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Evidently a queen excluder isn't needed when using this gadget because the cells are of an incorrect size that queens will not lay any eggs in, although workers will fill them with honey. So there's that. However, I'm a little dubious about claims that using this hive part is "less stressful for the bees". For one thing, the hive needs to be taken apart occasionally anyway, as part of normal hive inspection and management. For another, the "flow" supers need to be removed at the end of the year, and doing so requires emptying them of bees in exactly the same way that traditional supers do when they're removed for harvesting. So you're not actually causing the bees less stress, you're simply postponing the stress event until later in the year - which might not necessarily be a good thing, depending on how early a particular colony starts slowing down and consolidating for winter.