Professional-Grade Models
Many entry and mid-range hoods are suitable for normal domestic installations where the duct is short, smooth, and around 15cm to 18.5cm.
For more demanding installations, such as long ducting, ducting upstairs, several bends or more complex outlet routes, a professional-grade or higher-static-pressure model is more suitable.
| Installation condition | Suggested model type |
|---|---|
| Original 18.5cm duct / short route | Entry, mid-range or professional models. |
| 15cm opening / one smooth bend | Many entry or mid-range models may be suitable. |
| Long duct route | Higher-pressure model recommended. |
| Upstairs ducting | Higher-pressure or professional-grade model recommended. |
| Multiple bends | Higher-pressure or professional-grade model recommended. |
| 10cm wall opening | Enlarge the wall opening. Do not rely on the hood alone. |
Important Installation Disclaimer
This guidance is a broad reference only. The actual air volume moved by the hood after installation can vary depending on the full site condition. It should not be used as the only buying decision.
| Factor | Possible impact |
|---|---|
| Duct length | Longer routes create more resistance. |
| Number of bends | More bends make airflow less direct. |
| Bend angle | Soft bends perform better than sharp bends. |
| Duct diameter | Larger ducting supports smoother airflow. |
| Wall grille | Some grilles are more restrictive than others. |
| Backdraft flap | May add resistance. |
| Duct material | Smooth rigid ducting is usually better. |
| Duct age and cleanliness | Old or dirty ducting may affect airflow. |
| Reducer shape | Smooth reducers are preferred. |
| Installation quality | Poor sealing or crushed ducting affects performance. |
| Filter condition | Clean filters help maintain extraction. |
| Outdoor wind pressure | May affect outlet performance. |
Final Recommendation
For the best result, we always recommend using the original duct size supplied with the hood wherever possible. If the existing wall opening is between 15cm and 18.5cm, suitable adaptor rings or reducer accessories can usually help with installation.
If the existing wall opening is smaller than 15cm, especially a common 10cm outlet, we strongly recommend arranging for a qualified person to enlarge the wall opening before installation.
Ideal setup: original duct size where possible, short duct route, smooth bends, clean outlet, and no unnecessary restriction.