Static Pressure Reset; Related Points - Carrier 48AJ Operating And Service Manual

Operation — The ComfortLink™ controls support 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).
The Linkage Master then calculates the amount of supply
static pressure reduction 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 negligi-
ble 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 required airflow at the worst case termi-
nal but never more than necessary, so that the primary air
dampers do not have to operate 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 reduction
variable and the value is reduced. Because the reset value is
subtracted from the controlling set point at the equipment, 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 results 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 static pressure set point
programmed 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 calculate the worst case pressure drop at design condi-
tions and then hope that some intermediate condition does not
require a higher 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 dynami-
cally 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
load conditions. It will always be the lowest value under the
ITEM
Inputs
→ 4-20 → SP.M
Static Pressure mA
→ 4-20 → SP.M.T
Static Pressure mA Trim
→ 4-20 →SP.R.M
Static Pressure Reset mA
→ RSET →SP.RS

Static Pressure Reset

Outputs
→ Fans → S.VFD
Supply Fan VFD Speed
Table 70 — Static Pressure Reset Related Points
EXPANSION
4-20
-2.0 - +2.0
4-20
0.0-3.0
0-100
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
always 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
limited to zero). The result of the subtraction of the SPRESET
variable from the configured set point is limited so that it
cannot be less than zero. The result is that the system will
dynamically determine the required duct static pressure based
on the actual load conditions currently in the space. It elimi-
nates the need to calculate the design supply static pressure set
point. It also saves the energy that is the difference between the
design static pressure set point and the required static pressure.
Third party 4 to 20 mA input — It is also possible to perform
static pressure reset via an external 4 to 20 mA signal connect-
ed to the CEM board where 4 mA corresponds to 0 in. wg reset
and 20 mA corresponds to 3 in. wg of reset. Please note that the
static pressure 4 to 20 mA input shares the same input as the
analog OAQ sensor. Therefore, both sensors cannot be used at
the same time. To enable the static pressure reset 4 to 20 mA
sensor, set (Configuration
Enabled.
RELATED POINTS — These points represent static pressure
control and static pressure reset inputs and outputs. See Table 70.
Static Pressure mA (SP.M) — This variable reflects the value
of the static pressure sensor signal received by the
ComfortLink control. 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 20 mA static pressure trans-
ducer signal, and can be used to calibrate a transducer.
Static Pressure Reset mA (SP.R.M) — This input reflects the
value of a 4 to 20 mA static pressure reset signal applied to
TB6 terminals 9 and 10 on the CEM board, from a third party
control system.
Static Pressure Reset (SP.RS) — This variable reflects the
value of a static pressure reset signal applied from a CCN sys-
tem. The means of applying this reset is by forcing the value of
the variable SPRESET through CCN.
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.
RANGE UNITS
mA
mA
mA
in. wg
%
56
→ → →
UNIT SENS SP.RS)
CCN POINT DEFAULT
SP_MA
SPMATRIM
SPRST_MA 0.0
SPRESET 0.0
SFAN_VFD
to