I am not sure that's the right term, but it's a device that expands with higher temps. Such a device can be used to control the doors on the ash box and at the flue on back of the boiler on mostly hand fired units. My old "Red Square" boiler had one, the device had a cast iron body, and screwed into a water port. What I'm calling the aneroid was made of brass. When it expanded, the linkage would move and open/close the doors as needed. A weight was used on the linkage for adjustment. It worked very well.
Painter's boiler looks to have one. Page 12 and 13. Pictures Of Your Boiler
I have been asked where such a device could be bought. Anyone know if these are sold for less than an arm/leg? Like a dummy, I gave the one away from my rotted up from running it at too low a temp red square. I wish that I would have kept it for a conversation piece, if nothing else.
Totally non related, but during the "we must deal with emissions" mid/late 70s, some carburetors used such a device. It was used as an altitude compensating device of a sort.