Installation Guidelines for Supply and Return Air Grilles

When choosing the proper size grilles for any system, primary cosideration should be given to the capacity of that system, primary consideration should be gien to the given to the capacity of that system and its necessary air flow (CFM). Reduced or restricted air flow due to undersized grilles or poor installation will result in loss of performance and /or inefficient operation of the system.

Standard air flow for any system is normally rated at approximately 400 CFM (cubic feet per minute) per ton (12,00 BTU/Hr). In order to maintain this air flow return air grilles are to be a minimum of 140 square inches of surface area per ton. If the grille is in close proximity to hte evaporator coil, it should match the actual dimensions of the coil as closely as possible to prevent obstruction of air flow through part of the coil which would result in loss of performance.

Supply air grilles are to maintain of 70 square inches of surface area per ton. This can maintained using a single grille or a combination of smaller grilles, provided proper installation methods are followed. In either case, even and unrestricted air flow across the entire surface of the grilles(s) is the ulimate goal in any installation.

Placement of grilles is important to ensure proper system performance. Supply air grilles are to be located as high as possible in any cabin area. They should also be located to distribute air across the cabin as evenly as possible. Air flow across the ceiling of any area provides for even temeperature (no hot spots) and complete circulation of the air.

When supply air grilles cannot be located high and must be installed lower (i.e., countertop, etc.), they are to be installed in such a way as to channel air flow up and across the conditioned area. Properly directed louvers and built-up wedges both help to achieve proper air circulation in this application.

In no insatance shoulf a supply air discharge be directed towards a return air grille. This will cause the system to prematurely cycle off(short cycle) and prevent the cabin area from achieving the desired temperature.

Return air grilles are to be installed low in a cabin area and in close proximity to the evaporator coil of the system. If any plenum, chase or duct work is constructed between the return air grille and the face of the evaporator coil, all cross-sectional areas must maintain a minimum surface area equivalent to that of the return air grille. If any open area exists between the grille and the coil, care must be used to prevent this area from being obstructed(stowage, parts, etc.). REturn air should only be available to the system from the cabin area(s). Units should not be exposed to engine rooms, bilges or other areas where fumes or odors could be drawn into the system.

Compartments housing equipment should be completely selaed from these areas.

Some return air grilles are manufactured with a removable filter attached. This filter must be used to catch and trap particles in the air (dust, lint, etc.) that otherwise would be deposited on and in the evaporator coil, restricting the air flow through it. The louvre section of each grille is designed to be easily removed (clips or bullet catches) to access the filter material for cleaning on a regular basis. If no filter is attached to the grille, then a filter must be installed on the a/c unit.

Clip-on filters are available for applications where standard return air grilles are not used. These filters attach directly to the front of the evaporator coil, and must be checked on a regular basis.