Mythbusters did a test to see if a water heater would shoot through the roof of a two story building (home). They close all orfices, heated the water and it soared through the roof. They computed the final pressure as 85,000 pounds. As I recall they computed this by multiplying the area of the cylinder times its length times psi, then by another number, which they did not explain. 244. Maybe it was temperature (F). What law was used?