Please don't attach a lot of credence to Myth busters, the methods they use to produce spectacular shock/horror television are somewhat suspect from the scientific/engineering point of view. Any un-vented heating system must have an expansion vessel AND A SAFETY VALVE. It is obvious that the system demonstrated here did not have one. Usual un-vented system pressure is 1.5 bar, the safety valve will let go at twice the working pressure, ie 3 bar (1bar =14.4lbs/sq") The reason that wood burners etc are not suitable for un-vented systems has more to do with the very wide temperature fluctuations, and the fact that in an un-vented system, and water blown out of the safety valve is not automatically replaced as it is in a vented system. The problems with a vented system are that due to over-pumping, or overheating, very hot water can be forced into the header tank and raise the header tank temperature to a point where the plastic tank becomes unstable, and the loft space gets wet due to condensate dripping off the inside of the felt or damp proof membrane under the tiles!