30 November 1998
(4) Life Safety system interlocks.
(5) Special interlocks (such as for freeze protection).
b. Control system variations. These instructions show some of the possible HVAC system
equipment and control system variations, and provide guidance and examples to show how the designer
can modify control loops and systems for applications not specifically shown. The HVAC equipment and
system variations for which control system guidance is provided include:
(1) Outside air preheat coils using hot water or glycol.
(2) Outside air preheat coils using steam.
(3) One-hundred percent outside air in lieu of outside air/ return air economizer.
(4) Deleting economizer control.
(5) Return fans.
(7) Humidity controls.
(8) Smoke dampers in HVAC supply air and return air ducts.
(9) Override of control of valves and dampers for freeze protection or smoke control systems.
(10) Startup and shutdown of HVAC fan systems by external systems such as smoke control.
(11) Variable speed fan drives.
(12) Combining systems in a common control panel.
(13) Unoccupied mode space temperature setback control of HVAC equipment.
(14) Building purge and recirculation modes.
(15) Variations in the use of control valves.
c. Project applicability. The HVAC control systems shown in these instructions are applicable to new
construction building projects, building addition projects, building renovation projects, and (as further
described in chapter 6) building retrofit projects.
d. Types of HVAC equipment covered. These instructions provide control system guidance for
HVAC systems for heating, cooling, humidity control, ventilation and air delivery, terminal units, and
small packaged unitary systems. Terminal units include Variable Air Volume (VAV) boxes, duct coils,
fan coil units, unit heaters, gas-fired infrared heaters, and radiators.
e. Exceptions. These instructions do not cover control systems for HVAC equipment such as boilers
and chillers, which usually have controls integral to the equipment.
6. DESIGN CONCEPT. The guidance contained in these instructions adheres to a particular concept for
designing HVAC control systems. This concept includes the use of standard control systems that
incorporate standard control loops. These instructions then show these control loops implemented in two
different ways. One is with standard control system devices which are housed in a standard HVAC
system control panel. This design concept also includes the use of single-loop digital controllers (SLDC)
for the control of air handling systems and hydronic systems. The use of these controllers for such
systems has been tested in the laboratory and in the field. The other method of implementation of the
standard control loops in these instructions is Direct Digital Control (DDC) systems. Where DDC
implementation differs from implementation with single-loop controllers, the DDC description will follow
the descriptions of control via single-loop controllers. DDC control systems are widely available and
have been in use for HVAC control for many years. However, these systems utilize proprietary hardware
and software and, in general, are not compatible from one vendor's system to another.