Concentric Ducting Reducer

Duct reducers for your ventilation system

The duct reducer is the fitting that connects two ventilation components of different diameters. In an ideal world, every part of your extraction circuit would share the same size — in practice, that is rarely the case. Manufacturers of extractor fans and carbon filters use different standards, and when expanding an existing installation you often need to connect new components to older equipment with different dimensions. A reducer solves this elegantly without forcing you to replace the rest of your system.

Why the conical shape matters

Not every diameter transition is created equal. An abrupt transition (for instance a flat plate with two differently sized holes) creates turbulence at the point where airflow must suddenly narrow or widen. That costs effective airflow: your fan works harder for less useful throughput. A well-designed reducer uses a gradual conical profile in which the airflow transitions from one diameter to the other over several centimetres smoothly. The air stays laminar, pressure loss stays minimal, and you hear the difference immediately — a correctly installed conical system runs noticeably quieter than an abrupt reduction. All our reducers are manufactured from galvanised sheet steel resistant to the high humidity of an active grow room.

Eleven combinations for every configuration

A duct reducer always has two different diameters: a large end and a small end. Choose the combination that matches your extractor outlet and filter inlet (or vice versa). All eleven sizes are in stock:

• 100 → 125 mm — for compact fans on standard filters
• 100 → 150 mm — small fan, medium-sized filter
• 100 → 160 mm — significant jump in a single piece
• 125 → 150 mm — minimal transition, minimal pressure loss
• 125 → 160 mm — widely used when upgrading to a stronger fan
• 125 → 200 mm — large reduction for professional setups
• 150 → 160 mm — for intermediate brand-specific sizes
• 160 → 200 mm — typical when expanding from 120×120 cm to a larger filter
• 160 → 250 mm — large reduction for commercial installations
• 200 → 250 mm — smooth transition for high-end systems
• PVC 80-100-125-150 mm — universal multi-step reducer, PVC construction

The multi-step PVC reducer as a flexible solution

The PVC reducer 80-100-125-150 mm is a special product in our range. Instead of a single tapered transition, it features four steps, letting one product cover four different reductions. You simply cut it to the desired diameter and use only the part you need. Useful for growers who regularly switch between setups or want one reducer that covers multiple applications. The PVC construction is slightly lighter than galvanised steel and easier to work with standard tools, though the temperature range is lower (around 70 °C vs 140 °C for steel). More than sufficient for suction and exhaust systems in grow spaces.

When to combine with a duct coupling

A reducer has a solid steel connection at both ends where you slide ducting directly. However, if you need to join two same-diameter ducts in addition to the reduction, add a duct coupling on one side. Typical scenario: your extractor is in the loft, your filter hangs inside your grow tent, and you need to build a long duct run with a diameter change somewhere in between. Reducer + coupling + clamps give you exactly the configuration you need without improvised solutions.

Installation tips and common mistakes

Always install a reducer in the direction of airflow: air prefers to flow from small to large (a diffusing transition is more efficient than a converging one). When exhausting from a filter to a larger duct, the direction is automatically correct. Slide the duct at least 5 cm over each end of the reducer and secure with a matching duct clamp on the overlap area. Common mistake: placing the reducer directly against a 90° bend. Leave at least one duct-diameter of straight run between reducer and bend to avoid turbulence. Test the seal with your palm while the system runs — no air flow should be perceptible along the seams.

FAQ - Frequently asked questions about duct reducers

Order your duct reducer today at Plant & Flower. Fast, discreet delivery anywhere in Belgium.