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 be-
tween 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
position has changed at least +/- T24ECMDB %. A very small
changed in position shall not be considered movement.
The determination of whether the economizer is mechani-
cally disconnected shall occur SAT_SEC/2 seconds after the
economizer 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
movement 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-
100%..
3. Economizer reported position <=5% and SAT is not ap-
proximately 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 ap-
proximately 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
automatically 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 posi-
tion (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=YES
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 installed as standard on
these units, the user may select this option, E.SEL = 1, to per-
form 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 Table 53:
Table 53 — Differential Dry Bulb Cutoff Control
The status of differential dry bulb cutoff shall be visible un-
der Run Status
ECON
DISA
DDBC.
There shall be hysteresis where OAT must fall 1 deg F low-
er than the comparison temperature when transitioning from
DDBCSTAT=YES to DDBSTAT=NO.
ECONOMIZER SELF TEST — It is possible for one actua-
tor to become mechanically disconnected while the other(s)
continue to work properly, the following 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 power than a fan that is mov-
ing a lesser quantity of air. In this test, each actuator/damper as-
sembly is commanded independently while the fan and motor
characteristics are monitored.
E.SEL (ECON_SEL)
DDB.C
(EC_DDBCO)
OAT/RAT
Comparison
DDBC
(DDBCSTAT)
NONE,
OUTDR.ENTH,
DIF.ENTHALPY
N/A N/A NO
DIFF.DRY BULB
0 degF OAT>RAT YES
OAT<=RAT NO
–2 degF OAT>RAT-2 YES
OAT<=RAT-2 NO
–4 degF OAT>RAT-4 YES
OAT<=RAT-4 NO
–6 degF OAT>RAT-6 YES
OAT<=RAT-6 NO
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