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RT-PRC010-EN10
Application
Considerations
Figure AC-1 — Plan View of Modulating 100 Percent Exhaust System
2
100 Percent Exhaust System
Competitive rooftops use a return air fan
system for controlling the amount of
exhaust air during economizer
operation. The return fan is in series with
the supply fan and must operate
whenever the supply fan is operating.
During economizer operation, the
economizer outdoor air dampers control
the position of the return and exhaust air
dampers, to exhaust the proper amount
of air. The disadvantage of a return air
fan is that it runs continuously, versus an
exhaust fan system which runs only
when needed to lower or maintain
building static pressure. Also, the return
fan must discharge air in two directions,
through the return air dampers and/or
exhaust air dampers, resulting in less
efficient operation compared to an
exhaust fan.
The IntelliPak® Rooftop unit offers
modulating 100 percent exhaust system.
This fan system has performance
capabilities equal to the supply fan. The
FC exhaust fans are started by the
economizer’s outdoor air damper
position and the exhaust dampers track
the economizer outdoor air damper
position. The amount of air exhausted by
this fan is controlled by modulating
discharge dampers at the fan outlet. The
discharge damper position is controlled
by a signal that varies with the position
of the economizer dampers. When the
exhaust fans start, the modulating
discharge dampers are fully closed, and
exhaust airflow is 15 to 20 percent of
total exhaust capabilities.
3
50 Percent Exhaust System
The 50 percent exhaust system is a
single FC exhaust fan with half the air-
moving capabilities of the supply fan
system. The experience of The Trane
Company is that a non-modulating
exhaust system selected for 40 to 50
percent of nominal supply CFM can be
applied successfully.
The 50 percent exhaust system generally
should not be selected for more than 40
to 50 percent of design supply airflow.
Since it is an on/off nonmodulating
system, it does not vary exhaust CFM
with the amount of outside air entering
the building. Therefore, if selected for
more than 40 to 50 percent of supply
airflow, the building may become under-
pressurized when economizer operation
is allowing lesser amounts of outdoor air
into the building. If, however, building
pressure is not of a critical nature, the
non-modulating exhaust system may be
sized for more than 50 percent of design
supply airflow.
4
Barometric Relief Dampers
Barometric relief dampers consist of
gravity dampers which open with
increased building pressure. As the
building pressure increases, the pressure
in the unit return section also increases,
opening the dampers and relieving air.
Barometric relief may be used to provide
relief for single story buildings with no
return ductwork and exhaust
requirements less than 25 percent.