Installation manual

035-16682-000-E-0303
Unitary Products Group 77
the duct pressure floats within the null range, neither
contact is closed and ACT3 holds the inlet guide vanes
in position. When a call for heating is signaled by relay
FR1, ACT3 - terminal #3 is powered and the inlet guide
vanes fully open, regardless of the duct pressure or the
status of the ACT3 - terminal #2 input (an input at termi-
nal #3 always overrides any input at terminal #2).
Duct Pressure Control - VFD (Variable Frequency Drive)
Option: The duct pressure controls are powered with a
call for the supply fan. Once powered, the VFD responds
to a VDC control signal from the DSP (Duct Static Pres-
sure transducer). In the heating mode the VDC DSP
pressure signal is replaced with a 24 VAC signal to force
the fan to full volume.
15. Electronic Economizer Option: ELM (Economizer Logic
Module) directly controls the position of the outside air /
return air dampers through a 2-10 VDC signal to ACT1
(economizer actuator). ELM is powered only after supply
fan operation has been verified (APS closed) and when
CM-VAV is powered. When ELM is not powered, the out-
side air dampers spring return closed. When ELM is
powered and there is no Y1 call or if free cooling is
unavailable, ELM opens the economizer dampers to the
adjustable minimum position setting.
With a Y1 call where the outside air is appropriate for
free cooling (see below), ELM closes a contact between
ELM terminals 3 and 5 to engage relay RY1 and RY2
(Cooling Stage Control relays) and free cooling is used
for first stage cooling. (Please see the compressor stag-
ing chart above.) ELM modulates the economizer damp-
ers to maintain a Mixed Air Temperature (Return Air /
Outside Air Mixture, sensed between the filters and the
DX coil) between 50°F and 56°F.
ELM uses one of three methods to determine when free
cooling is available. The method used is determined by
the unit wiring and each method functions as listed
below.
Dry Bulb Changeover
: A dry bulb temperature sensor is
wired to ELM terminals So and +, in place of the Outdoor
Enthalpy sensor and a 620W resistor is connected to ter-
minals Sr and +, in place of the Return Air Enthalpy sen-
sor. The dry bulb changeover point is set through a
potentiometer on ELM labeled with specific points
marked A, B, C, and D, with the following approximate
correspondence: A [45,55°F], B [58,68°F], C [72,82°F],
and D [85,95°F]. When the potentiometer is set to one of
the reference points, ELM will allow free cooling when
the outdoor temperature falls below the indicated tem-
perature range. Free cooling will be available until the
outdoor temperature rises above the indicated tempera-
ture range. Free cooling is disabled when the outdoor
temperature rises above the temperature range and can-
not be enabled until the outdoor temperature falls below
the indicated temperature range. The changeover point
may be set between the reference points, thus allowing
customization for each application.
Single Enthalpy Changeover:
A 620W resistor is con-
nected to terminals Sr and +, in place of the Return Air
Enthalpy sensor. The enthalpy changeover point is set
through a potentiometer on ELM labeled with specific
points marked A, B, C, and D. At 50% relative humidity,
these points correspond to the following temperatures:
A-73°F, B-70°F, C-67°F, and D-63°F. The changeover
point may be set between the reference points, thus
allowing customization for each application. When the
outdoor enthalpy is below the changeover setpoint, free
cooling is available.
Dual Enthalpy (Comparative/Differential Enthalpy)
Changeover: Both the Outdoor Air Enthalpy and the
Return Air Enthalpy sensors are connected to ELM. The
changeover setpoint potentiometer is turned fully clock-
wise (past the D reference). When the outdoor air
enthalpy is less than the return air enthalpy, free cooling
is available. ELM is equipped with a high ambient lock-
out that will disable free cooling and force compres-
sorized cooling when the outside air temperature
exceeds 75°F at 75% relative humidity. Free cooling will
remain locked-out until the outdoor temperature drops
below 73.5°F (also at 75% RH).
FIGURE 32 - ECONOMIZER RESPONSE EXAMPLE
ELM set to point B
Free cooling O N /O FF & O utdoor Tem perature
ON
OFF
68 F
O
58 F
O