30 November 1998
HVAC CONTROL SYSTEM EQUIPMENT, EQUIPMENT USES AND HVAC CONTROL LOOPS
1. GENERAL. The design of HVAC control systems is implemented by defining the operating modes of
the HVAC equipment, defining the control loops required, and selecting the control system equipment to be
used in the loop. The process of selecting the control system equipment includes calculations by the
designer to specify the flow capacity of control devices, the physical size of control devices and the electric
service required. This chapter describes the operating modes, process variables, control modes, control
system devices and their features, control system equipment applications, and inter-connection of control
devices. This chapter provides criteria and guidance for selecting and sizing control devices.
2. CONTROL SYSTEM OPERATING MODES AND PROCESS VARIABLES.
a. Control system operating modes. Control systems start and stop the HVAC system equipment
according to a time schedule, and at specific outside air temperatures and specific indoor temperatures. In
addition, the control systems operate the HVAC systems in the following modes of operation:
(1) Occupied mode is initiated automatically to allow HVAC systems to start in sufficient time to
bring the space to the proper temperatures at the start of occupancy.
(2) Ventilation delay mode is initiated automatically to prevent the use of outside air when the unit
is started prior to occupancy, to cool down or warm up the area served.
(3) Unoccupied mode is initiated automatically to prevent unnecessary operation of HVAC-system
equipment during periods of non-occupancy except for special purposes such as operation to maintain
minimum space temperatures for freeze protection.
(4) Heating or cooling modes are initiated manually to provide either heating or cooling media to
b. Control system process variables. While the HVAC systems are in operation, the process variables
commonly sensed and controlled by HVAC control systems are:
Static pressure of air.
Differential pressure of air.
Air flow rate.
c. Constraints on process variables by operating modes. The constraints placed on the control of
HVAC process variables by the operating modes are:
(1) Cooling and humidification are shut off during the unoccupied mode.
(2) Outside air is not supplied to the space during the unoccupied and ventilation delay modes.
d. Modulating control. The amount of heat delivered to a space (or removed from a space) from
certain types of HVAC equipment is regulated by varying the heat exchanger capacity from zero to one-
hundred percent in response to the variation of a continuous, gradual input signal. This is called modulating
control. Heat exchanger control valves, mixing dampers, fan inlet vanes, variable speed drives, and
humidifier valves are examples of HVAC equipment that are controlled by modulating control.