All grilles are undersized from the nominal-duct dimensions in order to fit ductwork, and the linear units are no exception. The stack head is the part of the grille that is installed into the wall opening and associated ductwork or plenum. We start our sizing from the inside of the stack head because this is the business part of the unit through which the air passes and performance data is consistent regardless of the border width.
The inside of the linear bar grille stack head is undersized from the nominal, or duct dimension by ¾”. The floor frames, type 5 and 6, are the exception at 3/8". The undersized dimension represented in our marketing literature is D-¾”, or “Duct” minus ¾”. If a dimension is specified as 12, the inside of the stack head will be 11-¼”; This applies to the length and width of a unit. The outside or overall dimension is 11-¼” plus two border widths (Plus two mounting-frame widths for the combination frames that use both a border and frame).
Cut dimensions for mounting the units can vary from the loose fit of the specified D dimension to a tight fit of D-5/8”. The tighter fit is beneficial to the screw-mounting option as it provides the most overlap or “meat” into which the screw is installed, particularly for sheet-rock surfaces. The loose fit of a D cutout is flexible and should be used for combination frame & borders, spring-clip mounting, concealed-mounting, or to provide clearance for a plenum boot -- by others -- to be installed. Titus does not provide plenum boots for linear bar diffusers.
Floor frames (Type 5, 6 & 15) and combination frames that utilize a mounting frame (Types 1 through 4) are a little different in that the outside of the stack head or mounting frame correlates exactly to D. For these units, the cutout should be slightly oversized at D+1/8” to provide clearance for the weld beads at the corners of the frames.
There are two particular notes I would like to make involving narrow frame styles 7, 11 and 12. First, the type 11 and 12 frames offer a screw-mounting option (A). When the A option is used, it is important to consider upsizing the grille or making sure that the tight cutout used will provide a secure base for screw-mounting. If the D dimension is used, the screw holes provided in the frames may coincide with the opening; Installation becomes much more complicated. Concealed mounting is the preferable mounting option for these frames.
The type-7 frame does not offer the screw-mounting option, so a concealed fastening must be used. This frame style was originally designed for the type-4 combination frame, but the narrow border width makes it attractive to those desiring the least amount of exposed grille for aesthetic purposes. You will note that because of its primary use as the core of a combination frame, the D dimension actually falls outside the overall (O) dimension. Therefore, a unit size of 12" x 12” should not be installed in a 12" x 12” cutout. The tight fit cutout of D-1/2” should be used, or the unit needs to be oversized by 1/2” if the D cutout will be used.