For enclosed racks with solid sides, you also get the benefit of a longer thermal gradient for them to use since most people run the heat tape along the back, regardless which orientation the rack is.

So a 33x18 tub the usual way has about 18" of heat tape near the back end. If done the other way, you place 33" of heat tape across the long side in the back; this position would be needed to insert shorter tubs and have them all have a hot spot over the heat tape.

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