How does the diameter of an isolation valve effect the calculated results of a WaterCAD / WaterGEMS model?
The diameter is used to determine the velocity through the valve. If no minor loss coefficient is entered for the isolation valve, the only effect this will have would be the reported velocity at the valve location (reported in the isolation valve properties.) However, if you enter a minor loss coefficient (which you should) the diameter will effect the headloss through the valve. This is because the headloss equation considers velocity and as the diameter decreases, the velocity increases.
Minor losses for Isolation Valves
Change the diameter of all isolation valve to match the diameter of its referenced pipe