Is the rack enclosed? If not that's what I would do first to try to trap some heat. Try moving the probe to different areas of heat tape or even different shelves bc heat tape will often have different temps on the edges and even on other shelves. You may not be able to get them all where you want them and will have to find a happy medium with your temps or have a thermostat for each shelf. If you can't get your cool side up you may need to run heat tape for the cool side as well.

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