Turning Vanes: Necessary Component, or Efficiency Reduction Device? Posted on December 11, 2009 The installation of turning vanes in HVAC ductwork is perhaps one of the greatest sources of contention between mechanical contractors and HVAC engineers. Why? Because many mechanical 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 time and expense to the overall 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 airflows. In some cases when an HVAC system is having particular difficulty in supplying the required amounts of airflow to all zones, many mechanical contractors will recommend the removal of every other turning vane at each fitting in the system in order to “reduce the friction” in the duct. This practice is a violation of SMACNA ® turning vane spacing requirements, and has also been condemned by the Air Conditioning Contractors of America, because it decreases the uniformity of the airflow and increases the pressure drop in the system). The question is, does reality match up with popular belief? THE FACTS: When airflow changes direction in a duct that lacks turning vanes, the walls of the duct must absorb the sudden impact of the air in order to reorient the airflow to the direction desired. Turning vanes assist the airflow in making a smoother and more gradual change in direction, resulting in less of an impact, and thus less force transferred (as airflow velocity increases, this effect becomes more pronounced). While the turning vane surfaces do add a small amount of friction, the amount of energy lost to friction from the vanes is nothing compared to the energy lost in the impact resulting from the airflow taking an abrupt or significant change in direction. Figures 1.1 and 1.2 below illustrate the airflow resistance that occurs in a 90° square elbow with and without t urning vanes. Figure 1.1