Why Reducing the Diameter of the Ventilation Path is Not Worth It
Smaller Ducting = Bigger Problem
When installing ventilation, owners or installers sometimes decide to reduce the diameter of the ducting—for example, due to space limitations, aesthetics, or easier installation. While this may seem like a practical solution at first glance, in reality, it fundamentally reduces the effectiveness of the ventilation and overloads the fan.
What Happens When You Reduce the Duct Diameter
A ventilation system is always designed for a specific duct cross-section. If you reduce it, the air must flow faster to maintain the same volume flow rate. This leads to:
Increased air resistance in the ducting
Overheating and wear of the fan
Higher system noise
Lower performance and worse airflow
Reducing the duct diameter by as little as 25% can cause the volume of exhausted air to drop by up to 40%. In practice, this means that the air in the bathroom, kitchen, or toilet will not be exchanged sufficiently—leading to moisture, odour, and mold formation.
Noisier, Weaker, and Less Efficient Ventilation
When the fan has to overcome greater air resistance, it runs at a higher power and produces more noise. In the long term, this means higher energy consumption and a shorter motor lifespan. Therefore, reducing the diameter of the ventilation path is strongly discouraged in any case—even when transitioning to flat ducts.
How to Choose the Right Duct Size
For ventilation to work effectively, always adhere to the recommended duct diameter according to the fan type. Haco offers a complete range of circular and flat ventilation ducts that are fully compatible with each other:
Circular Ducting | Corresponding Flat Duct |
Ø 100 mm | 110×55 mm |
Ø 125 mm | 220×55 mm |
Ø 150 mm | 220×90 mm (New from October 2025) |
Thanks to these dimensions, you can easily adapt the air routing to the space without having to reduce the cross-section and lose performance.
Summary
Reducing the diameter of the ventilation path means reducing system efficiency, increasing noise, and shortening the fan's lifespan. If you want your ventilation to work quietly, economically, and reliably, stick to the prescribed duct dimensions—whether you choose Haco's round or flat solution.