Most ventilation problems do not start with the fan.
They start with what connects the duct.
After more than 35 years working with contractors across the UK and Ireland, we see the same pattern. The fan is sized correctly. Airflow calculations are sound. Yet the system still falls short.
Follow it back, and the issue nearly always sits in the fittings.
This guide covers the mistakes we see most often on site, and the habits that prevent them.
Why Duct Fittings Matter More Than Expected
Every fitting changes how air behaves.
A bend shifts direction. A T piece splits flow. A reducer increases velocity. A take off introduces a branch.
Each one adds resistance or creates turbulence, depending on how it is used.
Get the fittings right and the system works as intended. Get them wrong and even the best fan will struggle.
Duct fittings are not just connectors. They shape the performance of the entire system.
Mistake 1: Tight Bends Instead of Long Radius Bends
A sharp 90 degree bend is compact and easy to install. That is why it gets used so often.
The problem is airflow does not handle sharp turns well. It separates, creates turbulence, and increases pressure loss.
A long radius bend, or two 45 degree bends with a short straight between them, allows a smoother transition. Airflow improves. Noise reduces.
Use long radius bends as standard. Only use tight bends where space leaves no alternative.
In many cases, this single change improves performance more than upgrading the fan.
Mistake 2: Reducing Duct Size Too Early
Reducers have their place, but they are often installed too soon in the run.
Reducing diameter near the start forces the system to carry full airflow through a smaller duct. That drives up velocity, increases noise, and adds pressure.
Keep the main run at full size. Reduce closer to the terminals or branch points where airflow has already dropped.
Late reductions maintain efficiency and keep the system balanced.
Mistake 3: Poorly Sealed Take Offs
Air leakage at take offs is one of the most common issues on site.
Saddle take offs themselves are reliable. The problem usually comes from how they are fitted.
Openings cut too large. Poor seating. Incorrect sealing.
On systems with multiple branches, small leaks add up quickly. Overall performance drops below design levels. Airtightness classes for commercial systems are set out in the BESA DW/144 Specification for Sheet Metal Ductwork.
Cut accurately. Seat the fitting properly. Use the correct sealing method. Where required, test the system.
Mistake 4: Overusing Flexible Duct
Flexible duct has a role, but it is often pushed too far.
Common issues include:
Long runs instead of rigid duct Tight bends formed from flexible sections Compressed duct forced through tight spaces Used to correct alignment problems
Even when stretched, flexible duct creates resistance due to its internal profile.
Use it for short connections and minor adjustments. Let rigid duct and proper fittings define the route.
A well planned rigid system will always perform better.
Mistake 5: Mismatched Fitting Sizes
Accuracy matters in spiral systems.
Even small mismatches create leakage paths, noise, and sometimes vibration.
Check whether diameters are internal or external. Confirm compatibility across all components. Use the correct sealing method for each connection.
Spiral ductwork is designed to be airtight. That only holds true when everything fits as intended.
Mistake 6: Poor Support Around Fittings
Fittings are often heavier than the duct either side.
If support spacing is based only on straight duct, problems appear over time.
What we often see:
Sagging sections around fittings Stress at joints Dampers that no longer operate freely Failures after handover
Support fittings properly. Reduce hanger spacing where needed. Do not allow system weight to sit on dampers or terminals.
Mistake 7: Using the Wrong Fitting
Not all fittings serve the same purpose.
A balancing damper is not a fire damper. A T piece is not the same as a Y piece. An access panel is not a substitute for a proper inspection hatch.
Substitutions usually happen when the correct part is not available on site.
The replacement may look similar. It will not behave the same.
If a change is needed, confirm it properly. Do not swap components without checking.
Mistake 8: Ignoring Access Requirements
Some fittings need to be reached after installation.
Dampers require testing. Branches need inspection. Filters need changing.
Access is often overlooked early, then becomes impossible once ceilings are closed. Access panel requirements for commercial ductwork are detailed in CIBSE Guide B2: Ventilation and Ductwork.
Plan for it from the start.
Identify what needs access. Confirm physical space. Include access panels in the design.
If a system cannot be accessed, it cannot be maintained.
How to Avoid These Problems
The same issue sits behind most of these mistakes.
Fittings are treated as secondary.
The systems that perform well tend to follow a few simple rules:
Long radius bends built into the design Reductions placed later in the run Properly sealed connections Rigid duct forming the main structure Correct sizing across all components Adequate support throughout Specified fittings installed as intended Access planned early
None of this is complex. It just needs attention before installation begins.
Final Thoughts
Fans, duct sizing, and terminals all matter. They only deliver results when the fittings connecting them are chosen and installed properly.
If you are planning a project and want practical advice, our team at Fresh Air Supplies can help. With over 35 years of experience, in house manufacturing of galvanised spiral fittings, and strong stock availability across bends, T pieces, reducers, and take offs, we support contractors from design through to installation. Get in touch with our team today to discuss your project.