Cassette indoor units are made primarily for drop tile type ceilings where there is existing head space above the grid line. They work wonderfully for convenience stores, server rooms and closets, and even smaller, single use buildings such as a work shop or hunting lodge. Cassette units come in two sizes, a two-foot by two foot from 9,000 BTU up to 18,000 BTU, and a three-foot by three foot, running from 24,000 BTU up to 48,000 BTU.