Abstract: A thermally responsive valve device having non-linear flow characteristics. The thermally responsive valve device includes movable and stationary valve structure. A movable valve member is movable to adjust the open area of a flow passage, but movement of the movable valve member provides non-linear relationship between the area of the flow passage created and the distance of travel of the movable valve member. The movable and/or the stationary valve member is provided with an irregularly shaped portion so that with slight opening movement of the movable closure member less fluid flow than normal is permitted. As the movable valve member continues to move in an opening direction, the fluid flow increases at a predetermined non-linear rate until a specific flow rate is achieved. This operating characteristic eliminates or dramatically reduces temperature fluctuations in the system.