61
Setting up the System — The relevant set points for temper-
ing are located at the local display under Setpoints:
Operation — First, the unit must be in a vent mode, a low cool,
or a high cool HVAC mode to be considered for a tempering
mode. Secondly, the tempering mode is only allowed when the
rooftop is configured for staged gas or hydronic heating (Con-
figuration
→
HEAT
→
HT.CF=3 or 4).
If the control is configured for staged gas or hydronic heat-
ing and the control is in a vent, low cool, or high cool HVAC
mode, and the rooftop control is in a situation where the econo-
mizer must maintain a minimum position/minimum cfm, then
the evaporator discharge temperature (EDT) will be monitored.
If the EDT falls below a particular trip point then the 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 vent trip
point is T.CL.
• If in a pre-occupied purge mode (Operating Modes
→
MODE
→
IAQ.P=ON), then the trip point is T.PRG.
• If in an occupied mode (Operating Modes
→
MODE
→
IAQ.P=ON), then the 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.
If OAT is above the chosen tempering set point, tempering
will not be allowed. Additionally, tempering mode is locked
out if any stages of mechanical cooling are present.
A minimum amount of time must pass before calling out
any tempering mode. In effect, the EDT must fall below the
trip point value –1° F continuously for a minimum of 2 min-
utes. Also, at the end of a mechanical cooling cycle, there must
be a minimum 10 minutes of delay allowed 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 or hot water) will attempt to maintain
leaving-air temperature (LAT) at the tempering set point used
to trigger the tempering mode. The technique for modulation of
set point for staged gas and hydronic heat is the same as in a
heat mode. More information regarding the operation of heat-
ing can be referenced in the Heating Control section.
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 tem-
pering set point which may cause the control to continue or exit
tempering 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 accommodate the
needs of the terminals, and therefore to meet the varying
combined airflow requirement. The unit design includes two
alternative 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 either an optional variable frequency drive (VFD) or inlet
guide vanes (IGV) for the supply fan. The speed of the fan or
the position of the IGV 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 RCB board and is then used in a PID control routine that
outputs a 4 to 20 mA signal to the VFD, or a digital LEN signal
to the IGV.
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 either VFD or IGV
and a proper static pressure 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 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 cal-
culates 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 IGV or 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
TB203 terminals 6 and 7 (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 52.
ITEM EXPANSION RANGE UNITS CCN
POINT DEFAULT
T.PRG Tempering
Purge SASP
–20-80 dF TEMPPURG 50
T.CL Tempering in
Cool SASP
5-75 dF TEMPCOOL 5
T.V.OC Tempering Vent
Occ SASP
–20-80 dF TEMPVOCC 65
T.V.UN Tempering Vent
Unocc. SASP
–20-80 dF TEMPVUNC 50