Download the "ENGINEERING THE PLUMBING SYSTEM" Excel Spreadsheet
You have to have Excell 2000 or newer with the solver add-in installed.
This thing is cool. It uses Darcy, Hazen Williams, Haaland, Swamee-Jain equations, and every pump P/Q curve you can think of (Iwaki, Ehiem, ect..) to figure head losses due to plumbing.
There has to be an easy way to modify this (with creators permission) to include radiator and waterblock resistances. Doing so would let us tweat a virtual system. "I have this pump, this rad, X ft of X ID tubing - I wonder what would happen if i swapped blocks."
It's also good for demonstrating what happens when you put to big of a pump in.