Daikin MICROTECH 4 Operation Manual page 54

COOLINg
Figure 20: Cooling DAT Setpoints
When Airflow, Hum1, or Hum2 is selected, the values "Min Clg
Spt@" and "Max Clg Spt @" are entered as percentage values .
When ExtmA is Selected, the values "Min Clg Spt@" and "Max
Cl Spt @" are entered as mA values . When Ext VDC is selected,
the values "Min Clg Spt@" and "Max Cl Spt @" are entered as
VDC values .
If Ext mA or Ext V is selected as the type of reset, the Min Clg
Spt@ value may be set above the Max Clg Spt@ value to cause
a decrease in the DAT set point as the external signal or the Min
Clg Spt @ value may be set below the Max Clg @ Spt Value to
cause an increase in the DAT set point as the external set point
increases .
The Min Clg Spt@ value can be set below the Max Clg Spt @
Value for all types of reset, but it only makes sense for external
reset .
Cooling DAT Reset Application Considerations
These examples are just some common uses of DAT reset,
other building/application factors may require different values or
schedule sources .
Table 20: Cooling DAT Reset
1
OAT Clg Airflow
DAT
Min 45F 60F 40%
Max 65F 55F 60%
1 . OAT: OAT is an effective reset control strategy if all the
zones served have similar loads and are mostly dependent
on outdoor air temperatures . Examples would be west
facing perimeter zones with the same lighting/equipment/
occupancy loads . This reset strategy is problematic
if zones require cooling regardless of outdoor air
temperature . This could be due to internal gains or general
interior spaces of large buildings .
2 . Airflow: This is an effective reset control strategy for single
zone or multizone VAV units because airflow is a close
indicator of cooling demand. If a typical VAV unit supply fan
speed range is 40-100% a good airflow capacity to being
to reset the DAT is roughly 60-65% . By the time the fan
speed is down to 50% the DAT can be reset to a warmer
temperature .
3 . Network, ExtSig: It is recommended that a network control
the DAT reset schedule when a rooftop unit is applied in a
multizone VAV system with diverse loads. In this case, the
DAIKIN APPLIED
2 3
Clg DAT ExtSig Clg DAT
65F 0V 65F
55F 10V 50F
network can combine a building specific reset sequence
based on the polling of VAV boxes and synchronizing the
reset with a duct static pressure reset that maximizes
energy efficiency.
DX Coil Bypass Damper Operation
A compressorized cooling unit may be equipped with a set of DX
coil bypass dampers . When the Unit state is Heating, MinDAT
or Fan Only, the bypass damper will be 100% open to minimize
the air pressure drop and supply fan power consumption . The
DX Coil Bypass Damper will operate in Cooling operation only as
described in the description below .
Cooling
During Cooling Operation, if a unit is equipped with a DX Coil
Bypass Damper; the unit will control the compressors to maintain
the DXBP LCT Set point . The DX Coil Bypass Damper will
modulating to maintain the Effective Clg DAT Set point.
Cooling + Dehumidification
During Cooling Operation when the unit is equipped with hot gas
reheat or liquid subcool reheat and Dehumidification is active,
the DX Coil Bypass Damper will shut and all of the supply air will
flow through the DX Coil to maximize dehumidification using the
reheat source .
DX Coil Bypass Damper Application
Considerations
Operational settings should only be made with the advisement of a qualified
person; changing key configurations away from factory settings may result
in damage equipment or surrounding property. Recommended settings may
vary based on application specific requirements.
Should I use a Bypass Damper?
DX Coil Bypass is ideal for applications with high sensible
loads, or that have very light dehumidification requirements.
DX Coil Bypass Dampers use some bypass air for decreasing
the sensible load to the space, while providing some
dehumidification. DX bypass dampers are not recommended for
applications with large latent and sensible loads that occur at the
same time . Geographic location / Climate, and the SHR of the
space will determine if DX bypass dampers are right for you . As
shown in Table 21 on page
a sensible BP Damper Design if you are in Climate Zones B or
C any zone number and the application SHR (Sensible Heat
Ratio) is > than 0 .8, meaning the load is mostly sensible . Your
application is considered a Latent Bypass Application if you are
in Climate Zones A, B, or C, and/or you have a SHR <0 .8 .
54
WARNING
55, your application is considered
MICROTECH UNIT CONTROLLER