An air handling unit is isolated with internal springs, and the fan discharge inside the AHU is flex connected to the AHU housing. The indoor AHU is floor mounted to a 6″ concrete housekeeping pad in the mechanical room on the ground or second floor of a two story building. Are flex connections required for the supply and return air duct connections to the AHU housing? Is it redundant? What are the pros and cons of having the flex connections?
The answer to the first questions is YES. If you want optimal reduction of noise and vibration in your facility, flex connectors on the supply and return to the AHU housing are required. Even though the fan in the AHU is internally isolated and equipped with a flex connector, it does not mitigate the casing-radiated noise that travels from the AHU into the building structure and the supply and return ductwork. Casing-radiated noise, sometimes referred to as panel resonance, is structure-borne noise (vibration) caused by the airborne sound of the fan and other equipment in the AHU impinging on the walls of the AHU. This airborne sound energy along with any turbulence in the AHU causes the AHU housing to vibrate. Flexible connectors between the AHU and the supply and return ductwork will reduce the transmission of vibration from the AHU into the ductwork. For this same reason, we also recommend the use of external vibration isolation. The internal vibration isolators supplied to isolate the fan only isolates vibrations from the fan. External vibration isolation will eliminate vibration coming from the AHU housing. If you think you have or may have a vibration problem, we suggest you contact your local Acoustical Consultant before you start your repairs. A trained acoustical engineer can help you identify your problem(s) and potentially save you a bundle in the end. If using internal or external isolation, please refer to the question “Should both internal and external vibration isolation be used in a single application?” below.