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?

Thanks,

Mike