That's a good point. I was gonna go with belly heat. Just one long piece zig-zagged down each shelf. Somehow I don't see how a single strip of 4" belly heat would be able to provide enough heat for a 88+ degree hot spot AND a 78+ degree cool side. Especially since the cool side of the tub is 25-30 inches away from the heat source. 12" really seems like the way to go. A strip of 12" down each side might prove effective also. Maybe I should start a poll.