Carrier WeatherExpert 48N2 Controls, Start-Up, Operation, Service, And Troubleshooting page 74

The control shall test for a mechanically disconnected damper
if all the following conditions are true:
1. An economizer is installed.
2. The supply fan is running.
3. Conditions are good for economizing.
4. The difference between RAT and OAT > T24RATDF. It is
necessary for there to be a large enough difference
between RAT and OAT in order to measure a change in
SAT as the damper modulates.
5. The actuator has moved at least T24ECSTS %. A very
small change in damper position may result in a very small
(or non-measurable) change in SAT.
6. At least part of the economizer movement is within the
range T24TSTMN % to T24TSTMX %. Because the mix-
ing of outside air and return air is not linear over the entire
range of damper position, near the ends of the range even a
large change in damper position may result in a very small
(or non-measurable) change in SAT.
Furthermore, the control shall test for a mechanically discon-
nected actuator after T24CHDLY minutes have expired when
any of the following occur (this is to allow the heat/cool cycle
to dissipate and not influence SAT):
1. The supply fans switches from OFF to ON.
2. Mechanical cooling switches from ON to OFF.
3. Reheat switches from ON to OFF.
4. The SAT sensor has been relocated downstream of the
heating section and heat switches from ON to OFF.
The economizer shall be considered moving if the reported po-
sition has changed at least ± T24ECMDB %. A very small
change in position shall not be considered movement.
The determination of whether the economizer is mechanically
disconnected shall occur SAT_SEC/2 seconds after the econo-
mizer has stopped moving. The control shall log a "damper not
modulating" alert if:
1. SAT has not decreased by T24SATMD degrees F SAT_SET/
2 seconds after opening the economizer at least
T24ECSTS %, taking into account whether the entire move-
ment has occurred within the range 0-T24TSTMN %.
2. SAT has not increased by T24SATMD degrees F
SAT_SET/2 seconds after closing the economizer at least
T24ECSTS %, taking into account whether the entire
movement has occurred within the range T24TSTMX to
100%.
3. Economizer reported position ≤5% and SAT is not approx-
imately equal to RAT. SAT not approximately equal to
RAT shall be determined as follows:
a. SAT<RAT-(2*2(thermistor accuracy) + 2 (SAT increase
due to fan)) or
b. SAT>RAT+(2*2(thermistor accuracy) + 2 (SAT increase
due to fan))
4. Economizer reported position ≥95% and SAT is not
approximately equal to OAT. SAT not approximately
equal to OAT shall be determined as follows:
a. SAT<OAT-(2*2(thermistor accuracy) + 2 (SAT increase
due to fan)) or
b. SAT>OAT+(2*2(thermistor accuracy) + 2 (SAT increase
due to fan))
Except when run as part of a self-test, the control shall not au-
tomatically clear "damper not modulating" alerts on units.
Test for stuck/jammed actuator
The control shall test for a jammed actuator as follows:
• If the actuator has stopped moving and the reported position
(ECONxPOS, where x is 1,2) is not within ± EC_FLGAP%
of the command position (ECONOCMD) after EC_FLTMR
seconds, a "damper stuck or jammed" alert shall be logged,
i.e., abs(ECONxPOS - ECONODMD) > EC_FLGAP for a
continuous time period EC_FLTMR seconds.
• If the actuator jammed while opening (i.e., reported posi-
tion < commanded position), a "not economizing when it
should" alert shall be logged.
• If the actuator jammed while closing (i.e., reported posi-
tion > command position), the "economizing when it
should not" and "too much outside air" alerts shall be
logged.
The control shall automatically clear the jammed actuator
alerts as follows:
• If the actuator moves at least 1%, the alerts shall be cleared.
Alternate Excess Outdoor Air Test
For units configured with outdoor air measuring stations
(OCFMSENS=YES):
Configuration  ECON  CFM.C  OCF.S=enabled (set to 1
or 2)
Under the following conditions:
1. Unit is not performing free cooling.
2. OACFM sensor is detected as good.
3. IAQ is not overriding CFM.
4. Purge is not overriding CFM.
If OACFM > (ECMINCFM + EX_ARCFM) for EX_ARTMR
seconds, the "excess outside air" alert shall be logged.
DIFFERENTIAL DRY BULB CUTOFF CONTROL
Differential Dry Bulb Changeover
As both return air and outside air temperature sensors are in-
stalled as standard on these units, the user may select this option,
E.SEL = 1, to perform a qualification of return and outside air in
the enabling/disabling of free cooling. If this option is selected
the outside-air temperature shall be compared to the return-air
temperature to dis-allow free cooling as shown in Table 53:
Table 53 — Differential Dry Bulb Cutoff Control
E.SEL (ECON_SEL)
(EC_DDBCO)
NONE,
OUTDR.ENTH,
DIF.ENTHALPY
DIFF.DRY BULB
The status of differential dry bulb cutoff shall be visible under
Run Status  ECON  DISA  DDBC.
There shall be hysteresis where OAT must fall 1°F lower
than the comparison temperature when transitioning from
DDBCSTAT=YES to DDBSTAT=NO.
ECONOMIZER SELF-TEST
It is possible for one actuator to become mechanically discon-
nected while the other(s) continue to work properly. The fol-
lowing self-test utilizes fan characteristics and motor power
measurements to determine whether the dampers are properly
modulating. A fan that is moving more air uses additional pow-
er than a fan that is moving a lesser quantity of air. In this test,
each actuator/damper assembly is commanded independently
while the fan and motor characteristics are monitored.
It shall be possible to manually start the self-test:
74
DDB.O OAT/RAT
Comparison (DDBCSTAT)
N/A N/A
OAT>RAT
0 degF
OAT<=RAT
OAT>RAT-2
–2 degF
OAT<=RAT-2
OAT>RAT-4
–4 degF
OAT<=RAT-4
OAT>RAT-6
–6 degF
OAT<=RAT-6
DDBC
NO
YES
NO
YES
NO
YES
NO
YES
NO