Hotels, apartment buildings, hospitals and other facilities that need constant supplies of hot water usually have a storage tank that has been sized to meet peak demands, even though the usage at these times exceeds the output of the heater. This is achieved by selecting a tank of sufficient size, with adequate insulation, and designed to prevent incoming cold water from mixing with the hot water supply. These tanks are normally lined to prevent rusting, and the standard design working pressure is 125 psi, which exceeds typical city water main pressure.