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control setting, only mechanical cooling
will function and outside air dampers will
remain at their minimum position.
Mechanical Cooling
If the zone temperature cannot be
maintained within the setpoint deadband
using the economizer option or if there is
no economizer, the RTM sends a cooling
request to the MCM. The compressor
module checks the compressor
protection circuit before closing stage
one. After the first functional stage starts,
the compressor module monitors the
saturated refrigerant temperature and
closes the condenser fan output contact
when the saturated refrigerant
temperature rises above the lower limit
setpoint.
Air-Cooled Units Only
The compressor module closes the
condenser fan output contact when the
saturated refrigerant temperature rises
above the lower limit setpoint.
Water-Cooled Units Only
The WSM modulates the condenser coil
water valves to maintain condenser
temperature, if applicable. Otherwise, it
will check the entering condenser water
temperature to ensure it is greater than
54 F or if not, it will lock out cooling.
Auto Changeover (Units with Heat )
When the system mode is in auto, the
mode will change to cooling or heating as
necessary to satisfy the zone cooling and
heating setpoints. The zone cooling and
heating setpoints can be as close as 2 F
(1.1 C).
Occupied Zone Temperature - Heating
Relies on input from a sensor directly in
the space, while a system is in occupied
heating mode or an unoccupied period, to
stage electric heat on and off or modulate
the hydronic heating valve as required to
maintain the zone temperature within the
heating setpoint deadband. The supply
fan will operate when there is a request
for heat.
Electric Heat
On units with electric heat, the zone
temperature can be controlled to a
heating setpoint during the occupied
mode by cycling a single stage electric
heater. An interface is provided for field
supplied single stage electric heat. The
zone temperature heating setpoint and
94
Owner
deadband are user defined at the HI
panel.
Hydronic Heat: Hot Water or Steam
On units with hot water or steam heating,
the zone temperature can be controlled
to a heating setpoint during the occupied
mode. The zone temperature heating
setpoint and deadband are user defined
at the HI panel or zone sensor. VAV
occupied heating initiates by closing a
field-supplied switch or relay contacts
connected to the changeover input on the
RTM. Supply air static pressure is
maintained.
Supply Air Setpoint Reset (VAV Units
Only)
Supply air reset can be used to adjust the
supply air temperature setpoint on the
basis of a zone temperature or outdoor
air temperature. Supply air reset
adjustment is available at the HI panel for
supply air heating and supply air cooling
control.
Reset based on outdoor air temperature
Outdoor air cooling reset is sometimes
used in applications where the outdoor
temperature has a large effect on
building load. When the outside air
temperature is low and the building
cooling load is low, the supply air setpoint
can be raised, thereby preventing
subcooling of critical zones. This reset can
lower usage of mechanical cooling, thus
savings in compressor kW, but an
increase in supply fan kW may occur.
Outdoor air heating reset is the inverse of
cooling, with the same principles applied.
For both outdoor air cooling reset and
heating reset, there are three user
defined parameters that are adjustable
through the human interface panel.
• Beginning reset temperature
• Ending reset temperature
• Maximum amount of temperature reset
Reset based on zone temperature
Zone reset is applied to the zone(s) in a
building that tends to overcool or
overheat. The supply air temperature
setpoint is adjusted based on the
temperature of the critical zone(s). This
can have the effect of improving comfort
and/or lowering energy usage. The user-
defined parameters are the same as for
outdoor air reset.
Sequence of
Operation
Supply Air Tempering (Hot Water and
Steam VAV Units Only)
When supply air temperature falls below
the supply air temperature deadband low
end, the heating valve modulates open to
maintain the minimum supply air
temperature setpoint.
Daytime Warmup (Units with Supply Air
Temperature Control Only)
During occupied mode, if the zone
temperature falls to a preset, user-
defined zone low limit temperature
setpoint, the unit is put into daytime
warmup. The system changes over to CV
heating, the VAV boxes drive full open.
However, unit airflow modulation control
operates to maintain duct static setpoint,
and full heating capacity is provided until
the daytime warmup setpoint is reached.
The unit is then returned to normal
occupied mode.
Supply Air Tempering
Supply air tempering is available on units
without volume control and with hot
water, steam, or electric heat or units with
supply air temperature control with
steam or hot water heat. When the unit is
in heat mode but not actively heating, if
the supply air temperature drops to 10 F
(5.5 C) below the occupied zone heating
temperature setpoint, electric heat will
stage on or the hydronic valve will
modulate to maintain a minimum supply
air temperature. The unit transitions out
of heat mode if the supply air
temperature rises to 10 F (5.5 C) above
the occupied zone heating temperature
setpoint.
Changeover
This mode only functions on units with
supply air temperature control and
hydronic heat. When the changeover
binary input is closed the unit will control
to a discharge air heating setpoint. This
setpoint is entered at he HI, and can be a
higher temperature than the supply air
cooling setpoint. This function maintains
duct static pressure.
SCXG-SVX01B-EN
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