The storage tanks of these water-heaters cannot withstand the pressure of city water, so when the consumer turns the hot water tap on, the water pressure is much lower than the cold-water pressure. Non-pressure water heaters are also known as vacuum tube. This type of water-heater is cheaper due to the use of thinner and single-walled sheets. moreover, because of double-walled pipes made with vacuum pipe technology, there is no need for antifreeze.