They exhaust the steam into the stack as it creates draft, the speed of the steam leaving creates a low pressure inside the boiler, drawing in more air.

A "supercharger" if you will but, in reverse. LOL...

I enjoy your ramblings because you can describe what you see so that I can understand and appreciate the vision... and I'll admit I'm a bit jealous that I can't get there myself... YET! LOL..