30 November 1998
Figure 5-4. Ladder diagram variations for exhaust fans.
c. The designer will check the exhaust fan selection for the rated shutoff static pressure to determine
whether a damper end switch should be applied to the control of the exhaust fan starter circuit. If the
shutoff static pressure is 1 inch of water column or higher, the designer will incorporate an end switch in
the design as described later in this manual under variations for smoke dampers.
4. CONTROL SYSTEM VARIATIONS FOR SMOKE DAMPERS.
a. Smoke dampers can be used with any HVAC air delivery system, but generally the smoke
dampers are required only when the HVAC unit exceeds a given air capacity. An example of the use of
smoke dampers is shown in figure 5-5. Actuators DA-XX04 and DA-XX05 will be powered from some
source other than the fan starter holding coil transformer, such as the HVAC control panel. The
contacts of relay R-XX12 must be closed to allow the smoke dampers to open.
Figure 5-5. Schematic variations for smoke dampers.
b. Figure 5-6 shows the ladder diagram variations for smoke dampers. When relay coil R-XX12 on
line 105 is energized, contacts in the power circuits to DA-XX04 and DA-XX05 are closed allowing the
smoke dampers to open. When the smoke dampers open, end switches ES-XX01 and ES-XX02 close
their contacts on line 20 to energize relay coil R-XX11 . When relay coil R-XX11 is energized, contacts
on lines 104 and 204 close, to allow starter holding coils MO1 and MO2 to energize and start the fans.
Tripping either end switch will shut down both fans. Relay coil R-XX12 is the only device that will be
powered from the starter holding coil transformers. Relay coil R-XX12 must be wired through the
overload relay contacts and must be powered in the "HAND" and "AUTO" positions of the
"HAND-OFF-AUTO" switch; it also must be powered in the event that the remote safety override circuit
is closed. Both dampers must open before either fan can start,
Figure 5-6. Ladder diagram variations for smoke dampers.
c. The designer will modify HVAC control panel layouts to show the relays and terminal blocks
associated with these variations.
5. CONTROL SYSTEM VARIATIONS FOR VARIABLE SPEED DRIVES.
a. Figure 5-7a shows variable speed drives in lieu of the supply and return fan inlet vanes. Relay
contacts in the supply duct static pressure control loop and the return fan volume control loop, which
open on supply fan shutdown, are not necessary because there are no inlet vanes.
Figure 5-7a. Schematic variations for variable speed drives.
b. The variable speed drive must accept a 4 to 20 milliamperes signal as an input.