The Ducting Bible: Why Size Matters (125mm vs 150mm)
You have chosen the perfect cooker hood. It looks stunning, the motor is powerful, and it’s ready to be the centrepiece of your kitchen. But there is one invisible factor that can ruin everything: The Ducting.
At Luxair, the number one reason for customer inquiries regarding "noisy hoods" or "poor suction" isn't a fault with the hood—it is incorrect ducting. Many kitchen fitters default to standard 125mm (5") ducting because it is easier to install, but for high-performance motors, this is a critical mistake.
This guide explains exactly why upgrading to 150mm (6") is the single best investment you can make for your kitchen's air quality.
The Science: What is "Back Pressure"?
Imagine trying to run a marathon while breathing through a drinking straw. You would gasp for air, your face would turn red, and you would make a lot of noise. This is exactly what happens to a powerful cooker hood motor when connected to small ducting.
When you restrict the airflow:
- Noise Increases: The air becomes turbulent as it fights to squeeze through the pipe.
- Motor Strain: The motor has to work twice as hard, reducing its lifespan.
- Performance Drops: You might pay for 850 m³/hr extraction, but you may only get 400 m³/hr.
The Comparison: 125mm (5") vs 150mm (6")
The difference of one inch (25mm) might seem small, but in terms of airflow volume, it is massive.
| Duct Size | Efficiency Status | Luxair Verdict |
|---|---|---|
| 150mm (6") Round | 100% Efficiency | The Gold Standard. Required for all Luxair motors over 750 m³/hr. |
| 125mm (5") Round | ~35% Loss | Acceptable for lower power hoods, but will increase noise levels. |
| 100mm (4") Round | Major Failure | Do not use. This will void your warranty and suffocate the motor. |
Why does 125mm cause a 35% loss?
It comes down to cross-sectional area. A 150mm pipe holds roughly 44% more air volume per metre than a 125mm pipe. By stepping down to 5", you are physically choking the extraction rate of your new appliance.
The "Ceiling Void" Solution: Rectangular Ducting
We understand that not everyone has the space for large round pipes, especially in modern flats or when installing Ceiling Cooker Hoods within floor joists.
If you cannot use 150mm round ducting, the correct equivalent is 220mm x 90mm Rectangular Ducting. This flat channel provides nearly the same internal area as the 6" round pipe, allowing the air to flow smoothly without the noise penalties of smaller circular pipes.
Key Installation Tips for Joiners
- Avoid "Flexi-Hose" where possible: Flexible ribbed ducting causes friction (noise). Use rigid smooth pipe for 90% of the run, and only use flexi for the final connection.
- Reduce Bends: Every 90° bend in your ducting run reduces efficiency by roughly 33%. Try to keep the run as straight as possible.
- Check the Vent: Ensure your external wall vent has flaps that open fully. We often see vents that are clogged or painted shut!
The Luxair Guarantee
All Luxair Cooker Hoods are designed with powerful motors that are capable of clearing the air in large, open-plan kitchens. To protect this performance—and your 7-Year Warranty—we highly recommend planning your ducting route before you fit the kitchen cabinets.
Need help calculating your route? Contact our technical team for advice on the best ducting kits for your specific model.
