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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 dis-
connected 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
change 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 to T24T-
STMN%.
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 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))
The control shall test for a jammed actuator as follows:
• If the actuator has stopped moving and the reported position
(ECONOPOS) is not within ± 3% of the commanded posi-
tion (ECONOCMD) after 20 seconds, a "damper stuck or
jammed" alert shall be logged.
• If the actuator jammed while opening (i.e., reported position
is less than the commanded position), a "not economizing
when it should" alert shall be logged.
• If the actuator jammed while closing (i.e., reported position
is greater than the 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 jammed while opening, when ECONOPOS
is greater than the jammed position the alerts shall be
cleared.
• If the actuator jammed while closing, when ECONOPOS <
jammed position the alerts shall be cleared.
DIFFERENTIAL DRY BULB CUTOFF CONTROL (Dif-
ferential 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 disallow free cooling as shown below:
E.SEL
DDB.C
(ECON_SEL)
(EC_DDBCO
NONE,
N/A
OUTDR.ENTH,
DIF.ENTHALPY
0 deg F
–2 deg F
DIFF.DRY
BULB
–4 deg F
–6 deg F
The status of differential dry bulb cutoff shall be visible under
Run Status
ECON
There shall be hysteresis where OAT must fall 1 deg F lower
than the comparison temperature when transitioning from
DDBCSTAT=YES to DDBSTAT=NO.
SETTING UP THE SYSTEM — The economizer configura-
tion options are under the Local Display Mode Configuration
ECON. See Table 64.
Economizer Installed? (EC.EN) — If an economizer is not
installed or is to be completely disabled then the configuration
option EC.EN should be set to No. Otherwise in the case of an
installed economizer, this value must be set to Yes.
Economizer Minimum Position (EC.MN) — The configura-
tion option EC.MN is the economizer minimum position. See
the section on indoor air quality for further information on how
to reset the economizer further to gain energy savings and to
more carefully monitor IAQ problems.
Economizer Maximum Position (EC.MX) — The upper lim-
it of the economizer may be limited by setting EC.MX. This
value defaults to 98% to avoid problems associated with slight
changes in the economizer damper's end stop over time. Typi-
cally this will not need to be adjusted.
Economizer Position at Minimum VFD Speed (EP.MS) —
The configuration option EP.MS is the economizer command-
ed position at SP.MN (STATPMIN), which is the minimum
speed for the supply fan VFD. Typically the value is chosen to
maintain a minimum level of ventilation. See the section on in-
door air quality for further information on how to reset the
economizer further to gain energy savings and to more careful-
ly monitor IAQ problems.
Economizer Position at Maximum VFD Speed (EP.XS) —
The configuration option EP.XS is the economizer commanded
position at SP.MX (STATPMAX), which is the maximum
speed for the supply fan VFD. This is usually set to 100%
when CV.FD = Yes, the range is 33 to 67% with the default set-
ting of 67%. See the section on indoor air quality for further in-
formation on how to reset the economizer further to gain ener-
gy savings and to more carefully monitor IAQ problems.
58
OAT/RAT
Comparison
(DDBCSTAT)
N/A
NO
OAT>RAT
YES
OATRAT
NO
OAT>RAT-2
YES
OATRAT-2
NO
OAT>RAT-4
YES
OATRAT-4
NO
OAT>RAT-6
YES
OATRAT-6
NO
DISA
DDBC.
DDBC
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