Constant Volume (Cv) Units (Ftyp = 1-Speed); Cooling Operation; Cooling Mode Control - Carrier 48HC 04-28 Controls, Start-Up, Operation And Troubleshooting

Fan Speed - IAQ Override (FS.AQ)
This configuration defines the fan speed used when an IAQ
Override is active. This pertains only to the Override function

of IAQ (Configuration AIR.Q

or (Configuration AIR.Q
N/C), not the DCV or Minimum Position functions. When in
an IAQ override condition the fan will run at this speed unless
it is configured for less than the current commanded speed.
When the IAQ override condition is cleared the speed will go
to the next commanded speed.
Fan Speed - Free Cool Lo (FS.E1)
This configuration defines the initial fan speed used when in
Free Cooling. Refer to the Economizer Controls Operation sec-
tion for details on free cooling. The fan will stay at this config-
ured speed whenever the damper is being used for free cooling.
If the damper is at 100% for 5 minutes the fan will ramp to max
speed. It is locked there until the actual damper position falls
below 75% at which time it will ramp back down to this con-
figured speed.
Fan Speed - Mech Cooling (FS.CL)
This configuration defines the intermediate fan speed used
when one stage mechanical cooling is being requested. Fan
speed is based on how many cooling stages the unit has, how
many cooling stages are being requested, and how many reheat
stages are being requested. If the unit only has one circuit of
cooling, then the fan speed will be FS.MX whenever that stage
is requested. If the unit has more than one circuit of cooling but
only one is being requested, the fan speed will be set to this
FS.CL. Any time more than one stage is requested the fan
speed will be set to FS.MX. On Humidi-MiZer equipped units
fan speed is more complex. Refer to the FS.RH and Table 7 for
details.
Fan Speed - Reheat2 (FS.RH)
This configuration defines the fan speed used when Hot Gas
Reheat (reheat2) is being requested. Fan speed is based on how
many cooling stages the unit has, how many cooling stages are
being requested, and how many reheat stages are being re-
quested. If only reheat stages are being requested, then the fan
speed will be set to FS.RH. If the unit only has one circuit of
cooling, then the fan speed will be FS.MX when dehumidifica-
tion and cooling is being requested. If the unit has more than
one circuit of cooling and only one cooling stage is being re-
quested, the fan speed will be set to FS.CL. Any time more
than one cooling stage is requested the fan speed will be set to
FS.MX. If only one cooling stage is requested and reheat is re-
quested, then the fan speed will be set to the greater of the two
configurations (FS.CL or FS.RH). (See Table 7.)
Table 7 — Cooling Fan Speed Determination
Cooling
Number of
Stages
Circuits
Required Required
1
1 1
0
1
>1
>1
>1
1
0
OAU Fan Boost Enable (OAFB)
This configuration (Configuration
able when the unit is equipped with a factory installed Outside
Air Unit (OAU). If set to yes then the indoor fan can be used to
help the
OAU if there is a low CFM alarm. When the alarm is active for
10 minutes the fan will ramp to the next highest configured

IA.CF = 2) (Override IAQ)

II.CF = 3 or 4) (Override N/O or
Reheat
Stages HVAC Mode Fan Speed
0 Cooling FS.MX
>0 Reheat1 FS.MX
>0 Reheat2 FS.RH
0 Cooling FS.CL
0 Cooling FS.MX
>0 Reheat1 FS. MX
Reheat1/ FS.CL or
>0
Reheat 2 FS.RH
>0 Reheat2 FS.RH
 
OAU OAFB) is avail-
speed and wait 10 minutes. If the alarm is still active it will
ramp to the next highest configured speed for 10 minutes, and
so on until commanding FS.MX. If the alarm clears on its way
up to max speed or at max speed, the current speed will be
locked in until a standard speed driver commands something
different.

CONSTANT VOLUME (CV) UNITS (FTYP = 1-SPEED)

Single speed fan units are controlled by the Indoor Fan Relay 1
 
(Outputs FANS IDF.1) on the main base board (MBB),
which then operates the indoor fan contactor (IFC). The fan
will be either on or off depending on the unit control type and
the conditions above.
Thermostat Control
In thermostat mode, the IDF relay will be on in the following
situations: fan request G in ON, cooling request Y1 or Y2 is
ON, or heating request W1 or W2 is ON. If the G call is
dropped or was never on with an Y1, Y2, W1, or W2 call; the
IDF relay will turn off after a configurable time delay with re-
spect to the HVAC mode that is ending.
Space Sensor Control
In Space Sensor mode, the IDF relay will be on if the unit is in
Occupied mode and the indoor fan is configured to always run
while occupied (Configuration
the indoor fan is configured for intermittent fan (OC.FN = No),
the fan will only be on when there are cooling, heating, or de-
humidification stages running; or if there is an air quality de-
mand. During the unoccupied period, the fan will operate inter-
mittent. With intermittent fan, the IDF relay will turn off after a
configurable time delay with respect to the HVAC mode that is
ending.

Cooling Operation

The 48/50HC unit's cooling operation consists of: demand,
mode determination, staging request to satisfy the demand, and
handling a request with the unit's resources. These resources
can include compressors, an economizer, and a reheat coil.
This section covers mechanical cooling. For economizer and
reheat cooling, refer to the Economizer and Humidi-MiZer sec-
tions, respectively. The unit enters a cooling mode based on a
demand, decides how to satisfy the demand, executes its plan,
and then leaves the cooling mode.

COOLING MODE CONTROL

The cooling HVAC mode (Run Status
has six different expandable texts: Cooling, Free Cooling, Un-
occupied Free Cooling, Reheat1, Reheat2, or Reheat1/Reheat2.
These are all part of a general cooling mode and resemble the
specific type of cooling that is being performed at any given
time. All types of cooling are still performed under the general
cooling function, and the expanded text is for user reference
only. The control will display if it is ok to select the cooling
mode (Operating Modes
Thermostat Control
For the unit to enter cooling mode, three things must be true:
the indoor fan must be ok to use, the mode changeover time
guard must be expired, and there must be a cooling demand
(Y1, Y2, or dehumidification demand). The unit will remain in
cooling until the cooling demand is dropped or if any of the
above conditions turn false. If only a dehumidification demand
exists and a heat demand (W1, W2) occurs, the unit will end
cooling. The cooling mode can not officially end until all com-
pressors are off.
Space Sensor Control
For the unit to enter cooling mode, four things must be true: the
indoor fan must be ok to use, the mode changeover time guard
must be expired, the unit must have a valid space temperature,
and there must be a cooling or dehumidification demand. The
unit will remain in cooling for at least one minute and until the
20
 
I.FAN OC.FN = YES). If
 
MODE
 
COOL OK.CL= Yes).
HVAC=3)