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VAV-PRC011M-EN 99
DDC Controls
determining control action of the DDC controller. When configured for a CO
2
sensor, the value
of the input is used as a status-only input by Tracer SC.
Dual-duct support with two DDC controllers. One DDC controller controls the cooling air valve
and the other controller controls the heating air valve. With constant-volume sequences, the
discharge air volume is held constant by controlling discharge air volume with the heating
UCM.
Note: Flow sensor DP (in. wg) is measured at the flow ring to aid in system balancing and commissioning. See “Valve/Controller
Airflow Guidelines” in each section for unit performance.
DDC Remote Heat Control Options
When heat is added to the primary air at the VAV unit before it enters the zone, the air is said to
be reheated. The operating characteristics of the four basic types of VariTrane DDC terminal reheat
are discussed.
Single-Duct: On/Off Hot Water Reheat
Three stages of on/off hot water reheat are available. The water valves used are 2-position and are
either fully-opened or fully-closed. The heating minimum airflow setpoint is enforced during
reheat.
Stage 1 energizes when the space temperature is at or below the heating setpoint. When the zone
temperature rises above the active heating setpoint by 0.5°F (0.28°C), stage 1 is de-energized. Stage
2 energizes when the space temperature is 1°F (0.56°C) or more below the active heating setpoint,
and is de-energized when the space temperature is 0.5°F (0.28°C) below the active heating setpoint.
Stage 3 energizes when the zone temperature is 2°F (1.11°C) or more below the active heating
setpoint, and de-energizes when the space temperature is 1.5°F (0.83°C) below the active heating
setpoint. When reheat is de-energized, the cooling minimum airflow setpoint is enforced.
Single-Duct: Proportional Hot Water Reheat
Proportional hot water reheat uses 3-wire, floating-point-actuator technology.
When the space temperature drops below the active heating setpoint, the air valve open to the
Airflow Setpoint Reset Minimum Local Heat and the reheat valve modulates to maintain space
temperature at the active heating setpoint. Control of the water valve uses a separate proportional
plus integral control loop, and its position is dependent on the degree that the space temperature
Figure 6. Flow sensor signal vs. airflow delivery
0.01
0.1
1
5
10 100 1,000 10,000
Cfm
Flow Sensor DP (In. wg)
4"
12"
10"
8"6"
5"
14"
16"
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