A wax pellet element (4) in the thermostat expands when heated, and contracts when cooled. The pellet element connects through a piston (1) to a valve (2). When the pellet element is heated, pressure is exerted against a rubber diaphragm (3) which forces the valve to open. As the element is cooled, the contraction allows a spring (5) to close the valve. While the coolant is cold, the valve remains closed. This prevents circulation of coolant through the radiator. At this point, coolant is only allowed to circulate throughout the engine in order to quickly warm the engine.

As the engine warms, the pellet element expands and the thermostat valve opens. This permits coolant to flow through the radiator, where the heat dissipates through the radiator tube walls. This opening and closing of the thermostat permits enough coolant to enter the radiator in order to keep the engine within operating limits.