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I'm trying to size a window air conditioner for a room, but having trouble deciding if the usual guidelines are a good fit.

The room in question is about 230 sq. feet, and does not have full 8 foot ceilings, as part of the room is formed by the gable roof. Based on this I would pick a 6000 BTU unit. However, the room is also roofline with black asphalt shingles, east/west/southern exposure, the long dimension of the space being southern exposure with 2 south-facing skylights. There's no natural shading, and the room is also above an uncooled garage space. I typically see a 10% fudge factor recommended for "sunny rooms", but don't have a clear idea of what constitutes a "sunny room". Would this heat load significantly exceed that factor, and by how much?

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that's not a huge room. You should be OK. You could also consider blocking the skylights (I've done that in the past with foil faced insulation...not the prettiest, but worked) –  DA01 May 24 '12 at 14:50
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1 Answer

This page from Energy Star gives some good basic guidelines.

  • If the room is heavily shaded, reduce capacity by 10 percent.
  • If the room is very sunny, increase capacity by 10 percent.
  • If more than two people regularly occupy the room, add 600 BTUs for each additional person.
  • If the unit is used in a kitchen, increase capacity by 4,000 BTUs.
  • Consider where you install the unit. If you are mounting an air conditioner near the corner of a room, look for a unit that can send the airflow in the right direction.

I'd say your room fits the "very sunny" definition.

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