57
airflow from the unit, thereby effectively preventing overpres-
surization 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
SP.CF, which is the static
pressure control type, Configuration
SP
CV.FD, used to
indicate CV unit with VFD (staged air volume). Configuration
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 pres-
sure 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 pres-
sure. 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 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 ter-
minals (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 con-
trols will control the speed of the supply fan based on the oper-
ating 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 mini-
mum 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 heat-
ing stage is less than 75%. On units configured for two-stage
thermostat 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 Lo-
cal Display Mode Configuration
SP. See Table 59.
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 ComfortLink con-
trols. This setting 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.
In this latter case, a suitable means of control must be field in-
stalled. This setting must be used on CV units with VFD
(staged air volume).
Table 59 — Static Pressure Control Configuration
* Some defaults are model number dependent.
† 67 when CV.FD = Yes.
CAUTION
Failure to correctly configure SP.CF and SP.FN when oper-
ating in VFD Bypass mode will result in the indoor fan mo-
tor running continuously. Damage to unit could result.
ITEM EXPANSION RANGE UNITS CCN POINT DEFAULT
SP SUPPLY STATIC PRESS.CFG.
SP.CF Static Pres. VFD Control? 0, 1 STATICFG 0*
CV.FD Constant VOL IDF is VFD Yes/No CVIDFVFD No
SP.FN Static Pres. Fan Control? Yes/No STATPFAN Yes*
SP.S Static Pressure Sensor Enable/Disable SPSENS Disable*
SP.LO Static Press. Low Range –10 to 0 in. W.C. SP_LOW 0
SP.HI Static Press. High Range 0 to 10 in. W.C. SP_HIGH 5
SP.SP Static Pressure Setpoint 0 to 5 in. W.C. SPSP 1.5
SP.MN VFD Minimum Speed 0 to 100 % STATPMIN 20†
SP.MX VFD Maximum Speed 0 to 100 % STATPMAX 100
SP.FS VFD Fire Speed Override 0 to 100 % STATPFSO 100
HT.V.M VFD Heating Minimum Speed 75 to 100 % VFDHTMIN 75
SP.RS Stat. Pres. Reset Config 0 to 4 SPRSTCFG 0
SP.RT SP Reset Ratio (/dF) 0 to 2.00 SPRRATIO 0.2
SP.LM SP Reset Limit in iwc () 0 to 2.00 SPRLIMIT 0.75
SP.EC SP Reset Econo.Position 0 to 100 % ECONOSPR 5
S.PID STAT.PRESS.PID CONFIGS
SP.TM Static Press. PID Run Rate 1 to 200 sec SPIDRATE 2
SP.P Static Press. Prop. Gain 0 to 100 STATP_PG 20
SP.I Static Press. Intg. Gain 0 to 50 STATP_IG 2
SP.D Static Press. Derv. Gain 0 to 50 STATP_DG 0
SP.SG Static Press. System Gain 0 to 50 STATP_SG 1.0
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