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Unit Startup
Figure 40.
Auto changeover logic
1. This is the point at which the cooling capacity equals
0% and the error integrator starts to add up. Error
integration does not begin until the capacity is 0%. The
error integrator cannot be less than zero.
2. The Space Temperature must be less than Space
Setpoint before the controller can change to Heat. The
Table 35.
Mode of operation and applied control method
Heat Cool Mode
Cool
Heat
Morning Warm Up
Pre Cool
Night Purge
Unoccupied Cool
Unoccupied Heat
Single Zone VAV
Single Zone VAV equipment is configured with a variable
speed supply fan that modulates with Space Temperature
Control to minimize fan energy while maintaining space
comfort. When there is no demand for heating or cooling
the supply fan operates at the minimum speed setting while
providing ventilation according to occupancy setpoints.
Alternately, the supply fan can be setup to cycle off when
there is no demand for heating or cooling.
Single Zone VAV – Cooling
Cool — When there is a space cooling demand, DX
cooling will initialize and increase to satisfy space
temperature. The supply fan will continue to operate at a
low, fixed speed at low cooling capacities until the cooling
capacity requires additional airflow to keep compressor
operation optimal. The supply fan speed will increase as
the demand for cooling capacity increases. The minimum
supply fan speed is calculated by DX cooling capacity. As
the space requires additional cooling capacity, Space
Temperature Control will calculate a lower discharge air
temperature setpoint increasing DX cooling which in turn
76
Control Method
Space Temperature Control
Space Temperature Control
Discharge Air Control
Discharge Air Control
Discharge Air Control
Discharge Air Control
Discharge Air Control
Space Temperature must rise above the active space
Cool Setpoint before the controller can change to Cool.
3. The controller switches to Heat (from cool) after the
error integrator exceeds 900°F seconds or Space
Temperature <= (Heat Setpoint – 1°F).
Space Temperature Control Units
Single Zone VAV and Constant Volume Space Temperature
units use the Space Temperature Control sequence of
operation in Occupied Heat and Occupied Cool modes of
operation; however, the Discharge Air Control method is
used for all other modes. The table below summarizes
each mode of operation and the control method applied.
See Operating Modes section for more details
Discharge Air Temperature
Setpoint
Calculated
(cascade)
Calculated
(cascade)
Fixed
Fixed
Fixed
Fixed
Fixed
increases the supply fan speed (to prevent coil frost).
Cooling capacity and supply fan speed both can increase
to 100%.
If the unit discharge air temperature reduces to the
Discharge Air Temperature Minimum Cool Limit setting or
DX cooling reaches 100% capacity, cooling capacity
increases will hold while the supply fan speed will continue
to increase to 100% or modulate to manage space comfort
cooling.
Cool Economizer — If the unit is configured for a
modulating outdoor air damper and conditions are suitable
for economizer cooling, the supply fan will operate at
minimum speed while the economizer damper modulates
between the Outdoor Air Damper Minimum Position (or
Flow) Setpoint and 100% to satisfy the discharge air
temperature setpoint. If the economizer damper reaches
100% open and additional cooling capacity is required, the
supply fan will then modulate between minimum speed and
100% to provide additional cooling to the space.
Cool Economizer + DX —Cool – Economizer + DX is
reported as the Unit sub-mode of operation when both
Space/Discharge Setpoints
Space Temperature Setpoint Active
Discharge Air Temperature Setpoint
Active
Space Temperature Setpoint Active
Discharge Air Temperature Setpoint
Active
Morning Warmup Setpoint
Discharge Air Heating Setpoint Active
Pre Cool Setpoint
Discharge Air Cooling Setpoint Active
Unoccupied Cooling Setpoint
Discharge Air Cooling Setpoint Active
Unoccupied Heating Setpoint
Discharge Air Heating Setpoint Active
RT-SVX073A-EN
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