dampers are automatically controlled. An outdoor air dry bulb
sensor disables the economizer cycle and maintains minimum
ventilation air when outside air dry bulb temperature exceeds the
d) Unoccupied Mode: Cycle the equipment to maintain
minimum heating temperature (or maximum cooling temperature) in
the space while providing no ventilation. Outside air and
exhaust air dampers are closed and return air dampers are opened.
e) Maintenance and Safety Mode: A differential
pressure sensor measuring static pressure drop across the filter
shall initiate an alarm when the pressure drop exceeds the
A freezestat with extended surface sensing element
located ahead of the chilled water coil shall stop the supply
fan, return system components to their fail-safe position, and
initiate a low-temperature alarm if the temperature at any point
of the sensing element falls below the setpoint. Normal
operation shall occur only after the freezing condition is
corrected and the freezestat is manually reset.
f) 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, first gradually close
the heating coil valve, next gradually close the preheat coil
valve, then 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 opening the exhaust air damper, and finally 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. A dry bulb temperature low limit sensor located in the
supply air shall, on a fall in temperature, gradually open the
heating 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.
Multizone Unit Sequence of Operation (Sample).
Figure 34 and Table 16.
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