tepidarium

noun4

1. •
   Any room containing a warm bath.
2. •
   (historical) A warm room in a Roman baths that was usually heated by a hypocaust.
3. •
   A warm greenhouse typically used for plants from South Africa.

   • There is a pretty large glass-house, one-half being a caldarium, the other a tepidarium — 1851
4. •
   (historical) A boiler in which the water was heated.