Pressure gradient - difference between intake pressure and discharge pressure. If you are blessed with "good" hydrants like we are then it won't be much. We have hydrants that discharge at anywhere from 60 to 100 psi and the Husky system advocates flowing CAFS at 95 psi - so this isn't enough apparently to cool off the compressor. Therefore, the best way to create the necessary gradient or pressure difference would be to have 0 intake pressure by bringing the water through the tank. I am not as familiar with the Husky system as I am the Hale but this was what was told to me by the factory rep when I took the Husky class.