30 November 1998
c. The 100-percent outside air system may need a humidifier and its associated control loop. This
loop would function identically as discussed in paragraph 3-9 and shown in figure 3-13.
d. Figure 5-1 shows the variations that would occur for a 100-percent outside air unit. The outside
air preheat coil may be a heat recovery coil, which may be part of a glycol run-around system that
recovers heat in the air from one or more exhaust fans. The heating coil will not be part of the recovery
system. The system shown in figure 5-1 has a temperature/ humidity control sequence as described in
Chapter 4. If the system has a heat recovery coil, some additional controls may be required if they are
not part of a total heat recovery package. The exhaust fan is shown with a pneumatic damper actuator.
Device EP-XX02 is shown as field-mounted, but it may be located in a local fan starter or may be
included in the HVAC control panel. The designer will make this choice.
Figure 5-1. Schematic variations for 100-percent outside air systems.
e. The ladder diagram variations for the 100-percent outside air system are shown in figure 5-2,
which is similar to figure 4-21B, but modified to delete the relay and pilot lights associated with the
ventilation delay mode and with the system stops during the unoccupied mode. A smoke detector is
required in the supply fan discharge. Figure 5-2 shows an exhaust fan section of the ladder diagram on
lines 300 through 303. Each interlocked exhaust fan requires a contact of R-XX02 for safety shutdown
and a remote safety override, located at assigned terminal blocks in the HVAC control panel.
Figure 5-2. Ladder diagram variations for 100-percent outside air systems.
3. CONTROL SYSTEM VARIATIONS FOR EXHAUST FANS.
a. Schematic variations for an exhaust fan are shown in figure 5-3, for electric and pneumatic
actuators. Solenoid 3-way air valve EP-XX02 and damper actuator DA-XX02 will be energized to open
their respective dampers when their respective fans start. These devices will be powered from a source
other than the starter transformer, such as the HVAC control panel.
Figure 5-3. Schematic variations for exhaust fans.
b. Ladder diagram variations for exhaust fans are shown in figure 5-4. These variations are
somewhat different from the exhaust fan interlock shown in figure 5-2, which are intended for exhaust
fans that are interlocked to HVAC systems handling return air. One exhaust fan example is shown with a
pneumatic damper operator, for guidance in applying pneumatic actuators to shutoff dampers. No
positive positioner is required for the actuator. In this example, the fan is off and the dampers are closed
in the unoccupied and ventilation delay mode. Another example is shown with an electric damper
actuator, for guidance in applying electric actuators to shutoff dampers. In this example, the dampers
are open whenever the supply fan runs. The designer will add relays in parallel with R-XX01, R-XX03,
and R-XX11, as required to accommodate contacts to control additional exhaust fans. The designer will
show contact and coil references on the schematic and the ladder diagram for the additional relays.