Yes ambient temps in the room make a monster difference especially with a plastic tub. Typically you can only add 2-3º over the ambient temps in the room in the tub. If the room is 65º it is unlikely you will get ambient in the tub over 68ºF. They will effect the hot spot as well dropping this. There are limited heating options for tubs basically the easiest is the UTH and room temps typically tubs work best if the room is held around the 77-80º range with the thermostat controlled UTH.
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