We recently built out an additional 600sqft theater room in our attic. It’s a big room with 14’ high vaulted ceiling. We live in Houston and room heat load is fairly high. We installed a 2-ton unit that does a good job cooling and dehumidify the room. The main house is 2250sqft with a 5 ton unit, that also does a good job cooling and dehumidifying. The systems do not share any duct work.

There is a door to the theater room, and the cold air returns for each system are in the ceiling on either side of the door.

My question is, what happens if we leave the door open?

I’d think leaving the door open would make each system less efficient, because they would no longer be a “closed loop”. I also think with the higher heat load of the room, it’s better to leave the door shut and isolate the heat to that room rather sucking the hot air into the main unit.

  • open the door and hang long strips of toilet paper in the doorway ... that will give you an indication of any air movement between the systems
    – jsotola
    Commented May 20, 2022 at 16:52

1 Answer 1


Having lived in a similar type arrangement, the efficiency of each unit shouldn't drop any significant amount. Any registers should be aimed away from the door in question. I think you'll fine there's still a "closed loop" with the door open. Depending on how often you use the theater, you might think about not running that AC unit full time or at least at a higher temperature when not in use.

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