ASHRAE Journal - February 2021 - 16

TECHNICAL FEATURE

variable within a secondary control loop. The system will
measure the manipulated variable (i.e., airflow rate),
compare it to an airflow setpoint and adjust the damper
position accordingly.
However, we will need to reset that airflow setpoint
(the secondary setpoint) when the controlled variable
(i.e., space temperature) is not being maintained at its
setpoint. The components of the control loop for pressure-independent VAV boxes can be seen in Table 2.
The concept of a pressure-independent VAV box may
sound familiar; they are the norm. They ensure the VAV
box does not provide more than the maximum design
airflow rate. Additionally, such pressure-independent
VAV boxes can be used to ensure that minimum ventilation requirements are maintained during periods of low
or no cooling load on the space. Last, large fluctuations
in airflow due to variations in upstream static pressure can be identified well before they result in further
error from the space temperature setpoint. This further
improves the VAV box's ability to keep the space temperature at setpoint.
Though the theory behind cascade control may seem
excessively sophisticated, numerous applications of this
form of control exist in conventional HVAC systems today.

Conventional Applications
Several examples of conventional applications are
described below. The components of their control loops
are identified in Table 3.

Duct Mounted Reheat Coils
Historically, duct mounted reheat coil valves (either
located at a VAV box or stand-alone) were controlled
to maintain room temperature directly in closed-loop
control. Installing a discharge air temperature sensor
downstream of the reheat coil allows for the reheat
valve to control to a discharge air temperature setpoint
instead. That discharge air temperature setpoint can
be reset to maintain the room air temperature at its
setpoint.
This approach allows an upper limit to be placed on the
discharge air temperature setpoint. Discharge air that
is too warm, coupled with a ceiling supply and ceiling
return air distribution arrangement, will result in shortcircuiting of warm supply air directly into the return
grille. This will drive up energy consumption and reduce
the effectiveness of the HVAC system to heat the space.
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TABLE 2 Cascade control. Pressure-independent VAV box application.

Controlled Variable

Space Temperature

Sensing Element

Space Temperature Sensor

Setpoint

Space Temperature Setpoint

Secondary Setpoint

Airflow Setpoint

Control Signal

Commanded Damper Position

Controlled Device

VAV Box Damper

Manipulated Variable/
Secondary Controlled Variable

Supply Airflow to Space

Secondary Sensing Element

Airflow Sensor

Process Plant

Conditioned Space

FIGURE 2 Cascade control diagram. The diagram shows two separate controllers; however, each block merely represents a control response to error from
setpoint. Many applications of cascade control have a single physical controller capable of processing both responses.

Setpoint
Controller
Secondary
Setpoint

Manipulated
Control Controlled Variable Process
Plant
Signal Device

Controlled
Variable

Controller
Secondary
Sensing
Element

Secondary
Controlled
Variable

Sensing
Element

This is why ASHRAE/IES Standard 90.1-20193 prohibits
distribution of supply air more than 20°F (11.1°C) above
space temperature setpoint. ASHRAE Standard 62.1-20194
has even tighter requirements to keep the zone air distribution effectiveness value at 1.0, which is why ASHRAE
Guideline 36-2018 dictates controlling the reheat valve to
a resetting discharge air temperature setpoint in lieu of
controlling directly to the space temperature.2
An additional benefit is self-balancing the hot water
system, as it helps prevent more than the design amount
of hot water flow from passing through the reheat
coil.5 Reheat coils are almost always large enough to
meet their scheduled discharge air temperature for
the scheduled airflow rate. It is when the served space's
heating requirements exceed design values, and the
reheat coil is controlling directly to the space air temperature, that the reheat valve will fully open and the coil
will pull more than its fair share of hot water flow. This
is typically seen during morning warm-up or at zones

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ASHRAE Journal - February 2021

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