諸經要集 8.104

8.104

又十誦律云。外國浴室形圓。猶如圓倉。開戶通煙。下作伏瀆。出水內施。三擎閣齊。人所及處。以瓨盛水滿三重閣。火氣上升。上閣水熱。中閣水暖。下閣水冷。隨宜自取用。無別作湯。故云淨水耳。

The Sarvāstivāda-vinaya also states: "In foreign lands, bathhouses are circular in shape, resembling round granaries. An opening is made to vent smoke, and an underground conduit is constructed below to drain the water. Inside, three tiers of shelves are set up at a height reachable by people. Jars filled with water are placed across the three tiers of shelves. As the heat rises, the water on the top tier is hot, the water on the middle tier is warm, and the water on the bottom tier is cool. Each takes and uses it as appropriate; there is no separate preparation of hot water. Therefore, it is simply called 'clean water.'"