This month's Engineer's Notebook evaluates methods to reduce fan energy, which accounts for 30% to 40% of building HVAC system energy, from internal pressure drop along with the increased initial costs and reduction in annual operating costs. Most air-handling units are still selected at the typical 500 fpm (2.5 m/s) rule-of-thumb, regardless of application, says the author, but energy efficiency proponents say that lower velocity is better for high-performance operation.