An assembly includes a heat exchanger with an inlet portion. an outlet portion, and at least one fluid passage fluidly coupling the inlet portion to the outlet portion. The assembly additionally includes a movable blocker element having a first position with respect to the heat exchanger and a second position with respect to the heat exchanger. In the second position the blocker element reduces cross-sectional area of the at least one fluid passage relative to the first position. The assembly further includes a passive actuator coupled to the blocker element and configured to selectively move the blocker element from the first position to the second position in response to a thermal condition being satisfied.