Over-the-range microwave mounted above a stainless electric range
Generic photo: Curtis Adams · Pexels License

Over-the-range microwave duct run: size, equivalent length and when to recirculate instead

OTR microwave blowers are small, so the duct decides how well they vent. The common target on manufacturer install sheets is a 3-1/4 x 10 in rectangular duct or a round duct of at least 6 in through a rectangular-to-round transition, with the whole run kept under the maker's equivalent-length limit (GE sheets seen: 120 or 140 equivalent feet depending on the document). Every fitting counts as extra feet. This guide shows how to add up a run before you cut holes, and when the honest answer is to leave the unit on recirculate with a charcoal filter.

Cross-checked ~30 min moderateBrands: All

Before you start

  • Turn the circuit off before reaching into the cabinet above the unit; the OTR receptacle is usually there
  • Do not route a microwave duct into an attic, soffit or wall cavity without a proper exterior cap; grease-laden moist air must leave the building
  • Never use plastic or foil-vinyl flex duct for a cooking appliance exhaust; use metal duct
  • Cutting a roof or exterior-wall hole may need a permit and weatherproofing; if that is outside your trade, refer it

Diagnostic steps

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