16 • Space Temperature Control
026-1727 Rev 3
7.3. Space Temperature
Control
There are two active space temperature setpoints
in a DAC: a cooling setpoint and a heating setpoint. In
general, when the input rises above the cooling
setpoint, cooling mode begins, and when the input
falls below the heating setpoint, heating mode begins.
7.3.1. Terminal Load Calculation
As the space temperature rises above the active
cooling setpoint, or falls below the active heating set-
point, the system’s terminal load will increase, either
positively for cooling, or negatively for heating. The
terminal load can range from -100% to +100%, where
all negative values indicate a demand for heating and
all positive values indicate a demand for cooling.
When the terminal load rises above 20%, cooling
mode will be activated. Conversely, when the
terminal load falls below -20%, heating mode will be
activated. Once activated, both cooling and heating
modes will deactivate if the terminal loads goes back
to 0%.
7.4. Supply Temperature
Control
The DAC will determine the active supply air
temperature setpoint based on whether heating or
cooling is active as well as the amount of terminal
load.
When heating mode is active, the controller will
use the supply temperature heating setpoint as the
active supply temperature setpoint. Heat stages will
be turned on to maintain the supply air temperature at
the active setpoint. Conversely, when cooling mode is
active, the controller will use the supply temperature
cooling setpoint as the active supply temperature
setpoint. Cool stages will be turned on to maintain the
supply air temperature at the active setpoint.
7.4.1. Supply Air Setpoint Reset
The active supply air setpoint may be
automatically adjusted (reset) based on the heating or
cooling terminal load present. The active setpoint will
be increased or decreased up to the maximum amount
specified by the heating and cooling reset setpoints.
The maximum amount will be used when the terminal
load r100% for cooling and -100% for
heating.
7.4.2. Cooling Mode
The DAC supports both staged and modulating
mechanical cooling devices. Modulating devices are
treated as a cooling stage in the case of a unit having
a mixture of fixed and variable capacity compressors.
7.4.2.1.
Staged Cooling
Up to four stages of cooling can be used. Each
compressor stage can be configured to be fixed size,
digital scroll, VS, double size, or heat pump. The
fixed size and heat pump compressors assume a fixed
capacity while the digital scroll and VS compressors
assume a modulating capability. The double size
compressor indicates a fixed compressor which is
double in capacity compared to the other fixed
compressors.
When a compressor stage is defined as a heat
pump, it will turn on for both cooling and heating. A
reversing valve must also be used and can be defined
as active cooling or active heating.
7.4.2.1.1.
Staging On
When cooling is required, the cool stages will
stage on if the current supply air temperature is above
the active supply air cooling setpoint. The lowest
stage that is off will be turned on first. Additional
stages will be brought on according to the Cool On-
Delay setpoint. A cooling stage will not be turned on
unless it has satisfied its minimum off time.
Figure 7-1
- Terminal Load Graph
NOTE: The application supports a maximum
of two digital/VS compressor stages.
