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Static Pressure Control —
air-conditioning systems must provide varying amounts of air
to the conditioned space. As air terminals downstream of the
unit modulate their flows, the unit must maintain control over
the duct static pressure in order to accommodate the needs of
the terminals and meet the varying combined airflow require-
ment.
The static pressure control routine is also used on CV units
with VFD for staged air volume. The fan is controlled at dis-
crete speeds through the VFD by the unit ComfortLink controls
based on the operating mode of the unit.
A 48/50A3,A5 unit equipped with a duct pressure control
system is provided with a variable frequency drive (VFD) for
the supply fan. The speed of the fan can be controlled directly
by the ComfortLink controls. A transducer is used to measure
duct static pressure. The signal from the transducer is received
by the ECB-2 board and is then used in a PID control routine
that outputs a 4 to 20 mA signal to the VFD.
Generally, only VAV systems utilize static pressure control.
It is required because as the system VAV terminals modulate
closed when less air is required, there must be a means of
controlling airflow from the unit, thereby effectively prevent-
ing overpressurization and its accompanying problems.
A 48/50A2,A4 unit can be equipped with a VFD for staged
air volume control. The speed of the fan is controlled directly
by the ComfortLink controls based on the operating mode of
the unit. A 4 to 20 mA signal is sent to the VFD to control the
fan speed.
The four most fundamental configurations for most applica-
tions are Configuration
SP
sure control type, Configuration
cate CV unit with VFD (staged air volume). Configura-
tion
SP
SP.S, used to enable the static pressure sensor, and
Configuration
SP
SP.SP, the static pressure setpoint to be
maintained.
OPERATION — On VAV units equipped with a VFD and a
proper static pressure sensor, when SP.CF, SP.S and SP.SP are
configured, a PID routine periodically measures the duct static
pressure and calculates the error from setpoint. This error is
simply the duct static pressure setpoint minus the measured
duct static pressure. The error becomes the basis for the propor-
tional term of the PID. The routine also calculates the integral
of the error over time, and the derivative (rate of change) of the
error. A value is calculated as a result of this PID routine, and
this value is then used to create an output signal used to adjust
the VFD to maintain the static pressure setpoint.
Static pressure reset is the ability to force a lowering of the
static pressure setpoint through an external control signal.
The unit controls support this in two separate ways, through a 4
to 20 mA signal input wired to the unit's isolator board input
terminals (third party control) or via CCN.
When employing the CCN, this feature uses the communica-
tions capabilities of VAV systems with ComfortID™ terminals
under linkage. The system dynamically determines and
maintains an optimal duct static pressure setpoint based on the
actual load conditions in the space. This can result in a signifi-
cant reduction in required fan energy by lowering the setpoint to
only the level required to maintain adequate airflow throughout
the system.
OPERATION — On CV units equipped with a VFD (Staged
Air Volume) when SP.CF, CV.FD, SP.FN are configured, the
ComfortLink controls will control the speed of the supply fan
based on the operating mode of the unit. The VFD speed setting
points are SP.MN, SP.MX, HT.VM. When in LOW COOL
mode and the compressor stage less than 50%, fan will be as
SP.MN minimum speed. When in HIGH COOL, the fan will be
at SP.MX maximum speed. In heating mode, the fan will operate
at SP.MX maximum speed when the heating stage is 75% or
greater and at HT.VM heating minimum speed when the heating
Variable air volume (VAV)
SP.CF, which is the static pres-
SP
CV.FD, used to indi-
stage is less than 75%. On units configured for two-stage ther-
mostat operation, the fan will be at SP.MX on a call for W2 and
at HT.VM on a call for only W1.
SETTING UP THE SYSTEM — The options for static
pressure control are found under the Local Display Mode
Configuration
SP. See Table 60.
Failure to correctly configure SP.CF and SP.FN when
operating in VFD Bypass mode will result in the indoor fan
motor running continuously. Damage to unit could result.
Static Pressure Configuration (SP.CF) — This variable is
used to configure the use of ComfortLink controls for static
pressure control. There are the following options:
0 (None) — There will be no static pressure control by Com-
fortLink controls. This setting would be used for a constant vol-
ume (CV) application when static pressure control is not re-
quired or for a VAV application if there will be third-party con-
trol of the VFD. In this latter case, a suitable means of control
must be field installed. This setting must be used on CV units
with VFD (staged air volume).
Additionally, SP.CF must be set to 0 (None) when a unit is
equipped with optional VFD bypass and is operating in Bypass
mode. Failure to change this configuration in Bypass mode will
result in the indoor fan motor running continuously.
1 (VFD Control) — This will enable the use of ComfortLink
controls for static pressure control via a supply fan VFD.
Constant Vol IDF ia VFD? (CV.FD) — This variable enables
the use of a CV unit with VFD for staged air volume control.
Static Pressure Fan Control? (SP.FN) — This is automatical-
ly set to Yes when SP.CF = 1 or when CV.FD is set to Yes.
When the user would like the 4 to 20 mA output to energize
the VFD, as opposed to the fan relay, SP.FN may be set to Yes
when SP.CF = 0. When the control turns the fan ON, the con-
trol will send the SP.MX value of the 4 to 20 mA signal to the
third party VFD control.
Additionally, SP.FN must be set to NO when the unit is
equipped with optional VFD bypass and is operating in Bypass
mode. Failure to change this configuration in bypass mode will
result in the indoor fan motor running continuously.
Static Pressure Sensor (SP.S) — This variable enables the use
of a supply duct static pressure sensor. This must be enabled to
use ComfortLink controls for static pressure control. If using a
third-party control for the VFD, this should be disabled. This is
not used when CV.FD is set to Yes.
Static Pressure Low Range (SP.LO) — This is the minimum
static pressure that the sensor will measure. For most sensors
this will be 0 in. wg. The ComfortLink controls will map this
value to a 4 mA sensor input.
Static Pressure High Range (SP.HI) — This is the maximum
static pressure that the sensor will measure. Commonly this
will be 5 in. wg. The ComfortLink controls will map this value
to a 20 mA sensor input.
Static Pressure Setpoint (SP.SP) — This is the static pressure
control point. It is the point against which the ComfortLink con-
trols compare the actual measured supply duct pressure for deter-
mination of the error that is used for PID control. Generally one
would set SP.SP to the minimum value necessary for proper op-
eration of air terminals in the conditioned space at all load condi-
tions. Too high of a value will cause unnecessary fan motor pow-
er consumption at part load conditions and/or noise problems.
Too low a value will result in insufficient airflow.
VFD Minimum Speed (SP.MN) — This is the minimum speed
for the supply fan VFD. Typically the value is chosen to main-
tain a minimum level of ventilation.
54
CAUTION
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