Do you have a greenhouse roof vent that is no longer opening or not opening enough?
Over the years, the piston moving through the seals gradually wears the seal material away, allowing a gradual seepage of wax out of the cylinder. This reduces the pressure within the cylinder and needs to be replaced. But it could also be that the wax in the cylinder just needs a simple kick start and a warm bath. Here is a tip to see if you can get them working again:
Remove the cylinder and place it in the refrigerator, not the freezer, for 30 minutes. Immediately after removing them from the fridge, run the cylinders under hot water in the sink for 5-10 minutes.
Here is a video that explains how to install the cylinders back into the vent opener: