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Greetings. The Gree unit will not connect to the duct in the attic which is most likely way too small to service the sun room which will require 3 to 6 tons of cooling or more depending on where you are in the world and how cool you want it in the sun room.
Attach a sketch please and with the North arrow and West compass directions showing, and tell me the name of the nearest city, and what is the highest temperature you will be able to tolerate in the sun room
I will make some recommendations'
Cooling a sun room can run your electric bill way up, 3 to 4x current amounts, there are some options and compromises if cost of operation is an issue for you