Abstract: A ventilator actuator is provided for opening and closing a ventilator opening having a diaphragm secured to a closure member mounted for pivotal movement about an axis to form a reservoir for a heat responsive fluid between them, the diaphragm normally being retracted at a first temperature and extended at a second elevated temperature. An actuating member has one end secured to the diaphragm and spaced from the pivotal axis, the other end of the actuating member is anchored such that when the diaphragm expands the actuating member causes the closure member to pivot to open the ventilating opening. When the diaphragm retracts the closure member is caused to pivot back to close the ventilating opening.