This greenhouse replacement cylinder fits the Bayliss Standard Solar Power MK7 Roof Vent Openers. The Bayliss greenhouse vent openers automatically open and close due to the temperature inside the greenhouse. Once the temperature reaches about 62 degrees F, the roof vents begin to open. Consequently, as night temperatures fall, the vents will close. The cylinder needs to be replaced every two or three years. Shipped as package of 2 cylinders only.