thats to heat the water and keep the air around it condusive to water temp as well...as flames diminish and you have hot coals thats when you would get in it but if there were hot coals under the tub your feet would burn on the bottom when trying to get in never mind if you tried to kneel or sit...so thats why you dont way HOT under it... smile also around the tub would give even heat.
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CAPPY