of the sensing element falls below the setpoint. Normal
operation will occur only after the freezing condition is
corrected and the freezestat is manually reset.
f) Chilled Water Coil Discharge Air Mode: A dry bulb
temperature sensor located in the chilled water coil discharge
air shall, on a rise in temperature, first gradually close the
preheat coil valve, next gradually open the outside air damper
from its minimum ventilation air position while simultaneously
gradually close the return air damper (ensure thorough mixing)
and gradually open the exhaust air damper, and then gradually
open the chilled water coil valve. A dry bulb temperature low
limit sensor located in the mixed air (ensure thorough mixing)
shall, on a fall in temperature, gradually open the preheat coil
valve. An outdoor air dry bulb sensor disables the economizer
cycle and maintains minimum ventilation air when outside air dry
bulb temperature exceeds the changeover point.
g) Heating Coil Discharge Air Mode: A dry bulb
temperature sensor located in the heating coil discharge air
shall, on a rise in dry bulb air temperature, gradually close the
heating coil valve.
h) Room Control Mode: A room type dry bulb
temperature sensor located in the most critical spot shall, on a
rise in dry bulb temperature in the space, gradually open the
cold deck damper while simultaneously gradually close the hot
Variable Air Volume (VAV) Unit Sequence of Operation
(Sample). See Figure 35 and Table 17.
a) General: Sequences describe actions of the control
system in one direction of change in the HVAC process analog
variable, such as temperature, humidity, or pressure. Reverse
sequence shall occur when direction of change is reversed unless
b) Optimized Start Mode: Energize equipment at
maximum capacity at the latest time required for heating up (or
cooling down) the space to normal occupied temperature
conditions. Outside air and exhaust air dampers are closed and
return air dampers are opened. Fans of series type VAV terminal
units shall start.
c) Occupied Mode: Maintain occupied conditions
all load variations while providing required ventilation.
fan operates continuously. Outside air dampers are opened
minimum ventilation airflow by the air monitoring unit for