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Calibration, Operation Modes, and Control
Air Valve Control
Ventilation flow control uses the air valve as a constant volume device. The unit is given a constant
flow setpoint for air valve control (configured ventilation setpoint). The air valve only repositions
itself in response to changes in inlet static pressure. By using pressure-independent control for
ventilation purposes, a constant volume of fresh air can be maintained, regardless of small
fluctuations in inlet static pressure. Ventilation flow control unit can use a Ventilation Setpoint from
a BAS system, if it is valid. If the unit is standalone, the ventilation flow control uses one of the
following two (2) airflow setpoints:
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•
Staged Reheat Control (Electric and Hot Water)
The heat outputs of the controller are binary. Only discrete levels of discharge air temperature are
possible. Since discrete discharge air temperature levels do not always provide an instantaneous
temperature within the required band, staged reheat controls to a 30-minute average discharge air
temperature. The discharge air temperature setpoint is limited from 19°F to 70°F (-7.22°C to 21.11°C).
Staged Electric Reheat Control
Units that are equipped with electric reheat should be sized so that the maximum temperature rise
across the heating elements is from 40°F to 48°F (4.44°C to 8.88°C); it should never exceed 50°F
(10°C) for safety reasons. These values were selected to allow the largest control range without
damage to the heater elements.
For staged electric ventilation flow control, the number of installed stages can range from one to
three. Three stages are recommended, since finer control is available with more stages. In cases
where the outdoor air temperature is more than 48°F (8.88°C) below the discharge air temperature
setpoint, the controller cannot provide the requested control performance.
The controller provides no cooling capacity. If the outdoor air temperature (OAT) is above the
discharge air temperature setpoint, the discharge air temperature cannot be controlled and the
discharge air temperature is equal to the OAT.
Staged Hot Water Reheat Control
Units equipped with hot water coils should be sized so the maximum temperature of air leaving
the coil does not exceed 140°F (60°C). Since only one stage of hot water reheat is available, staged
hot water does not allow tight control of the discharge air temperature. Although the controller
operates in this configuration, it is not recommended.
Staged Reheat Control Algorithm (Integral Only)
An incremental form of the integral PID algorithm computes the desired level of reheat capacity.
The algorithm is run once every 10 seconds.
Modulating Reheat Control (Hot Water Only)
Units equipped with hot water coils should be sized so the maximum temperature of air leaving
the coil does not exceed 140°F (60°C). If hot water reheat is installed, the preferred method of
control is with a modulating valve. The modulating valve enables an instantaneous discharge air
temperature within the dead band of the discharge air temperature setpoint. The reheat capacity
algorithm for modulating hot water discharge air temperature control is the same algorithm used
to control modulating hot water valves for a space temperature controller.
Modulating Reheat Control Algorithm (Proportional-Integral)
An incremental form of the proportional integral algorithm runs once every 10 seconds and
calculates the reheat capacity required to meet the discharge air temperature setpoint. The
required capacity is equal to valve position.
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If no reheat being used, it uses the configured Ventilation Setpoint Local.
If reheat being used, it uses the configured Airflow Setpoint Minimum Local Heat.
VAV-SVX07B-EN