Originally, the first 23 sets were not fitted with any air-conditioning system due to structure gauge restrictions caused by the small tunnel size of the line. In 1990, Mitsubishi Electric developed a compact air-conditioning unit of thickness 240 mm and refrigeration capacity of 16.2 kW. A prototype air-conditioning unit was tested on set 16 in August and subsequent non-air conditioned sets were all retrofitted afterwards. To achieve this, the ceiling of the trains (both those already fitted with air-conditioning when new and those with retrofit) were lowered.