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22 RT-PRC031-EN
Exhaust/Return
Fan Options
When is it necessary to provide
building exhaust? Whenever an
outdoor air economizer is used, a
building generally requires an
exhaust system. The purpose of the
exhaust system is to exhaust the
proper amount of air to prevent over
or under-pressurization of the
building.
The goal is to exhaust approximately
10 percent less air than the amount
of outside air going into the building.
This maintains a slightly positive
building pressure.
The reason for applying either a
return, or exhaust fan is to control
building pressure. The Trane 100
percent modulating exhaust system
with Statitrac is an excellent choice
for controlling building pressure in
the majority of applications.
For more demanding applications,
Trane's 100 percent modulating
return fan system with Statitrac is an
excellent choice for systems with
high return static pressure losses, or
duct returns. Both systems employ
direct digital control technology to
maintain building pressure. Either
return or exhaust fan systems with
Statitrac may be used on any air
handler application that has an
outdoor air economizer.
A building may have all or part of its
exhaust system in the air handler
unit. Often, a building provides
exhaust external to the air handling
equipment. This external exhaust
must be considered when selecting
the air handler exhaust system.
With an exhaust fan system, the
supply fan motor and drives must be
sized to overcome the total system
static pressure, including return
losses, and pull return air back to the
unit during non-economizer
operation.
However, a supply fan can typically
overcome return duct losses more
efficiently than a return air fan
system.
Essentially, one large fan by itself is
normally more efficient than two
fans in series because of only one
drive loss, not two as with return fan
systems.
In a return fan system, the return fan
is in series with the supply fan, and
operates continuously whenever the
supply fan is operating to maintain
return air volume. The supply fan
motor and drives are sized to deliver
the design CFM based on internal
and discharge static pressure losses
only.
The return fan motor and drives are
sized to pull the return CFM back to
the unit based on return duct static.
Therefore, with a return fan system,
the supply fan ordinarily requires
less horsepower than a system with
an exhaust fan
IntelliPak™ II Rooftop Air
Handler Unit Offers Four
Types of Exhaust/Return Fan
Systems:
1
100 percent modulating exhaust with
Statitrac™ direct space sensing
building pressurization control (with
or without exhaust variable
frequency drives)
2
100 percent modulating exhaust
without Statitrac
3
100 percent modulating plenum
return airfoil fan with Statitrac direct
space sensing building
pressurization control with variable
frequency drive
4
100 percent modulating plenum
return airfoil fan without Statitrac
Drivers for applying either return or
exhaust fan systems range from
economy, to building pressure
control, to code requirements, to
generally accepted engineering
practices
Application
Recommendations
100 Percent Modulating Exhaust
with Statitrac Control, Constant
Volume and VAV Units
For both CV and VAV air handlers,
the 100 percent modulating exhaust
discharge dampers (or VFD) are
modulated in response to building
pressure. A differential pressure
control system, Statitrac, uses a
differential pressure transducer to
compare indoor building pressure to
atmospheric pressure.
The FC exhaust fan is turned on
when required to lower building
static pressure to setpoint. The
Statitrac control system then
modulates the discharge dampers
(or VFD) to control the building
pressure to within the adjustable,
specified deadband that is set at the
Human Interface Panel.
Economizer and return air dampers
are modulated independent of the
exhaust dampers (or VFD) based on
ventilation control and economizer
cooling requests.
Advantages:
• The exhaust fan runs only when
needed to lower building static
pressure.
• Statitrac compensates for
pressure variations within the
building from remote exhaust
fans and makeup air units.
• The exhaust fan discharges in a
single direction resulting in
more efficient fan operation
compared to return fan systems.
• When discharge dampers are
utilized to modulate the exhaust
airflow, the exhaust fan may be
running unloaded whenever the
economizer dampers are less
than 100 percent open.
Applications Considerations