Thanks for your replies. We got back in town late, so I couldn't investigate much. Yesterday, I took the cover off. After awhile, it did drop 1 degree. I left the tub on 80 degrees and we used it. I watched the control panel to see if the heat was coming on. After about an hour of use, the temp dropped to 99 and and the heater never came on. While we were gone, the daytime temps. were in the low 70's and the night temps in the 60's. Yesterday, it was rainy all day and the high temp. was 60. I'm assuming, now, that it's a tribute to full foam insulation and a really good cover.