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. 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.
Operation of Space Control (Multizone Unit). 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 deck damper.
Operation of Space Control (VAV Terminal Unit -
Pressure Dependent). 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 VAV terminal box
cold air valve.
Operation of Supply Duct Pressure Control. A supply
duct static pressure sensor located two-thirds the distance from
the fan of the length of the longest supply duct run shall, on a
rise in static pressure, gradually reduce the speed of the
electric motor variable speed drive and close the fan vortex
damper. A high-limit duct static pressure sensor located at the
fan discharge shall override the supply duct static pressure
sensor to limit pressures to design values.
Single Zone Unit Sequence of Operations (Sample).
Figure 33 and Table 15.
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.
c) Occupied Mode: Maintain occupied conditions under
load variations while providing required ventilation. Supply fan
operates continuously. Outside air dampers are opened to the
minimum ventilation air position. If an economizer outside air
cycle is provided, the outside air, return air, and exhaust air