7
Curb and Ductwork
This fan was specified for a specific CFM and static pressure. The ductwork attached to this unit will
significantly affect airflow performance. When using rectangular ductwork, elbows must be radius throat,
radius back with turning vanes. Flexible ductwork and square throat/square back elbows should not be
used. Any transitions and/or turns in the ductwork near the fan outlet will cause system effect. System
effect will drastically increase the static pressure and reduce airflow.
• Table 1 shows the minimum fan outlet duct sizes and straight lengths required for optimal fan
performance.
• Do not use the unit to support ductwork in any way. This may cause damage to the unit.
• Follow SMACNA standards and manufacturer's requirements for the duct runs. Fans designed
for rooftop installation should be installed on a prefabricated or factory-built roof curb.
• Follow curb manufacturer’s instructions for proper curb installation.
• The unit should be installed on a curb and/or rail that meets local code height requirements.
• Make sure the duct connection and fan outlet are properly aligned and sealed.
• Secure fan to curb through vertical portion of the ventilator base assembly flange. Use a minimum of
eight (8) lug screws, anchor bolts, or other suitable fasteners (not furnished). Shims may be required
depending upon curb installation and roofing material.
• Verify all fasteners are secure. Figure 3 and Figure 4 show different mechanical installations.
• Adequate building relief shall be provided so as not to over pressurize the building when the heating
system is operating at its rated capacity. This can be accomplished by taking into account, through
standard engineering methods, the structure’s designed infiltration rate; by providing properly-sized
relief openings; or by interlocking a powered exhaust system; or by a combination of these methods.
• Heaters installed with intake ductwork must be purged to replace at least four air changes of the
volume of the intake duct.
• If the failure or malfunction of this heater creates a hazard to other fuel-burning equipment in the
building (e.g., when the heater is providing makeup air to a boiler room), the unit is to be interlocked to
open inlet air dampers or other such devices.
• On outdoor installations, it is recommended that the discharge duct be insulated to prevent
condensation during the “OFF” cycle in cold weather.
• Flexible connectors should be used on all ductwork connections. Vibration isolators are optional and
can be supplied in the loose parts package.
• Units that are installed in airplane hangars should be installed in accordance with the Standard for
Aircraft Hangars, ANSI/NFPA 409. Units that are to be installed in public garages should be installed in
accordance with the Standard for Parking Structures, ANSI/NFPA 88A, or the Standard for Repair
Garages, ANSI/NFPA 88B, and with CAN/CGA B149 Installation Codes.
Table 1 - Required Supply Ductwork
Duct Size Straight Duct Length
12” x 12” 36”
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