another thing to add is with a sealed overflow the system is mow going to pressurize higher than the rad cap pressure. radiator builds pressure to 16 psi, then it vents air or coolant that air or coolant has to go somewhere so it goes into the overflow tank. the air pressure in the tank quickly reached 16psi and then if the coolant keeps getting hotter the entire system pressurizes higher then 16 till something blows (hopefully the overflow hose and the hood is shut)
Remember myth busters and the flying water heaters, all they did was close off the overpressure valve and the in and outlets and allowed the heater to heat