30 November 1998
CONTROL-SYSTEM DESIGN VARIATIONS
1. GENERAL. The control systems shown in Chapter 4 will be modified when required to account for
HVAC-system equipment variations. The variations covered in this chapter are as follows:
a. 100 percent outside air systems.
c. Smoke dampers.
d. Variable speed drives.
e. Steam preheat coil with face and bypass dampers.
f. Modulating hot water or hot glycol preheat coil.
g. Combining hydronic system and air system control systems in the same control panel.
h. Unoccupied mode space temperature setback control for terminal units.
I. Two-way shut-off valves on fan coil units.
j. Building purge/flush cycle.
k. EMCS initiated building purge and recirculation modes.
l. Smoke control and freeze protection.
m. Control systems without economizer modes.
n. Dual steam valves.
o. Hydronic systems with boilers requiring constant flow.
2. CONTROL SYSTEM VARIATIONS FOR 100 PERCENT OUTSIDE AIR (CONTINUOUS
a. HVAC systems that introduce 100-percent outside air are used as makeup air systems when large
quantities of air are exhausted from the space. Such systems are used in the heating and cooling of
such spaces as hospital operating rooms and laboratories, which cannot recirculate air from the space
through the system. Spaces that cannot use return air will usually have more air exhausted than is
supplied, to insure that the space is at a negative pressure with respect to surrounding spaces. This may
be accomplished by an exhaust fan interlocked with the supply fan.
b. A 100-percent outside air system does not require the economizer changeover and mixed air
temperature control loops, but does require an outside air preheat coil and associated temperature
control loop in most climates.