Using Ceiling Space as a Return Air Path in HVAC Systems

In all air systems, supply air from air handling units is provided through ducting network to areas where it is required. Location of AHU is at a distance from supply air grilles. For recirculation, air from occupied space is carried back to the unit. Different options are available for return air routes. Each option has advantages and disadvantages.

Just like supply air, return air may be fully ducted. Return grilles are located in the ceiling or on the wall. Return duct picks air from these locations. Branch return ducts are connected to main return trunk. This may run horizontally above ceiling space parallel to the supply ducting or it may be turned vertically up or down to AHU room location. Disadvantages of ducted return systems are the additional space above false ceiling. Floor height is greater in buildings with ducted return systems. As supply and return ducts may cross during distribution floor to ceiling height is selected accordingly. It means more building space is used for services. Architects and building owners sometimes show concern about extra space taken by ducted return system. Another disadvantage is the fan power requirement for return air duct system. Ducts and bends put some resistance to air flow. Return air fan capacity is higher for ducted return system.

 Advantages of this system include ease of cleaning and maintenance. Ducts can be cleaned when dust deposits inside. These are cleaned with suction through access doors. Indoor air remains clean. Fire spread risk is less when air is contained in ducts and insulated with fire rated material. With insulation heat loss and gain is very little.

Another option for return air is to use spaces above false ceiling as return air plenum. With this system there are no return ducts. Air from spaces is brought to ceiling void through return grilles. This plenum is sealed from sides. Return air is channeled to air handling unit room through masonry ducts or with small duct connections. With free flow of air, there is not much pressure drop. Return air fan energy consumption is low. Economizer cycle can be operated without relief fan. A damper is required at any location in the return plenum. During exhaust mode, this damper opens and air is discharged out due to pressure differential.

Disadvantages of this system is the difficulty of cleaning. As whole ceiling space is used as return air path, cleaning cavities becomes a cumbersome job. Where materials used for suspended ceiling are not fire rated, return air ceiling plenums are not used. In this case return air becomes serious fire spread hazard.