71
Static Pressure mA (SP.M)
This variable reflects the value of the static pressure sensor sig
-
nal received by
Comfort
Link controls. It may, in some cases,
be helpful in troubleshooting.
Static Pressure mA Trim (SP.M.T)
This input allows a modest amount of trim to the 4 to 20mA
static pressure transducer signal, and can be used to calibrate a
transducer.
Static Pressure Reset (SP.RS)
This variable reflects the value of the static pressure reset sig
-
nal applied from a CCN system.
Static Pressure Reset mA (SP.M)
This input reflects the value of the static pressure transducer re
-
set signal applied from a CCN system.
Static Pressure Reset Sensor (SP.RS)
This variable can be configured to allow static pressure reset
from a CCN system. See relevant CCN documentation for ad
-
ditional details.
Supply Fan VFD Speed (S.VFD)
This output can be used to check on the actual speed of the
VFD. This may be helpful in some cases for troubleshooting.
STATIC PRESSURE RESET
CCN Linkage
The
Comfort
Link controls supports the use of static pressure
reset. For static pressure reset to occur, the unit must be part of
a CCN system with access to CCN reset variable and the Link
-
age Master Terminal System Logic. The Linkage Master termi
-
nal monitors the primary air damper position of all the termi
-
nals in the system (done through LINKAGE with the new
Comfort
ID™ air terminals).
It then calculates the amount of supply static pressure reduc
-
tion necessary to cause the most open damper in the system to
open more than the minimum value (60%), but not more than
the maximum value (90% or negligible static pressure drop).
This is a dynamic calculation, which occurs every 2 minutes
whenever the system is operating. The calculation ensures that
the supply static pressure is always enough to supply the re
-
quired airflow at the worst case terminal but never more than
necessary, so that the primary air dampers do not have to oper
-
ate with an excessive pressure drop (more than required to
maintain the airflow set point of each individual terminal in the
system). As the system operates, if the most open damper
opens more than 90%, the system recalculates the pressure re
-
duction variable and the value is reduced. Because the reset
value is subtracted from the controlling set point at the equip
-
ment, the pressure set point increases and the primary air
dampers close a little (to less than 90%). If the most open
damper closes to less than 60%, the system recalculates the
pressure reduction variable and the value is increased. This re
-
sults in a decrease in the controlling set point at the equipment,
which causes the primary air dampers to open a little more (to
greater than 60%).
The rooftop unit has the design static pressure set point pro
-
grammed into the CCN control. This is the maximum set point
that could ever be achieved under any condition. To simplify
the installation and commissioning process for the field, this
system control is designed so that the installer only needs to
enter a maximum duct design pressure or maximum equipment
pressure, whichever is less. There is no longer a need to calcu
-
late the worst case pressure drop at design conditions and then
hope that some intermediate condition does not require a high
-
er supply static pressure to meet the load conditions. For
example, a system design requirement may be 1.2 in. wg, the
equipment may be capable of providing 3.0 in. wg, and the
supply duct is designed for 5.0 in. wg. In this case, the installer
could enter 3.0 in. wg as the supply static pressure set point and
allow the air terminal system to dynamically adjust the supply
duct static pressure set point as required.
The system will determine the actual set point required deliver
-
ing the required airflow at every terminal under the current
load conditions. It will always be the lowest value under the
given conditions, and as the conditions and airflow set point at
each terminal change throughout the operating period, so will
the equipment static pressure set point.
The CCN system must have access to a CCN variable (SPRESET
which is part of the equipment controller). In the algorithm for
static pressure control, the SPRESET value is always subtracted
from the configured static pressure set point by the equipment
controller. The SPRESET variable is always checked to be a posi
-
tive value or zero only (negative values are clamped to zero). The
result of the subtraction of the SPRESET variable from the config
-
ured set point is limited so that it cannot be less than zero.
The result is that the system will dynamically determine the re
-
quired duct static pressure based on the actual load conditions
currently in the space. It eliminates the need to calculate the
design supply static pressure set point (although some may still
want to do it anyway). It also saves the energy that is the differ
-
ence between the design static pressure set point and the re
-
quired static pressure (multiplied by the airflow). Normally, the
VAV system operates at the design static pressure set point all
the time; however, a typical VAV system operates at design
conditions less than 2% of the time. A significant saving in fan
horsepower can be achieved utilizing static pressure reset.
Third Party 4 to 20 mA Input
It is also possible to perform static pressure reset via an exter
-
nal 4 to 20 mA signal connected to the CEM board where 4
mA corresponds to 0 in. reset and 20 mA corresponds to 3 in.
of reset. The only caveat to this is that the static pressure 4 to
20 mA input shares the same input as the analog OAQ sensor.
Therefore, obviously both sensors cannot be used at the same
time. To enable the static pressure reset 4 to 20 mA sensor: Set
Configuration
UNIT
SENS
SP.RS
to “Enabled.”
Static Pressure Reset Sensor (SP.RS)
If the outdoor air quality sensor is not configured (
Configura
-
tion
IAQ
AQ.CF
OQ.A.C
= 0), then it is possible to use
that sensor’s location on the CEM board to monitor or perform
static pressure reset via an external 4 to 20 mA input. Enabling
this sensor will give the user the ability to reset from 0 in. to 3
in. of static, the supply static pressure set point (
Configura
-
tion
SP
SP.SP
), where 4 mA= 0 in. and 20 mA = 3 inches.
As an example: If the static pressure reset input is measuring 6
mA, then the input is resetting 2 mA of its 16 mA (4-20) “con
-
trol range.” This is essentially
2
/
16
of 3 in. or 0.375 in. of reset.
If the static pressure set point (
SP.SP
) = 1.5 in., then the static
pressure control point for the system will be 1.5 – 0.375 =
1.125 inches.
Fan Status Monitoring
GENERAL
The N Series
Comfort
Link controls offer the capability to de
-
tect a failed supply fan through either a duct static pressure
transducer or an accessory discrete switch. The fan status
switch is an accessory that allows for the monitoring of a dis
-
crete switch, which trips above a differential pressure drop
across the supply fan. But for any unit with an installed duct
static pressure sensor, it is possible to measure duct pressure
rise directly, which removes the need for a differential switch.
SETTING UP THE SYSTEM
There are 2 configurations of concern located in
Configura
-
tion
UNIT
.
See Table 48.
Summary of Contents for WeatherExpert 48N2
Page 135: ...135 Fig 18 48 50N Typical Power Schematic Nominal 075 Ton Unit Shown ...
Page 136: ...136 Fig 19 48 50N Typical Power Schematic Nominal Ton 90 150 Units Shown ...
Page 137: ...137 Fig 20 48 50N Main Base Board Input Output Connections ...
Page 138: ...138 Fig 21 48 50N RXB EXB CEM Input Output Connections a48 9307 ...
Page 139: ...139 Fig 22 48 50N EXV SCB Input Output Connections a48 9308 ...
Page 140: ...140 Fig 23 48N Typical Modulating Gas Heat Unit Control Wiring ...
Page 141: ...141 Fig 24 50N Typical Electric Heat Unit Control Wiring ...
Page 144: ...144 Fig 27 48N Typical Gas Heat Section Wiring Nominal Ton 120 to 150 Units ...
Page 145: ...145 Fig 28 48 50N Typical Power Component Control Wiring 460 v ...
Page 146: ...146 Fig 29 48 50N Component Control Wiring 575 v Nominal Ton 075 to 150 Units ...
Page 147: ...147 Fig 30 48 50N Component Arrangement Power Box ...
Page 148: ...148 Fig 31 48 50N Component Arrangement Control Box ...
Page 240: ...240 APPENDIX D VFD INFORMATION CONT Fig G VFD Bypass Wiring Diagram WHEN USED ...