Can someone please tell me what this is? I had a 35-year old oil furnace replaced recently and the new furnace is obviously more complex and has many safety features the old one didn't. I've looked it over and understand what all the new devices do... except this one. The red plate suggests it's a fire safety device but I can't figure out why it would have an adjustable wheel.
That is a thermally fused switch. There is a fuse in there that will cut off the power if the temperature in the room rises above a certain temperture. The wheel is to replace the fuse when it blows.
There's one on this page, Beckett 12501, not sure if it's the same as yours: