Installation manual

035-16682-000-E-0303
72 Unitary Products Group
stage heat. As the call heat (W1) has been satisfied, W2
will be de-energized and the APS will open disconnect-
ing power to the first stage of electric heat.
All electric heat modules have an auto-reset primary limit
and non-restartable back-up limits. Dirty filters, broken
belts, fan motor burn-out or running this unit below 6,000
CFM will cause the limit to trip on the auto-reset primary
limit. In addition to the primary limit, back-up limits may
trip under these conditions. The back-up limits are one-
trip devices and will have to be replaced should they trip.
10. 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 the
CM-CV Aux. Econo. Relay contact (A to Com) is closed.
Remember that CM-CV engages the Aux. Econo. Relay
only when CM-CV calls for the supply fan during the
occupied mode. When ELM is not powered, the outside
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 when 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 staging chart
above.) ELM modulates the economizer dampers to
maintain a Mixed Air Temperature (Return Air / Outside
Air Mixture, sensed between the filters and the DX coil)
between 50 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 20W 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 [85,95°F], B [72,82°F], C [58,68°F],
and D [45,55°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:
The Outdoor Air Enthalpy
Sensor is connected and a 620W resistor is connected
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).
11. Power Exhaust Option with Modulating Dampers: The
power exhaust controls are powered through relay MR
(supply air motor relay) with a call for the supply fan.
Once powered, null pressure switch SSP1 (Switch, Static
Pressure - Building) provides floating point control of the
exhaust dampers. A contact closure is provided (COM to
HI) whenever the building pressure exceeds the building
pressure setpoint of SSP1, which applies a 24 VAC sig-
nal to wire 4 of the power exhaust damper actuator
(ACT2, OPEN damper). A contact closure is provided
(COM to LO) whenever the building pressure falls below
the building pressure setpoint minus the null span adjust-
ment of SSP1, which powers wire 3 of ACT2 (Close
FIGURE 31 - 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