IM 178 / Page 11
Ventilation clearance
Following are minimum ventilation clearance recommenda-
tions. The system designer must consider each application
and assure adequate ventilation. If this is not done, the unit
will not perform properly.
Unit(s) surrounded by a screen or a fence:
1. The bottom of the screen or fence should be at least 1 ft.
(305 mm) above the roof surface.
2. The distance between the unit and the screen or fence
should be as described in “Service clearance.”
3. The distance between any two units within the screen or
fence should be at least 120" (3048 mm).
Unit(s) surrounded by solid walls:
1. If there are walls on one or two adjacent sides of the unit,
the walls may be any height. If there are walls on more than
two adjacent sides of the unit, the walls should not be
higher than the unit.
2. The distance between the unit and the wall should be at
least 96" (2438 mm) on all sides of the unit.
3. The distance between any two units within the walls
should be at least 120" (3048 mm).
Do not locate outside air intakes near exhaust vents or
other sources of contaminated air.
If the unit is installed where windy conditions are com-
mon, wind screens should be installed around the unit,
maintaining the clearances specified above. This is particu-
larly important to prevent blowing snow from entering out-
side air intakes.
Overhead clearance
1. Unit(s) surrounded by screens or solid walls shall have no
overhead obstructions over any part of the unit.
2. The following restrictions shall be observed for overhead
obstructions above the air handler section (see Figure 4):
a. There shall be no overhead obstructions above the
furnace flue, or within 9" (229 mm) of the flue box.
b. Overhead obstructions shall be no less than 2" (51 mm)
above the top of the unit.
c. There shall be no overhead obstructions in the areas
above the outside air and exhaust dampers that are
farther than 24" (610 mm) from the side of the unit.
Figure 4. Overhead clearance
24"
(610 mm)
Max.
2" (51 mm) Min.
Top of Unit
To Overhead
Obstruction
24"
(610 mm)
Max.
Overhead
Canopy
Flue Box
9" (229 mm)
Min. To Flue Box
Typical All Sides
The roof curb and unit must be located on a portion of the roof
that can support the weight of the unit. The unit must be
supported to prevent bending or twisting of the machine.
If building construction could allow the transmission of
sound and vibration into the occupied space, it is recom-
mended that the unit be located over a noncritical area. It is
the responsibility of the system designer to make adequate
provisions for noise and vibration in the occupied space.
The curb and unit must be installed level to allow the
condensate drain to flow properly.
Integral supply and return air duct flanges are provided
with the RDS roof curb, allowing connection of ductwork to
the curb before the unit is set. The gasketed top surface of
the duct flanges seals against the unit when it is set on
the curb. It is not recommended that these flanges support
the total weight of the ductwork. Refer to the “Installing
ductwork” section for details on duct connections. It is
critical that the condensate drain side of the unit be no higher
than the opposite side.
Assembly of a typical RDS roof curb is shown in Figures
5a and 5b. Parts A through H are common to all units having
bottom return openings. Depending on the unit length, Parts
L and M may be included with the roof curb kit to create the
correct overall curb length.
Roof Curb Assembly and Installation
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