Heat Pump Operation; Air Source Heat Pumps - Trane Horizon OABD Series Installation, Operation And Maintenance Manual

Hot Gas Reheat Operation with Standard
Scroll Compressors
Units equipped with only standard scroll compressors (not
digital scroll or variable speed) will enable the hot gas reheat
during cooling mode to provide precise temperature control
since staged compressors do not have modulating capacity to
maintain a temperature. Compressor control is determined by
offset the discharge setpoint down, and the compressors stage
to that temperature off the evaporator. The amount of offset
and deadband is determined by the number of compressor
stages to reduce cycling. In summary, the compressors to a
temperature just below the discharge air temperature setpoint
active, and the hot gas reheat is enabled, controlling to the
discharge air temperature setpoint active.
For 1 or 2 compressors, the deadband will be 5°; for 3 or 4
compressors, the deadband will be 5°. The setpoint offset will
be half the deadband.
Hot Gas Reheat Operation with Modulating
Capacity Compressors
Units equipped with modulating compressors (digital scroll or
variable speed) on the first circuit will enable the hot gas reheat
only during dehumidification mode. During dehumidification
mode, the compressors control to the dehumidification
temperature setpoint active, and the hot gas reheat controls to
the discharge air temperature setpoint active.
Hot Gas Reheat Purge Mode
When utilizing hot gas reheat, the unit must initiate a purge
mode to return oil back to the compressor(s). This purge
operation has been improved to reduce the impact on the
discharge air temperature, while still providing sufficient
velocity for oil return.
Upon entering the purge mode, the last setting is saved so that
the unit can return to that position immediately without having
to wait for it to adjust down. The valve position is set to 0% in
the final moments of the purge cycle to quickly cool off the coil
to reduce the impact on the supply air conditions.
Parallel Piped Reheat Circuit (Parallel to
Condenser)
After 30 cumulative minutes above 0%, a 4-minute purge cycle
is initiated. During Hot Gas Reheat Purge Mode, the Hot Gas
Reheat Valve Command is slowly ramped up to 90% for 2.5-
minutes, and then ramped down to 0% for the remaining 2.5-
minutes.
Series Piped Reheat Circuit (Series with
Condenser)
After 30 cumulative minutes above 0%, a 4-minute purge cycle
is initiated. During hot gas reheat purge mode, the hot gas
reheat valve command is slowly ramped up to 90% for 3-
minutes, and then ramped down to 0% for the remaining
minute.
OAU-SVX007A-EN
Sequence of Operation v12.0

Heat Pump Operation

On heat pumps, the reversing valve's default (de-energized)
state is in the heating position. On initial startup, if there's a
cooling demand, the reversing valve will switch into the cooling
position after the compressor status has been proven. If the
unit remains in cooling mode, but the first compressor is being
cycled, the reversing valve remains energized in the cooling
position, even while the compressor is off.
Once there is a heating demand following cooling operation,
the compressor typically won't be operating during the
switching of modes. To prevent the reversing valve from being
stuck, the valve will remain in the cooling position until the
compressor status is proven. Units with other forms of heating,
such as an energy recovery wheel, may show that the
reversing valve is in cooling mode for an extended period
during heating mode.
Power Failure
If the reversing valve switches while the compressor is not
operating, such as during a power failure, the reversing valve
will be toggled between heating and cooling after the
compressor is restarted to confirm that the valve is properly
seated in its position.
Supplemental Primary Heat
During heat pump operation, if the compressor heating
capacity reaches 100% and there is insufficient heating
capacity to meet the discharge air setpoint, the primary heater
will be engaged to supplement the discharge air temperature.
While the primary heat is engaged, the heat pump will be
locked at full capacity.

Air Source Heat Pumps

Frost Avoidance
During heating mode, outdoor air dewpoint is measured, and
suction pressure is monitored using transducers installed on
each refrigeration circuit. Using the measured saturated
refrigerant temperature, the compressor capacity is modulated
to maintain the saturated temperature slightly above the
outdoor air dewpoint. This allows the circuit to run for an
extended period without requiring defrost cycles. The frost
avoidance operation is disabled when the saturated
temperature rises above freezing.
Frost avoidance is used only on circuits with modulating
capacity, such as digital scroll, variable, or even staged
compressors, but not on single stage circuits. This operation is
restricted if the outdoor air dewpoint is within 5° of the outdoor
air temperature (~80%RH), as the limitation on the compressor
capacity typically causes additional energy consumption with
the primary heater.
Defrost Mode
With the frost avoidance method, defrost cycles occur
infrequently, and a typical defrost cycle lasts less than 5
minutes on average.
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