I have one of these, that's been sitting in the basement for decades. I pulled it out yesterday and patched up the clear plastic cover, but the thermostatic opener seems to not be working, it's open all the time (or at least partially open, I found a picture online and it seems to be open more than mine).

Dalen doesn't seem to make anything like this anymore, however I emailed them to see if they knew anything about parts availability. In the meantime I thought I would ask here to see if anyone knows anything about this or fixing it. I suppose it's possible that the thermostatic element is just stuck after being in the same position for 40 years due to the relatively constant basement temp.