Reducer

A reducer joins two duct sizes together. You need one as soon as your fan, filter and ducting are not all the same diameter, which happens more often than you would expect.

One size down is not free

The same volume of air through a narrower duct means the air has to move faster. Halve the cross section and the speed doubles, and resistance climbs with roughly the square of that speed. Going from 200 to 160 mm sounds like a small step, but the area drops by almost forty percent.

You notice it in two ways. Your fan delivers less than it can on paper, and you hear it: fast air hisses, and that noise is generated precisely at the restriction. Anyone chasing a quiet system looks at the narrowest point in the run first.

So reduce as late as possible and as little as possible. A reduction just behind the fan is nearly always better than one halfway along a long run, and one step from 250 to 200 beats two steps back to back.

Available sizes

Technical Specifications:

• Function: transition between two duct diameters
• Transitions: eight combinations, from 125 to 250 mm
• Build: suits spiral pipe and flexible ducting
• Fixing: tape, worm clamps or connecting sleeves

Frequently asked questions

Can I use it the other way, narrow to wide?
Yes. Expanding costs less than narrowing, so that is the kind direction.

Do I lose capacity through a reducer?
Little through the piece itself. The loss sits in the narrower duct behind it, not in the transition.

Could I just use tape instead?
You could, but a hose squeezed shut is exactly the sharp restriction you are trying to avoid.