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limit line in the custom curve of the psychrometric chart. This
is the outdoor air dewpoint limit boundary.
Custom Psychrometric Curves — Refer to the psychrometric
chart and the standard Honeywell A-D curves in Fig. 11. The
curves start from the bottom and rise vertically, angle to the left
and then fold over. This corresponds to the limits imposed by
dry bulb changeover, outdoor enthalpy changeover and
outdoor dewpoint limiting respectively. Therefore, it is now
possible to create any curve desired with the addition of one
outdoor relative humidity sensor and the options for change-
over now available. See Fig. 12 for an example of a custom
curve constructed on a psychrometric chart.
UNOCCUPIED ECONOMIZER FREE COOLING — This
Free Cooling function is used to start the supply fan and use
the economizer to bring in outside air when the outside
temperature is cool enough to pre-cool the space. This is done
to delay the need for mechanical cooling when the system
enters the occupied period. This function requires the use of a
space temperature sensor.
When configured, the economizer will modulate during an
unoccupied period and attempt to maintain space temperature
to the occupied cooling setpoint. Once the need for cooling has
been satisfied during this cycle, the fan will be stopped.
Configuring the economizer for Unoccupied Economizer
Free Cooling is done in the UEFC group. There are three
configuration options, FC.CF, FC.TM and FC.LO.
Unoccupied Economizer Free Cooling Configuration
(FC.CF) — This option is used to configure the type of unoc-
cupied economizer free cooling control that is desired.
0 = disable unoccupied economizer free cooling
1 = perform unoccupied economizer free cooling as available
during the entire unoccupied period.
2 = perform unoccupied economizer free cooling as available,
FC.TM minutes before the next occupied period.
Unoccupied Economizer Free Cooling Time Configuration
(FC.TM) — This option is a configurable time period, prior to
the next occupied period, that the control will allow unoccu-
pied economizer free cooling to operate. This option is only
applicable when FC.CF = 2.
Unoccupied Economizer Free Cooling Outside Lockout
Temperature (FC.L.O) — This configuration option allows
the user to select an outside-air temperature below which unoc-
cupied free cooling is not allowed. This is further explained in
the logic section.
Unoccupied Economizer Free Cooling Logic — The follow-
ing qualifications that must be true for unoccupied free cooling
to operate:
• Unit configured for an economizer
• Space temperature sensor enabled and sensor reading
within limits
• Unit is in the unoccupied mode
• FC.CF set to 1 or FC.CF set to 2 and control is within
FC.TM minutes of the next occupied period
• Not in the Temperature Compensated Start Mode
• Not in a cooling mode
• Not in a heating mode
• Not in a tempering mode
• Outside-air temperature sensor reading within limits
• Economizer would be allowed to cool if the fan were
requested and in a cool mode
• OAT > FC.L.O (1.0° F hysteresis applied)
• Unit not in a fire smoke mode
• No fan failure when configured to for unit to shut down
on a fan failure
If all of the above conditions are satisfied:
Unoccupied Economizer Free Cooling will start when both of
the following conditions are true:
{SPT > (OCSP + 2)} AND {SPT > (OAT + 8)}
The Unoccupied Economizer Free Cooling Mode will stop
when either of the following conditions are true:
{SPT < OCSP} OR {SPT < (OAT + 3)} where SPT = Space
Temperature and OCSP = Occupied Cooling Setpoint.
When the Unoccupied Economizer Free Cooling mode is
active, the supply fan is turned on and the economizer damper
modulated to control to the supply air setpoint (Setpoints
SASP) plus any supply air reset that may be applied (Inputs
RSET
SA.S.R).
FDD CONFIGURATIONS
Log Title 24 Faults (LOG.F) — Enables Title 24 detection
and logging of mechanically disconnected actuator faults.
T24 Econ Move Detect (EC.MD) — Detects the amount of
change required in the reported position before economizer is
detected as moving.
T24 Econ Move SAT Test (EC.ST)
amount the economizer must move in order to trigger the test
for a change in SAT. The economizer must move at least
EC.ST % before the control will attempt to determine whether
the actuator is mechanically disconnected.
T24 Econ Move SAT Change (S.CHG) — The minimum
amount (in degrees F) SAT is expected to change based on
economizer position change of EC.ST.
T24 Econ RAT-OAT Diff (E.SOD) — The minimum amount
(in degrees F) between RAT (if available) or SAT (with econo-
mizer closed and fan on) and OAT to perform mechanically
disconnected actuator testing.
T24 Heat/Cool End Delay (E.CHD) — The amount of time
(in minutes) to wait before mechanical cooling or heating has
ended before testing for mechanically disconnected actuator.
This is to allow SAT to stabilize at conclusion of mechanical
cooling or heating.
T24 Test Minimum Position (ET.MN) — The minimum po-
sition below which tests for a mechanically disconnected actu-
ator will not be performed. For example, if the actuator moves
entirely within the range 0 to ET.MN a determination of
whether the actuator is mechanically disconnected will not be
made. This is due to the fact that at the extreme ends of the ac-
tuator movement, a change in position may not result in a de-
tectable change in temperature. When the actuator stops in the
range 0 to 2% (the actuator is considered to be closed), a test
shall be performed where SAT is expected to be approximately
equal to RAT. If SAT is not determined to be approximately
equal to RAT, a "damper not modulating" alert shall be logged.
T24 Test Maximum Position (ET.MX) — The maximum po-
sition above which tests for a mechanically disconnected actua-
tor will not be performed. For example, if the actuator moves
entirely within the range ET.MX to 100 a determination of
whether the actuator is mechanically disconnected will not be
made. This is due to the fact that at the extreme ends of the ac-
tuator movement, a change in position may not result in a de-
tectable change in temperature. When the actuator stops in the
range 98 to 100% (the actuator is considered to be open), a test
shall be performed where SAT is expected to be approximately
equal to OAT. If SAT is not determined to be approximately
equal to OAT, a "damper not modulating" alert shall be logged.
SAT Settling Time (SAT.T) — The amount of time (in sec-
onds) the economizer reported position must remain un-
changed (± EC.MD) before the control will attempt to detect a
mechanically disconnected actuator. This is to allow SAT to
stabilize at the current economizer position. This configuration
sets the settling time of the supply-air temperature (SAT). This
typically tells the control how long to wait after a stage change
before trusting the SAT reading, and has been reused for Title
24 purposes.
60
—
The
minimum
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