Controlling temperatures in a cold frame or greenhouse is straightforward with this automatic control.

It incorporates a gas-charged cylinder of adjustable volume. As the temperature rises, the gas expands; it will open a 15 lb window, more than adequate in cold frames and greenhouses. A simple adjustment lets you control the operating range. As temperatures fluctuate, the 12 1/2" long unit will open and close a window as necessary to prevent disasters.

Made in England.
Non-rusting anodized aluminum and powder-coated steel.

https://www.leevalley.com/en-ca/shop/garden/planting/greenhouse/10543-heat-activated-window-opener?item=AM401