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minimum cfm, then the evaporator discharge temperature (EDT)
will be monitored. If the EDT falls below a particular trip point
then tempering mode may be called out.
HVAC mode = "Tempering Vent"
HVAC mode = "Tempering LoCool"
HVAC mode = "Tempering HiCool"
The decision making/selection process for the tempering trip set
point is as follows:
If an HVAC cool mode is in effect, then the tempering cool point
is SASP – T.CL.
If not in effect and unit is in a pre-occupied purge mode (Operat-
ing Modes
MODE
IAQ.P=ON), then the trip point is T.PRG.
If not in effect and unit is in an occupied mode (Operating
Modes
MODE
IAQ.P=ON), then trip point is TEMPVOCC.
For all other cases, the trip point is TEMPVUNC.
NOTE: The unoccupied economizer free cooling does not qualify
as a HVAC cool mode as it is an energy saving feature and has its
own OAT lockout already. The unoccupied free cooling mode
(HVAC mode = Unocc. Free Cool) will override any unoccupied
vent mode from triggering a tempering mode.
A minimum amount of time must pass before calling out any tem-
pering mode. In effect, the EDT must fall below the trip point val-
ue –1°F continuously for a minimum of 2 minutes. Also, at the
end of a mechanical cooling cycle, a 10 minute delay will be en-
forced before considering a tempering during vent mode in order
to allow any residual cooling to dissipate from the evaporator coil.
If the above conditions are met, the algorithm is free to select the
tempering mode (MODETEMP).
If a tempering mode becomes active, the modulating heat source
(staged gas, modulating gas, SCR electric heat, or hot water) will
attempt to maintain leaving-air temperature (LAT) at the temper-
ing set point used to trigger the tempering mode. The technique
for modulation of set point for staged gas, modulating gas, SCR
electric heat, and hydronic heat is the same as in a heat mode.
More information regarding the operation of heating can be ref-
erenced in the Heating Control section on page 63.
Recovery from a tempering mode (MODETEMP) will occur
when the EDT rises above the trip point. On any change in
HVACMODE, the tempering routine will re-assess the tempering
set point which may cause the control to continue or exit temper-
ing mode.
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 accommo-
date the needs of the terminals, and therefore to meet the varying
combined airflow requirement. The unit design includes two alter-
native optional means of accommodating this requirement. This
section describes the technique by which this control takes place.
A unit intended for use in a VAV system can be equipped with an
optional 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 RXB board and is
then used in a PID control routine to determine the required fan
speed. The required speed is then communicated to the VFD.
Generally only VAV systems utilize static pressure control. It is re-
quired because as the system VAV terminals modulate closed
when less air is required, there must be a means of controlling air-
flow from the unit, thereby effectively preventing overpressuriza-
tion and its accompanying problems.
The static pressure control routine is also used on CV units with
VFD for staged air volume. The fan is controlled at discrete
speeds through the VFD by the unit ComfortLink controls based
on the operating mode of the unit.
The four most fundamental configurations for most applications
are Configuration
SP
trol type, Configuration
with VFD, staged air volume control, Configuration
used to enable the static pressure sensor, and Configuration
SP
SP.SP, the static pressure set point to be maintained.
OPERATION FOR VAV
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 rou-
tine 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 calcu-
lated 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
further detail below, the control supports this in two separate
ways; through a 4 to 20 mA signal input wired to TB202 terminals
6 and 7 (thereby facilitating third-party control), or via CCN
controls.
In the latter case, this feature leverages the communications capa-
bilities of VAV systems employing ComfortID™ terminals under
linkage. The system dynamically determines and maintains an op-
timal duct static pressure set point based on the actual load condi-
tions in the space. This can result in a significant reduction in re-
quired fan energy by lowering the set point to only the level re-
quired to maintain adequate airflow throughout the system.
OPERATION FOR SAV
On CV units equipped with VFD (staged air volume) when the
unit is in VENT mode, the supply fan VFD will operate at min-
imum speed (SP.MN). If SAV is configured to optimize for de-
humidification (SAV-DEHUM=Yes) and the unit is in LOW
COOL or HIGH COOL mode and the compressor stage is be-
low 60%, the supply fan VFD will operate at minimum speed
(SP.MN). When the compressor stage is at or above 60%, the
supply fan VFD will operate at maximum speed (SP.MX).
ComfortLink will also perform an airflow check at minimum
fan speed (SP.MN) by comparing the SST to the SAT. If the
SST and SAT are close, indicating low airflow, the fan will be
commanded to the maximum fan speed (SP.MX) until suffi-
cient separation between SST and SATs has been achieved. In
the condition where the speed is changing up and down below
60% compressor capacity, the SP.MN may be set too low or
the airway may be obstructed, leading to higher than normal
temperature drop across the evaporator coil. If SAV is config-
ured to optimize for sensible control (SAVDE-HUM=No) and
the unit is in LOW COOL mode, the supply fan VFD will op-
erate at minimum speed (SP.MN). If the unit is HIGH COOL
mode, the supply fan VFD will operate at maximum speed
(SP.MX).
SETTING UP THE SYSTEM
The options for static pressure control are found under the Local
Display Mode Configuration
Static Pressure Configuration (SP.CF)
This variable is used to configure the use of ComfortLink controls
for static pressure control.
When set to disable, there is no static pressure control by
ComfortLink controls. 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.
In this latter case, a suitable means of control must be field
installed.
72
SP.CF, which is the static pressure con-
SP
SP.SV, used to indicate CV unit
SP. See Table 61.
SP
SP.S,
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