Just an FYI R&B, the boiling point of water is a function of temperature and pressure. If the system is sealed and there is no air in it, then there is no room for expansion and the pressure in the system rises rapidly without the water boiling.
Check out the table in this link:
Pressure and Boiling Points of Water
Of course, there is a point where the temperature will rise faster than the pressure it induces especially when hoses are expanding and effectively retarding the pressure rise, then boiling occurs but this can be well above 210 (your 250 point maybe). If there is air in the system, expansion is allowed to happen and the pressure doesn't rise fast enough to prevent boiling, so it will boil at a much lower temperature.
Adding salts or organics (like glycol) will raise the boiling point further but the system really needs be sealed tight and air free to allow the pressure to build and prevent localized boiling at the hottest point like in the block or head.