Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 79

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UNOCCUPIED ECONOMIZER FREE COOLING
This "Free Cooling" function is used to start the supply fan and
use the economizer to bring in outside air when the outside
temperature is cool enough to pre-cool the space. This is done
to delay the need for mechanical cooling when the system en-
ters the occupied period. Once the space has been sufficiently
cooled during this cycle, the fan will be stopped.
In basic terms, the economizer will modulate in an unoccupied
period and attempt to maintain space temperature to the "occu-
pied" cooling set point. This necessitates the presence of a
space temperature sensor.
Configuring the economizer for Unoccupied Economizer Free
Cooling is done in the UEFC group, with 3 configuration op-
tions, FC.CF, FC.TM, and FC.LO.
Unoccupied Economizer Free Cooling Configuration
(FC.CF)
This option is used to configure the "type" of unoccupied econ-
omizer free cooling control that is desired.
0 = disable unoccupied economizer free cooling
1 = perform unoccupied economizer free cooling as available
during the entire unoccupied period.
2 = perform unoccupied economizer free cooling as available,
FC.TM minutes before the next occupied period.
Unoccupied Economizer Free Cooling Time Configuration
(FC.TM)
This option is a configurable time period, prior to the "next oc-
cupied period," that the control will allow unoccupied econo-
mizer free cooling to operate. This option is only applicable
when FC.CF = 2.
Unoccupied Economizer Free Cooling Lockout Temperature
(FC.LO)
This configuration option allows the user to select an outside
air temperature, below which unoccupied free cooling is disal-
lowed. This is further explained in the logic section.
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Fig. 11 — Custom Changeover Curve Example
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Unoccupied Economizer Free Cooling Logic
Unoccupied free cooling can only operate with the qualifications:
• Unit configured for an economizer
• Unit in the unoccupied mode
• FC.CF set to 1 or FC.CF set to 2 and within FC.TM min-
utes of the next occupied period
• Not in the Temperature Compensated Start Mode
• Not in a cooling mode
• Not in a heating mode
• Not in a tempering mode
• Space temperature sensor enabled and sensor reading healthy
• Outside air temperature sensor healthy
• The economizer would be allowed to cool if the fan were
requested and in a cool mode.
• OAT > FC.LO (1.0 dF hysteresis applied)
• The rooftop is not in a fire smoke mode
• No fan failure when configured to shut the unit down on a
fan failure
If all of the above conditions are satisfied:
Unoccupied Economizer Free Cooling shall start when both of
the following conditions are true:
{SPT > (OCSP + 2)} AND {SPT > (OAT + 8)}
The Night Time Free Cooling Mode shall stop when either of
the following conditions are true:
{SPT < OCSP} OR {SPT < (OAT + 3)}
...where SPT = Space Temperature and OCSP = Occupied
Cooling Setpoint
When the Unoccupied Economizer Free Cooling mode is ac-
tive, the supply fan is turned on and the economizer damper
modulated to control to the supply air set point (Set-
points  SASP) plus any supply air reset that may be applied
(Inputs  RSET  SA.S.R).
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