Hey @kingdeking!

**Inlet Boundary Condition (assuming you mean velocity):**

The velocity inlet boundary condition defines an inflow condition based on the flow velocity. The inlet pressure and total (stagnation) values are not fixed, hence are calculated but the pressure gradients are fixed to zero value. For the velocity inlet you have to make sure to define another pressure condition for stability.

**InletOutlet Boundary Condition:**

The inletOutlet boundary condition is normally the same as **zeroGradient**, but it switches to **fixedValue** if the velocity vector next to the boundary aims inside the domain (backward flow). The value of that fixedValue is inletValue. (see picture below)

**FanPressure Boundary Condition:**

The fan pressure custom BC represents the situation where an artificial fan operates at a particular face.

It will create the pressure drop in the specified direction based on the flux/pressure curve. This means that the pressure drop applied on the face is dependent on the flow flux that passes through the BC.

Very nice post by @psosnowski on that topic: **Custom Boundary Condition - FanPressure**

As far as I know in order to get the operating point you can look that up in a booklet of the manufacturer (if the fan already exists ) and if you simulate it you need to know the resistance of your system. @PowerUsers_CFD, any ideas from your side?

**Additional information for the IntletOutlet BC:**

- If the velocity vector at the outlet points out of the domain it becomes a
**zeroGradient** boundary condition thus a **Neumann** boundary condition.
- If the velocity vector points into the domain it becomes an inflow with a
**constant value** thus a **Dirichlet** boundary condition.

**Sources:**