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Duct reducers

A duct reducer joins two different diameters in your ventilation system. Match both ends to the duct, filter or extractor connection. Our range contains 13 Spiro reducers with size combinations from 125 to 406 mm. We help you choose the correct diameters and fit for your grow tent or grow room ventilation.

13 products

Choose your connection sizes

Which duct reducer fits your ventilation?

Start with the two components you want to join. Note each connection diameter in millimetres. For 160 mm ducting and a 200 mm connection, look for a 160/200 mm reducer. A 125/160 mm reducer bridges a different pair of sizes: matching only one end is not enough.

Next, check the fit. Does the reducer need to slide inside flexible ducting, over a spigot or onto another fitting? Nominal diameter alone does not establish how two parts fit together. Measure existing connections if needed and leave enough room to secure them.

All 13 versions are from Spiro, with combinations from 125/160 to 355/406 mm. Select the pair you actually need and avoid joining several intermediate reducers.

Group photograph of metal duct reducers with two different connection diameters

Adapting an existing system

Replacing an extractor or carbon filter? Compare the old and new connection sizes before ordering. A reducer can bridge the difference, but it does not establish the capacity of the complete system. Check your ventilation ducting diameter and length as well.

Building new ventilation

Where possible, choose ducting, a filter and a fan with connection sizes that fit directly. If a size change is necessary, use one suitable reducer and assess the entire air path. For equal diameters, choose a duct connector instead of a reducer.

Connect your ducting with care

A reducer changes the connection size. It does not make undersized ducting suitable for every airflow. Required air volume, duct diameter, duct length, bends and the filter all contribute to resistance. A smaller diameter can increase pressure loss at the same airflow, so do not automatically choose the smallest duct.

Check before switching on

  • Both diameters and the available insertion space suit the connected components.

  • The flexible duct has no sharp kink at the connection.

  • Each duct joint has suitable fixing and sealing.

  • The ductwork is supported independently and does not pull on the filter or fan.

Use duct clamps with the correct adjustment range for flexible ducting. Fixing and sealing have separate jobs: a clamp holds the joint, while sealing closes the gap. After assembly, check for loose connections and air leaks.

Advice on sizing and installation

All duct fitting guides

Frequently asked questions about duct reducers

Unsure about your connection sizes?

Tell us the diameters and connection types on your ducting, filter or extractor. We will help you choose a suitable reducer and the fixing you need.

  • Both connection diameters recorded

  • Fit and insertion space checked

  • Clamp range matched to the joint

  • Duct route free from sharp kinks

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Complete your ventilation run