In article , drmrbrewer.414ca98
@gardenbanter.co.uk says...

We've inherited a very nice greenhouse from the previous owners of our
house, fitted out a few years back to a pretty high standard. However,
the automatic vent openers no longer seem to operate as they should,
mostly stuck closed.

I can't find any information as to how to maintain these, and whether I
just need a replacement black piston thingy (and if so, what I do to
replace it), or whether I need to replace the whole mechanism including
the arms etc, or whether in fact it is possible that some simple
adjustment would do the trick. How long do these things normally
last?

The black piston won't take kindly to being frozen, I always take mine
out every year and keep them in the house. As for WD, it's not a good
lubricant, use 3 in 1 or similar on the hinges and all moving parts.
How long they last depends, mine are at least ten or fifteen years old.