875—95.6 (89) Thermal expansion. If a system is equipped with a check valve or pressure-reducing valve in the cold-water inlet line, consideration should be given to installation of an airtight expansion tank or other suitable air cushion. If an expansion tank is provided, it shall be in accordance with a nationally recognized standard and rated for a maximum allowable working pressure equal to or greater than that of the temperature and pressure relief valve. Except for prepressurized tanks, provisions shall be made for draining the tank without emptying the system.