A device and method enable both an automated and a user controlled diversion of fluid flow from a first pathway to an alternate pathway. The automated method relies upon detection of an environmental quality, such as fluid temperature, fluid heat content, fluid viscosity, fluid chemical composition, flow rate, or other detectable parameter. Both the automated diversion and the user controlled diversion rely upon positioning of the piston relative to the two pathways. The piston defines an internal pathway and the position of the piston internal pathway determines whether fluid flows through the first pathway or the alternate pathway. A lever movably couples the piston to an actuator; in the automated method an actuator responsive to its environment moves the lever to reposition the piston. In the user controlled method an override is applied, preferably manually, to reposition the piston to cause the diversion from the first pathway to the alternate pathway.