YORK INTERNATIONAL
80
Sequence of Operation
FORM 100.50-NOM1 (604)
• The unit will also require a motorized exhaust air
damper that operates off a building pressure trans-
ducer.
• 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.
MODULATING GAS HEAT
Modulating gas heat can be ordered with a minimum
of two gas heat stages (1 heat exchanger section) and
a maximum of six stages (three heat exchanger sec-
tions).
Fig 44 shows the staging sequence for the modulat-
ing heat stages. As can be seen from the chart, Heat
section 1A (modulating) is always the
Þ
rst section on
and the last section off. Heat section 1B is always the
last section on and the
Þ
rst section off. The number of
stages between 1A and 1B operation is depended on the
number of heating sections installed in the unit. The
modulating heat section, 1A, must always modulate to
full
Þ
re before any additional stages can be brought on.
When additional stages are required, the modulating
heat section will go to minimum
Þ
re and the next stage
of heating will be brought on. If there continues to be a
demand for additional heat, the modulating heat section
re-modulates to high
Þ
re before any additional stages
can come on. During a decrease in heating demand,
the modulating heat section must be at a minimum
Þ
re
condition before any stages are turned off.
The following Primary Unit controller parameters are
required for modulating heat operation. These values
will be entered into the control at the factory. Under
the CONFIGURATION key of the OptiLogic control
panel the “
# of Heat Steps”
will be set to 0. This value
does not change regardless of the number of stages
installed. Under the CONFIGURATION key of the
OptiLogic control panel the “
Heating Type”
will be
set to HW, even though the unit is
Þ
red on gas. Under
the SETPOINTS key of the OptiLogic control panel
ON
OFF
OFF
OFF
ON-LOW
ON-HIGH
ON-LOW
ON-HIGH
MAX
MIN
Furnace 1A
(Modulating)
Furnace 1B
Furnace 2
Furnace 3
Less Heat
More Heat
When staging up Furnace 1B,
Furnace 1A can only be turned down
to Min. Volts HIGH and the Inducer
Fan must remain on HIGH speed.
FIG. 44 –
MODULATING GAS HEAT STAGING
LD09791
