Back in the old days when most folks burned coal, vacuum was just about everyone’s friend. That slow and steady coal fire produced a steam that gently nudged the air from the system.
Back then, the radiator air vents probably had tiny check valves in their outlets. Those checks would let the air out, but they wouldn’t let it back in. When the steam condensed in the radiators, a naturally induced vacuum formed and the water would start boiling at temperatures much lower than 212° F. This made the most of the diminishing coal pile.