Carrier 38AUM Series Installation, Operation & Maintenance Manual page 11

Vertical Separation (outdoor unit above indoor unit)
Vertical elevation difference of 60 m (200 ft) is permitted when the outdoor unit (38AUMZ or 38AUMD) is located
above the indoor unit.
Insulate Suction Lines
Apply closed-cell tubular insulation to all suction lines between evaporator coil connection and 38AUM unit's suction
service valve.
Hot Gas Bypass
Hot gas bypass, if used, should be introduced before the evaporator. (A bypass route that also bypasses the
evaporator circuit may lead to oil trapping in the evaporator circuit during low load conditions and then to oil slugging
as evaporator load increases.) Model 38AUM units do not include a hot gas stub connection; a tee must be field-
supplied and installed in the compressor discharge line. Run a 1/2-in OD line between outdoor unit and evaporator coil
inlet. Install an Auxiliary Side Connector at the evaporator between TXV and distributor (follow instructions for the side
connector part). Insulate the hot gas line.
38AUMD: Generally only one hot gas bypass system will be applied on a two-circuit unit. Connect the hot gas bypass
system to Circuit 1 (first-on/last-off, connected to the evaporator coil's bottom circuit).
38AUMD Piping Connections
The 38AUMD's two circuits are designated Circuit 1 and Circuit 2. Circuit 1 is controlled by the thermostat's Y1 (or
TC1) contact and will be the first circuit on and last circuit off. Circuit 2 is controlled by the thermostat's Y2 (or TC2)
contact and this circuit is always the "lag" circuit.
See Fig. 38AUMD Service Valve Locations for location of Circuit 1 and Circuit 2 service valves and field piping
connections. Circuit 1 is on the right-hand side of the service valve compartment; Circuit 2 is on the left. When a single
piece evaporator coil with two separate circuits is connected to a 38AUMD, the lower coil circuit should be connected
to the 38AUMD's Circuit 1 so that the evaporator's lower coil segment is first-on/last-off (to avoid re-evaporation of
condensate on dry lower coil segments).
Plan the Circuit 1 and Circuit 2 tubing segments carefully, mark each segment and check constantly as piping systems
are assembled to avoid piping errors.
38AUMD unit cannot be field-piped as a single-circuit/tandem system.
Connecting 40RUM to 38AUMD: The 40RUM fan coil in sizes 12, 14 and 16 is a face-split coil design that also has
its circuits designated as 1 and 2. See Fig: - Typical Evaporator Coil Connections (40RUM). Note that the lower coil
segment changes as the arrangement of the 40RUM changes. In a vertical arrangement, the 40RUM's lower coil
segment is segment 2; this segment should be connected to the 38AUMD's Circuit 1. In a horizontal arrangement, the
40RUM's lower segment is now segment 1; this segment should be connected to the 38AUMD's Circuit 1.
Note that refrigerant suction piping should be insulated.
Fig: - 38AUMD Service Valve Locations
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