Civil Engineering Reference
In-Depth Information
the minimum required VAV zones, or occupied mode becomes active, the direct
digital control system shall disable NHL. (This control prevents the building
temperature during the off hours of the building moves above a maximum allow-
able setting.)
• Typical sequence of operation for maintaining air handling unit discharge tem-
perature: The direct digital control system shall modulate the chilled water valve
to maintain air handling unit discharge air temperature set point. The direct digi-
tal control system shall reset the discharge temperature set point upward on a fall
in space temperature below 70 °F (adjustable) (this control allows the system to
deliver air at a preset value (typically 55 °F) and when possibility arises, to
increase this set-point to higher temperatures gradually in order to save energy).
• Typical sequence of operation for maintaining duct static pressure: The DDC
system shall modulate the variable frequency drive to maintain a pre-set duct
static pressure set point at all time (this control allows the system to run with
enough capacity in order to keep a pre-set static pressure level in duct and there-
fore proper amount of air be delivered to each space at all time).
• Typical sequence of operation for fan high pressure safety: Upon receiving a
signal of an air pressure of at least 5 in. of water column (adjustable) in supply
duct, the direct digital control system through a high-pressure switch located in
the supply air ductwork shall stop the fan (this control prevents the system from
over-pressuring and damaging the ductwork).
• Typical sequence of operation for maintaining mixing plenum static pressure:
The direct digital control system shall modulate the return air damper to maintain
mixing plenum static pressure and minimum required outside air quantity. This
static pressure shall be measured by a static pressure sensor installed inside the
mixed air plenum and adjusted at time of balancing the system. (A more accurate
and also more expensive method is installing an air measuring station in the
minimum outside air path to deliver the exact quantity of required minimum
outside air at all time.) (This control allows the proper amount of minimum out-
door air at all times be introduced to the system).
• Typical sequence of operation for maintaining the preheat coil discharge tem-
perature (freeze protection): The direct digital control system shall modulate the
preheat coil hot water valve to maintain coil discharge temperature set-point.
This temperature shall be sensed by an averaging temperature sensor (at all time,
even during the system shutdown) which is located downstream of the preheat
coil (this control allows the air temperature leaving the preheat coil and passing
over the cooling coil maintain enough temperature and therefore prevent freez-
ing of the cooling coil).
• Typical sequence of operation for airside economizer mode: The direct digital
control system shall initiate economizer mode operation when the outdoor air
temperature (dry bulb) drops below a set point, approximately (65 °F adjustable).
When the air handling unit is operating in economizer mode, the direct digital
control system shall modulate the relief and maximum outside air dampers to
maintain air handling unit discharge air temperature set-point. When the maxi-
mum outside air damper is fully open and the discharge air temperature set point
