Precooling refrigerator coils

I will be installing a heat pump water heater soon. It will move a lot of cold to the interior, but was thinking of using some of this to feed into the fridge's heat rejection coils. Does anybody have ideas on duct size to feed air to the coils? I already had a metal shop create a manifold box to direct the cool air to the cellar and to help dry the clothes.

Kurt

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