I know what you're thinking. York's got to be great! You have high expectations. There's the Minster! There's the city walls to walk around! There's the medieval part! There's the Viking stuff! How could anything be beyond expectation?
I went to the National Railway Museum. I know some people are really into trains, but I'm just not. Who cares except that it gets you from point A to point B safely and quickly at not too much cost? I don't.
Until I saw this:
At the bottom here, is where the ashes could be moved and air let in, and the arrows up indicate where the hot air is going. And that's really hot air, remember.
What we have so far then is a big space filled with burning coal, surrounded mostly by a double wall filled with water getting hot, and with the hot air rising. But the hot air doesn't just rise anywhere, no indeedy. It's forced by pressure and hotness to go into those many, many tubes that you see the entry for in this picture.
The blue here indicates the part filled with water, and of course you have to imagine that the cut away tubes are actually there. And as the water turns to gas, it goes up, and that's indicated by the light blue. But that hot steam gas is then forced to go BACK through the small tubes inside tubes that you see on the right part of this picture, which superheats it! (Enbiggen the picture and you'll see the blue tubes inside the yellow/orange tubes) (This is so cool!)
Meanwhile, that hot, sooty air hits the big strainer looking thing, which catches the embers (mostly), and keeps them from flying up the smoke stack and catching the whole country on fire.
Now, you can tell why I found this unexepectedly and amazingly cool. Who would have thought?