There are two ways. One is with the bypass valve. There is a spring that creates or loosens tension with the handle which directly affects the pressure inside the system. It is normal to hear chatter on initial start-up with a bypass valve. Once everything is up to speed and rotation is verified on the pump, the chatter will quiet down. The other way is with a VFD, coupled with a transducer; it will maintain constant psi in your system by speeding up or slowing down the motor. Before you start adjusting the pressure in the system, which we factory set with every chiller at 20psi, be sure to verify that the jacket pressure ratings with the vessel manufacturer you went with.