In the Fan:VariableVolume object are coefficients for how power varies with flow. Set up these coefficients to provide a cubed output of power versus flow. At a flow fraction of 1, this curve output should be 1. At a flow fraction of 0.5 this curve output should be 0.125. Set coefficient 4 = 1 and the others = 0. Note that Coeff 1 = constant, Coeff 2 = flow fraction, Coeff 3 = flow fraction^2, Coeff 4 = flow fraction^3 and Coeff 5 = flow fraction ^4. So for an ideal fan only Coeff 4 is used.

```
Fan:VariableVolume,
N8 , \field Fan Power Coefficient 1
\note all Fan Power Coefficients should not be 0.0 or no fan power will be consumed.
\note Fan Power Coefficents are specified as function of full flow rate/power
\note Equation:
N9 , \field Fan Power Coefficient 2
N10, \field Fan Power Coefficient 3
N11, \field Fan Power Coefficient 4
N12, \field Fan Power Coefficient 5
```

@mcdemarchi it sounds like you're using EnergyPlus, but can you confirm that? If so, please mention EnergyPlus in your question title and/or body, as well as add the

`energyplus`

tag so that others can provide better help.Thanks for the observation. I use Energy Plus.