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Sequence of Operation
48 RTU Open
Heat Pump operation
The RTU Open can control heat pumps HP O/B and Y1/W1.
HP O/B provides a separate output (BO-7) to control a reversing valve. The reversing valve control may be
configured to be energized with a call for heating
(B), or energized with a call for cooling (O).
The sequence of operations are as previously described for heating and cooling except that the Y1 and Y2
outputs are compressor outputs, energizing mechanical heating or cooling, depending on the state of the
reversing valve.
W1 and W2 are used for auxiliary heat. Up to two stages are available.
Selection Y1/W1 is for heat pumps that do not require a O terminal to energize the reversing valve. The
sequences of operations are as described for Heating (page
47) and Cooling (page 43). The reversing valve
output is not utilized in this application.
W1 and W2 are used for auxiliary heat. Up to two stages are
available.
Dehumidification
The RTU Open provides occupied and unoccupied dehumidification on units that are equipped with the
Carrier Humidi-MiZer™ option from the factory. This requires a space relative humidity sensor or a humidistat
for control.
The following conditions must be true for the dehumidification control to operate:
• The Outside Air Temperature is greater than the Cooling Lockout Temperature setpoint
• The Indoor Fan has been ON for at least 30 seconds
• The unit has a valid Supply Air Temperature input
• The unit has a valid Space Temperature input
• The unit has a valid Space Relative Humidity Sensor or Humidistat input
• Heat mode is not active and the time guard between modes has expired
When using a relative humidity sensor to control dehumidification, occupied and unoccupied dehumidification
setpoints are used.
When using a humidistat, the setpoints are not used. The humidistat indicates a high-humidity condition.
When a high indoor relative humidity condition is indicated and the above conditions are satisfied, the RTU
Open enters the dehumidification mode, energizing the Humidi-MiZer™ output.
The mode continues until the space relative humidity falls below the active setpoint by a 5% fixed Hysteresis
when a humidity sensor is used, or when there is no longer a call for dehumidification where a humidistat is
used.
See the base unit / Humidi-MiZer™ operations manual for additional information.