> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/user-guide/2d-building/building/hvac/space-hvac/fan-coil-unit/air-loop.md).
# Air Loop
## System
### Fan Speed Method
Specify the Fan Speed Method to choose how the fan speed can be varied, with options of Continuous or Discrete. Discrete indicates specific speed settings, while Continuous allows smooth variation up to the Design Maximum Air Flow Rate.
### Fan Power Minimum Flow Rate Fraction
Specify the Fan Power Minimum Flow Rate Fraction to define the lowest operational flow rate fraction for variable speed fans. This value is a fraction of Design Maximum Air Flow Rate and is utilized in calculating fan power at low flow rates, but it doesn't enforce a lower limit during simulation. This field is applicable only when Speed Control Method is set to Continuous.
### Fan Motor Efficiency
Specify the Fan Motor Efficiency by entering the motor efficiency value, calculated as the shaft power divided by the electric power consumed by the motor.
### Fan Motor In Air Stream Fraction
Specify the Fan Motor In Air Stream Fraction as the fraction of motor heat added to the air stream. A value between 0 and 1 is accepted, where 0 means the motor is outside the air stream, and 1 means the motor is entirely inside, adding all of its heat to the air stream.
### Fan Design Power Sizing Method
Specify the Fan Design Power Sizing Method by choosing from:
**PowerPerFlow:** For Power Per Flow, use the Electric Power Per Unit Flow Rate to size the Design Electric Power Consumption.
**PowerPerFlowPerPressure:** For Power Per Flow Per Pressure, the Electric Power Per Unit Flow Rate Per Unit Pressure is used for sizing.
**TotalEfficiencyAndPressure:** For Total Efficiency And Pressure, use the input Fan Total Efficiency and Design Pressure Rise to size the power.
### Fan Total Efficiency
Specify the Fan Total Efficiency by entering the motor efficiency value, calculated as the shaft power divided by the electric power consumed by the motor.
### Fan Design Pressure Rise
Specify the Fan Design Pressure Rise as the total system pressure rise experienced by the fan in Pascals at full flow rate and altitude-adjusted standard density of dry air at 20 degrees Celsius dry bulb.
## Sizing Method
Specify the Sizing Method by choosing from Autosize or Manual. Manual sizing requires completion of all numerical fields with valid entries.
### Fan Design Maximum Air Flow Rate
Specify the Fan Design Maximum Air Flow Rate as the design volume flow rate of the fan in the HVAC system, in m3/s. This rate, converted to a mass flow rate using an altitude-adjusted standard density of dry air at 20 degrees Celsius dry bulb, represents the full-speed flow rate and acts as the upper limit for variable flow rate fans. This field can be autosized.
### Fan Design Electric Power Consumption
Specify the Fan Design Electric Power Consumption as the electric power consumption at the full Design Maximum Air Flow Rate and Design Pressure Rise, in Watts. This value determines the fan efficiency. The field is autosizable and defaults to auto-size. When autosized, three options for the method used to size the design power can be selected in the following field.
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