The passage about Jantāghara from Minor matters
Now at that time monks made a fireplace in the middle of a small bathroom, and there was no access. “ I allow you, monks, to make a fireplace at one side of a small bathroom, in the middle of a large one.” The fire in the bathroom scorched their faces. “I allow, monks, clay for the face.” They moistened the clay with their hands. “I allow, monks, a tub for the clay.” The clay came to smell nasty. “I allow you, monks, to cure it.” The fire in the bathroom scorched their bodies. “I allow you, monks, to take in water.” They took in water in dishes and bowls. “I allow you, monks, a receptacle for water, a saucer for the water.” A bathroom with a grass roofing did not make them sweat. “I allow you, monks, having lashed on (a roof), to give it a smearing inside and outside.” The bathroom became swampy. “I allow you, monks, to spread three (kinds of) spreadings: a spreading of bricks, a spreading of stones, a spreading of wood.” Even so it became swampy. “I allow you, monks, to wash it.” Water remained. “I allow, monks, a drain for the water.” Now at that time monks sat down on the ground in a bathroom and they got pins and needles in their limbs. “I allow, monks, a chair for the bathroom.” Now at that time a bathroom was not fenced in. “I allow, monks, three (kinds of) fences to fence it in with: a fence of bricks, a fence of stones, a fence of wood.”
If Jantāghara was a bathroom there is no need of making fire in the middle. In fact this was allowed to get rid of bad health along with walking paths over a recommendation by Jīvaka Komārabhacca a physician.
Jīvaka Komārabhacca spoke thus to the Lord:
“At present, Lord, monks are very ill with their bodies full of (bad) humours. It were well, Lord, if the Lord allowed the monks a place for pacing up and down in and a bathroom. Thus will the monks come to have few afflictions.” Then the Lord gladdened, rejoiced, roused, delighted Jīvaka Komārabhacca with talk on dhamma . Then Jīvaka Komārabhacca, gladdened … delighted by the Lord with talk on dhamma , rising from his seat, having greeted the Lord, departed keeping his right side towards him. Then the Lord on this occasion, in this connection, having given reasoned talk, addressed the monks, saying:
“I allow, monks, a place for pacing up and down in, and a bathroom.”
This indicates that this was used to improve health; induce fat burning with steam and dry heat (?). The passage also explains a place more like a sauna with the use of fire and water.
This sounds like a sauna rather than a bathroom.
A sauna, or sudatory , is a small room or building designed as a place to experience dry or wet heat sessions, or an establishment with one or more of these facilities. The steam and high heat make the bathers perspire.