79
YORK INTERNATIONAL
FORM 100.50-NOM1 (604)
5
• The speed of the indoor blower and the opening of
the exhaust air damper will both in
ß
uence the speed
at which the return air fan operates.
Fully Modulating Economizer Option with VFD
Supply Air Fan but No Exhaust Option
This con
Þ
guration would be used for applications where
the supply fan performance cannot meet the total static
requirement of the duct system employing a modulat-
ing outdoor and return air damper system without
exhaust. This allows the external static pressure to be
split between the return air and supply air fan while
having the capability of 100% outdoor air for cooling.
Building pressurization would have to be handled by
an external means.
• Applications uses two return air fans each with
there own motor being controlled by a single VFD
drive.
• The return fan VFD drive is controlled using a pres-
sure transducer with a static pressure probe located
in the mixed air section of the unit.
• The return fan VFD is programmed to maintain -
.03 to -.05 inches of WC static at the probe. These
values are programmed into the VFD at the factory.
The value can be veri
Þ
ed by checking item # 204
and 205 in Menu 2 (References and Limits) in the
VFD program. Refer to Installation, Operation
& Maintenance Form 100.40-N04 for additional
information.
• The return fan static pressure setpoint is reset based
on the supply air fan VFD output.
◦
When the supply air fan output is at minimum,
the setpoint will be –0.1 to 0.2 inches of WC.
◦
When the supply air fan output is at maximum,
the setpoint will be –0.5 to –0.6 inches WC.
• The change in return fan static pressure setpoint
allows the unit to maintain a linear and consistent
outside and return air
ß
ow at all supply air
ß
ow
rates.
• The supply air VFD is controlled by the duct static
transducer.
• The supply air blower and return air fan will always
be energized together.
• As the pressure in the return air section becomes
more negative the return air fan will speed up. As
the pressure in the return section increases above
the setting the return air fan will slow down.
• The return air damper will never go to a totally
closed position.
Fully Modulating Economizer Option with VFD
Supply Air Fan and Motorized Exhaust Damper
This con
Þ
guration would be used for applications where
the supply fan performance cannot meet the total static
requirement of the duct system employing a modulating
outdoor and return air damper system with motorized
exhaust. This allows the external static pressure to be
split between the return air and supply air fan while
having the capability of 100% outdoor air for cooling.
• Applications uses two return air fans each with
there own motor being controlled by a single VFD
drive.
• The return fan VFD drive is controlled using a pres-
sure transducer with a static pressure probe located
in the mixed air section of the unit.
• The return fan VFD is programmed to maintain -
.03 to -.05 inches of WC static at the probe. These
values are programmed into the VFD at the factory.
The value can be veri
Þ
ed by checking item # 204
and 205 in Menu 2 (References and Limits) in the
VFD program. Refer to Installation, Operation
& Maintenance Form 100.40-N04 for additional
information.
• The return fan static pressure setpoint is reset based
on the supply air fan VFD output.
◦
When the supply air fan output is at minimum,
the setpoint will be –0.1 to 0.2 inches of WC.
◦
When the supply air fan output is at maximum,
the setpoint will be –0.5 to –0.6 inches WC.
• The change in return fan static pressure setpoint
allows the unit to maintain a linear and consistent
outside and return air
ß
ow at all supply air
ß
ow
rates.
