By Norm Grusnick, PEng
Commercial Sales Rep, PACAIRE HVAC Suppliers Ltd.
Turning vanes in HVAC ductwork are perhaps one of the greatest sources of conflict between sheet metal contractors and HVAC engineers. Why? Because many mechanical/sheet metal contractors believe that turning vanes can cause the ductwork to become less efficient by increasing the pressure drop in the system as well as adding expense and time to the installation.
This belief seems to be based in simple logic: when there is more surface area exposed to the airflow, the amount of friction will be increased, and the harder the fan must work to achieve the required airflow. In some cases where the HVAC system is having difficulty in supplying the required volume to all zones, many contractors want to recommend the removal of every second turning vane to reduce the friction in the duct. This practice is in violation of SMACNA turning vane spacing requirements because it decreases the uniformity of the airflow and increases system pressure drop. The real question is, does reality match up with popular beliefs?
When there is a change in direction in a duct that has no turning vanes, the duct walls must absorb the sudden impact of the air in order to re-direct the airflow to the desired direction. Turning vanes assist the airflow in making a smoother and more gradual change in direction, thus less impact and less force transferred to the duct walls. While the turning vane surfaces do add a small amount of friction, the amount of energy lost to friction from the vanes is very little compared to the energy lost in the impact resulting from the airflow taking an abrupt change in direction. Figures 1(a) and 1(b) show the airflow resistance that occurs in a 90-degree elbow with and without turning vanes.
From these figures, we can see that the elbow with turning vanes is much more effective than the same elbow without the vanes. If the client desires a less expensive installation, the design engineer may specify a radius elbow without turning vanes. A radius elbow is still highly effective, but with a recommended 1.5 center line radius it occupies a lot more space. Figure 1(c) shows a radius elbow.
In all cases noted, it is clear that as the airflow changes direction more gradually, the fitting pressure drop decreases and with it the energy required by the system fan to supply the desired airflow volume.
Turning vanes have been proven very valuable for reducing pressure losses and increasing system efficiencies. Designers should always specify the highest efficiency fittings possible to increase system efficiency at every available opportunity. Contractors should never take it upon themselves to add or remove turning vanes from the engineer’s design.
References
American Society of Heating, Refrigeration, and Air-Conditioning Engineers Inc. (2025). ASHRAE Handbook-Fundamentals.
Sheet Metal and Air Conditioning Contractors National Association Inc. (2020). HVAC Duct Construction Standard-Metal and Flexible (4th edition).■
