How to Size Return Air Boxes


In a central air conditioning unit, heated and cooled air is transported behind ceilings, walls and floors through duct work. The air enters individual rooms through return air boxes. If the boxes are too small or too large, the central unit does not work efficiently. They should be 10 percent larger than the size of the main supply duct. You must measure the size of the air return boxes to determine this.

Things You'll Need

  • Measuring tape
  • Phillips screwdriver
  • Unscrew and remove the cover on the return air box.

  • Measure the width of the opening behind the cover.

  • Measure the height of the opening behind the cover. Record your measurements; this is the size of the return air box.

  • Place the cover back on the return air box and screw it in place.

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