If you want your greenhouse vent to automatically open & close when you are away, I recommend this "Harbor Freight greenhouse vent opener featuring a gas charged cylinder."

At about $25, it lasts as long as other openers costing 3 times as much.

Constructed of durable steel & aluminum, this opener features an adjustment knob to change the open/close settings according to your greenhouse’s needs.

• Gas charged cylinder & piston rod, opens & closes greenhouse window vents
• Opening temperature range 59 degrees to 77 degrees Fahrenheit
• Max. operating temperature: 122 degrees Fahrenheit
• Lift capacity: 15 lbs.
• 90 degree maximum opening angle