In Revit 2012, there is a new Calculations parameter in the System type properties. You can use it to limit or turn off system calculations. The Calculations parameter has three options: All, Flow Only, and None, depending on the system type.
Depending on the option, the affected parameters may not be calculated, and their values are displayed as “Not Computed”. The affected parameters include flow, fixture units, velocity, velocity pressure, friction, friction factor, pressure drop, static pressure, and others.
See also Customizing System Types.