54
Static Pressure Control —
Variable air volume (VAV)
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 simply maintain
control over duct static pressure in order to accommodate the
needs of the terminals, and therefore to meet the varying
combined airflow requirement.
A 48/50AK,AY unit equipped with a duct pressure control
system is provided with an optional variable frequency drive
(VFD) for the supply fan. The speed of the fan can be con-
trolled directly by the
Comfort
Link™ control. A transducer is
used to measure duct static pressure. The signal from the trans-
ducer 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.
The three most fundamental configurations for most appli-
cations are
Configuration
→
SP
→
SP.CF
, which is the static
pressure control type,
Configuration
→
SP
→
SP.S
, used to
enable the static pressure sensor, and
Configuration
→
SP
→
SP.SP
, the static pressure set point to be maintained.
OPERATION — On units equipped with a VFD and a proper
static pressure sensor, when
SP.CF
,
SP.S
and
SP.SP
are config-
ured, a PID routine periodically measures the duct static
pressure and calculates the error from set point. This error at
any point in time is simply the duct static pressure set point
minus the measured duct static. The error becomes the basis for
the Proportional 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 set point.
Static pressure reset is the ability to force a lowering of the
static pressure set point through an external control signal.
Explained in detail further below, the control supports this in
two separate ways; through a 4 to 20 mA signal input wired to
the unit’s isolator board input terminals (thereby facilitating
third party control), or via CCN.
In the latter case, this feature leverages the communications
capabilities of VAV systems employing ComfortID™ termi-
nals under linkage. The system dynamically determines and
maintains an optimal duct static pressure set point based on
the actual load conditions in the space. This can result in a
significant reduction in required fan energy by lowering the set
point to only the level required to maintain adequate airflow
throughout the system.
SETTING UP THE SYSTEM — The options for static
pressure control are found under the Local Display Mode
Configuration
→
SP
. See Table 69.
Static Pressure Configuration (
SP.CF
) — This variable is
used to configure the use of
Comfort
Link™ control for static
pressure control. It has the following options:
0 (None) — No static pressure control by
Comfort
Link
control. This would be used for a constant volume (CV)
application when static pressure control is not required or for a
VAV application if there will be third-party control of the VFD
or IGV. In this latter case, a suitable means of control must be
field installed.
1 (VFD Control) — This will enable the use of
Comfort
Link
for static pressure control via a supply fan VFD.
Static Pressure Fan Control? (
SP.FN
) — This is automatical-
ly set to Yes when
SP.CF
= VFD Control. When the user
would like the 4 to 20 mA output to trigger a VFD on, as op-
posed to the fan relay,
SP.FN
may be set to Yes when
SP.CF
=
none. When the control turns the fan ON, the control will send
an “
SP.MX
”% of the 4 to 20 mA signal to the third party VFD
control.
Static Pressure Sensor (
SP.S
) — This variable enables the use
of a supply duct static pressure sensor. This must be enabled to
use
Comfort
Link control for static pressure control. If using a
third-party control for the VFD, this should be disabled.
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
Comfort
Link control 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
Comfort
Link control will map this value
to a 20 mA sensor input.
Static Pressure Set Point (
SP.SP
) — This is the static pres-
sure control point. It is the point against which the
Comfort
Link control compares the actual measured supply
duct pressure for determination of the error that is used for PID
control. Generally one would set
SP.SP
to the minimum value
necessary for proper operation of air terminals in the condi-
tioned space at all load conditions. Too high of a value will
cause unnecessary fan motor power consumption at part-load
conditions and/or noise problems. Too low a value will result in
insufficient airflow. Additional information will be found on
page 55, under Static Pressure Reset.
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.
NOTE: Most VFDs have a built-in minimum speed adjustment
which must be configured for 0% when using
Comfort
Link
control for static pressure control.
VFD Maximum Speed (
SP.MX
) — This is the maximum
speed for the supply fan VFD. This is usually set to 100%.
VFD Fire Speed Override (
SP.FS
) — This is the speed that
the supply fan VFD will use during the fire modes; pressuriza-
tion, evacuation and purge. This is usually set to 100%.
Static Pressure Reset Configuration (
SP.RS
) — This option
is used to configure the static pressure reset function. When
SP.RS
= 0, there is no static pressure reset via an analog input.
When
SP.RS
= 1, there is static pressure reset based on the
CEM 4-20MA input and ranged from 0 to 3 in. wg. When
SP.RS
= 2, there is static pressure reset based on RAT and
defined by
SP.RT
and
SP.LM
. When
SP.RS
= 3, there is
static pressure reset based on SPT and defined by
SP.RT
and
SP.LM
. When
SP.RS
= 4, there is VFD speed control where
0 mA = 0% speed and 20 mA = 100% (
SP.MN
and
SP.MX
will
override).
Static Pressure Reset Ratio (
SP.RT
) — This option defines
the reset ratio in terms of static pressure versus temperature.
The reset ratio determines how much is the static pressure
reduced for every degree below set point for RAT or SPT.
Static Pressure Reset Limit (
SP.LM
) — This option defines
the maximum amount of static pressure reset that is allowed.
This is sometimes called a “clamp.”
NOTE: Resetting static pressure via RAT and SPT is primarily
a constant volume application which utilizes a VFD. The rea-
soning is that there is significant energy savings in slowing
down a supply fan as opposed to running full speed with
supply air reset. Maintaining the supply air set point and
slowing down the fan has the additional benefit of working
around dehumidification concerns.
Summary of Contents for WEATHERMAKER 48AJ020
Page 95: ...95 Fig 13 Typical Main Control Box Wiring Schematic A48 7787 ...
Page 96: ...96 TO NEXT PAGE Fig 14 Auxiliary Control Box Wiring Schematic A48 7294 ...
Page 98: ...98 Fig 15 Typical 2 Stage Gas Heat Wiring Schematic Size 051 and 060 Units Shown A48 6866 ...
Page 102: ...102 TO NEXT PAGE Fig 18 Typical Power Schematic Size 051 and 060 Units Shown A48 7298 ...
Page 104: ...104 Fig 19 Controls Option Wiring Schematic A48 7810 ...
Page 105: ...105 Fig 20 Small Chassis Component Location Size 020 035 Units A48 7301 ...
Page 106: ...106 Fig 21 Large Chassis Component Locations Size 036 060 Units A48 7302 ...