Reducers
Reducers for galvanised spiral ducting — over 160 diameter combinations, stepping down from every starting size between 100mm and 1000mm. A reducer transitions the duct smoothly from one diameter to another, keeping air velocity where the design needs it as branches take their share of the flow.
Finding the right reducer
Reducers are organised by their larger (starting) diameter — browse the subcategory for your main duct size and every available step-down is listed. Combinations run from small steps such as 250mm to 200mm, through to major transitions like 1000mm to 500mm, so both gentle staged reductions and single big steps are catered for. Each reducer connects to standard spiral duct and fittings in the matching sizes, so a reduction drops into the run like any other component.
Where reducers fit in a system
The classic layout reduces the main duct after each branch take-off: as a T-piece or Y-piece sends part of the airflow down a branch, the main run steps down to keep transport velocity up — especially important in dust extraction, where slow air drops its load. Reducers also adapt fans, silencers and filter boxes to duct of a different size, and connect new ductwork to existing systems of another diameter.
When specifying, quote the two diameters in order — large end to small end — and check whether a single step or two staged reductions suits the run better; a gentle pair often flows better than one abrupt change. Stocked sizes ordered by 3pm are despatched for next-day delivery, and UK mainland delivery is free on orders over £400 ex VAT.
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From 100mm
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From 125mm
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From 150mm
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From 160mm
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From 180mm
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From 200mm
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From 224mm
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From 250mm
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From 280mm
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From 300mm
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From 315mm
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From 355mm
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From 400mm
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From 450mm
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From 500mm
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From 560mm
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From 600mm
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From 630mm
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From 710mm
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From 762mm
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From 800mm
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From 900mm
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From 1000mm
What Are Duct Reducers?
Duct reducers (also called tapers or transitions) are fittings used to connect two different sizes of ductwork. They provide a smooth transition between diameters, minimising turbulence and pressure loss at the size change. Reducers are one of the most commonly used fittings in any ductwork system.
Concentric vs Eccentric Reducers
Concentric reducers have both the large and small ends centred on the same axis. They are the standard choice for most applications and are typically used on horizontal and vertical duct runs where the duct centreline needs to remain consistent.
Eccentric reducers have the small end offset to one side, so that one face of the reducer is flat. They are used where the duct needs to maintain a flat bottom (for example, running along a ceiling) or where condensation drainage is a concern — the flat bottom prevents water pooling inside the reducer.
Sizing
Reducers are specified by their two end diameters — the large end and the small end. For example, a “200-150 reducer” transitions from 200mm to 150mm. Both ends have male (crimped) spigots that insert into the female ends of the connecting ductwork.
Why Choose Alpha Air?
We stock the full range of concentric and eccentric reducers in all standard DW/144 size combinations. Over 90% of orders dispatch same day when placed before 3pm. Free delivery on orders over £400.
Frequently Asked Questions
When should I use an eccentric reducer instead of a concentric one?
Use an eccentric reducer when you need to maintain a flat bottom on the ductwork — for example when running along a ceiling soffit. The flat face sits flush against the ceiling while the diameter change occurs on the opposite side. Eccentric reducers also prevent water pooling in horizontal runs.
Which end of a reducer is male and which is female?
Both ends of a standard DW/144 reducer have male (crimped) spigots. They insert into the female (plain) ends of the spiral duct on either side. This is the standard convention for all spiral duct fittings.