Static Pressure Reset - Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service And Troubleshooting Instructions

STATIC PRESSURE RESET

CCN Linkage — The ComfortLink 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 Linkage Master Terminal System Logic. The
Linkage Master terminal monitors the primary air damper posi-
tion of all the terminals in the system (done through LINKAGE
with the new ComfortID™ 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 two 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 en-
ter 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 sup-
ply 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 de-
livering the required airflow at every terminal under the current
ITEM
Inputs
 
4-20 SP.M Static Pressure mA
 
4-20 SP.M.T Static Pressure mA Trim
 
RSET SP.RS Static Pressure Reset
 
4-20 SP.RM Static Pressure Reset mA
 
RSET SP.RS Static Pressure Reset Sensor
Outputs
 
FANS S.VFD Supply Fan VFD Speed
Table 47 — Static Pressure Reset Related Points
EXPANSION
4-20
-2.0  +2.0
0.0-3.0
4-20
Enable/Diable
0-100
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, and 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 al-
ways subtracted from the configured static pressure set point
by the equipment controller. The SPRESET variable is always
checked to be a positive value or zero only (negative values are
clamped to zero). The result of the subtraction of the SPRESET
variable from the configured set point is limited so that it can-
not be less than zero.
The result is that the system will dynamically determine the
required duct static pressure based on the actual load condi-
tions 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
difference between the design static pressure set point and the
required 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 4-20mA Input — It is also possible to perform
static pressure reset via an external 4-20 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 4-20 mA input shares the same input as
the analog OAQ sensor. Therefore, obviously both sensors can-
not be used at the same time. To enable the static pressure reset
4-20 mA sensor: Set Configuration
to "Enabled."
Static Pressure Reset Sensor (SP.RS) — If the outdoor air
quality sensor is not configured (Configuration

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 pres-
sure reset via an external 4-20 mA input. Enabling this sensor
will give the user the ability to reset from 0 in. to 3 in. of static,
the supply static
 
tion SP SP.SP), where 4 mA= 0 in. and 20 mA = 3 inches.
As an example: If the static pressure reset input is measur-
ing 6 mA, then the input is resetting 2 mA of its 16 mA (4-20)
"control range." This is essentially
reset. If the static pressure setpoint (SP.SP) = 1.5 in., then the
static pressure control point for the system will be 1.5 - 0.375 =
1.125 inches.
RANGE UNITS
mA
mA
in. wg
mA
%
71
  
UNIT SENS

pressure setpoint (Configura-
2 / of 3 in. or 0.375 in. of
16
CCN POINT DEFAULT
SP_MA
SPMATRIM
SPRESET 0.0
SPRST_MA 0.0
SPRSTSEN Disable
SFAN_VFD
SP.RS

IAQ