Split/Dual Exhaust And Return Air Paths - Trane Horizon OABD Series Installation, Operation And Maintenance Manual

Sequence of Operation v12.0
Defrost mode for the outdoor coil occurs whenever the
temperature difference between the outdoor air temperature
and the saturated temperature rises above a pre-determined
point that indicates heavy frost accumulation has occurred.
This setpoint varies based on a variety of factors but is
generally around a 25° difference.
During defrost mode, the reversing valves are switched to
cooling, outdoor fans are disabled, the compressor staging is
locked to prevent compressors from turning off or on, and any
modulating compressors are taken to 80% command. After the
coil is defrosted, the condenser fans are enabled to minimum
flow to allow continued drying of the coil, before switching the
reversing valve back to heating and returning the condenser
fans back to full capacity.
The primary heater (if installed) will be engaged during defrost
operation and control to the discharge air temperature setpoint
active. The primary heater has full modulating during
compressor operation.

Split/Dual Exhaust and Return Air Paths

Units equipped with dual air paths for the return and exhaust
will have an additional function in addition to the exhaust fan
operation. During all operating conditions, the exhaust fan
controls to a pressure differential across the damper between
the two air paths to always have air leakage from the return,
into the exhaust, so that bathroom exhaust does not leak into
the return air stream. During economizer mode, the damper
between the two paths will open fully, closing the return air
damper, and exhausting all of the air through the ERV and out
through the exhaust air.
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