Specifications

53
ECONOMIZER DIAGNOSTIC HELP — Because there are
so many conditions which might disable the economizer from
being able to provide free cooling, the control has a display
table to identify these potentially disabling sources. The user
can check ACTV, the “Economizer Active” flag. If this flag is
settoYesthereisnoreasontocheckDISA (Economizer
Disabling Conditions). If the flag is set to No, this means that at
least one or more of the flags under the group DISA are set
to Yes and the user can discover what is preventing the econo-
mizer from performing free cooling by checking the table.
The economizer’s reported and commanded positions
are also viewable, as well as outside air temperature, relative
humidity, enthalpy and dew point temperature.
The following information can be found under the Local
Display Mode Run Status
→ ECON. See Table 62.
Economizer Control Point Determination Logic
— Once the
economizer is allowed to provide free cooling, the economizer
must determine exactly what set point it should try to maintain.
The set point the economizer attempts to maintain when “free
cooling” is located at Run Status
→ VIEW → EC.C.P.Thisis
the economizer control point.
The control selects set points differently, based on the
control type of the unit. This control type can be found at
Configuration
→ UNIT → C.TYP. There are 6 types of control.
C.TYP =1 VAV-RAT
C.TYP =2 VAV-SPT
C.TYP = 3 TSTAT Multi-Staging
C.TYP =4 TSTAT2Stage
C.TYP = 5 SPT Multi-Staging
C.TYP =6 SPT2Stage
If the economizer is not allowed to do free cooling, then
EC.C.P =0.
If the economizer is allowed to do free cooling and the
Unoccupied Free Cooling Mode is ON, then EC.C.P =
Setpoints
→ SASP + Inputs → RSET → SA.S.R.
If the economizer is allowed to do free cooling and the
Dehumidification mode is ON, then EC.C.P = the Cooling
Control Point (Run Status
→ VIEW → CL.C.P).
If the C.TYP is either 4 or 6, and the unit is in a cool mode,
then
If Stage = 0 EC.C.P = the Cooling Control Point (Run
Status
→ VIEW → CL.C.P)
If Stage = 1 53.0 + economizer suction pressure reset (see
below)
If Stage = 2 48.0 + economizer suction pressure reset (see
below)
NOTE: To check the current cooling stage go to Run Status
→ Cool → CUR.S.
If the C.TYP is either 1,2,3 or 5, and the unit is in a cool
mode, then EC.C.P = the Cooling Control Point (Run Status
→
VIEW → CL.C.P).
Economizer Suction Pressure Reset for Two-Stage
Cooling — If the unit’s control type is set to either 2-stage ther-
mostat or 2-stage space temperature control, then there is no
cooling control point. Stages 1 and 2 are brought on based on
demand, irrespective of the evaporator discharge temperature.
In this case, the economizer monitors suction pressure and
resets the economizer control point accordingly in order to
protect the unit from freezing. For those conditions when the
economizer opens up fully but is not able to make set point, and
then a compressor comes on, it is conceivable that the coil
might freeze. This can be indirectly monitored by checking suc-
tion pressure. Rather than fail a circuit, the control will attempt
to protect the unit by resetting the economizer control point
until the suction pressure rises out of freezing conditions.
If either circuit’s suction pressure drops to within 5 psig
of the low suction pressure trip point, the control will start
adding reset to the economizer control point if it is active. It
will be possible to reset the control point upwards, 10 degrees
(2 degrees per psig), between the low suction pressure trip
point of 48 psig and 5 psig above it. If this does not work, and if
the suction pressure drops below the trip point, then the control
will further reset the control point 1 degree every 15 seconds
up to a maximum of 10 degrees. The resulting effect will be to
warm up the mixed air entering the evaporator, thereby raising
the suction pressure.
Fig. 9 — Custom Changeover Curve Example