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Sizing Your Air Conditioner Properly

Proper sizing of a room air conditioner is important to achieve the best in comfort, energy efficiency and cost of operation. You should resist the urge to buy the largest unit available since, in the case of room air conditioners, bigger is not better. A unit that is too large for the space it is cooling will cycle on and off frequently. This increases the costs of operation, reduces its operational efficiency and limits the unit's ability to dehumidify the air, the primary determinant of comfort.

The following table gives you a simple way to determine the size room unit you need based on BTU's (British Thermal Units), the most common way in which room units are marketed. Once you have the proper BTU rating, look for the highest efficiency rating available and an EnergyStar label. The Department of Energy Offers additional information on sizing and operation, on product information such as efficiencies and on possible rebates.

Sizing Room Air Conditioners

To figure out which size unit is best for your cooling needs:

1. Determine the square footage of the area to be cooled using the following formulas:
• For square and rectangular rooms, multiply the length of the area by its width
• For a triangular area, multiply the length of the area by the width and divide by 2
Most rooms can be further divided into these basic shapes to determine the square footage.
2. Using the square footage and the chart below, determine the correct cooling capacity. Cooling capacity is measured in British thermal units (BTU's) per hour.

Area to be Cooled
(sq. feet)

Capacity Needed
(BTU's/hour)
100 to 150
5,000
150 to 250
6,000
250 to 300
7,000
300 to 350
8,000
350 to 400
9,000
400 to 450
10,000
450 to 550
12,000
550 to 700
14,000
700 to 1,000
18,000
1,000 to 1,200
21,000
1,200 to 1,400
23,000
1,400 to 1,500
24,000
1,500 to 2,000
30,000
2,000 to 2,500
34,000

3. Make any adjustments for the following circumstances:
• 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.

Proper sizing is equally important for central air conditioning systems as well. Unlike room units, central systems will most likely require the assistance of a professional heating and cooling contractor to determine the correct size. That said, you can still do some homework to know what you want in a system and use this data to select your contractor.

First of all, decide what level of efficiency you want. The DOE has a document prepared for use by Federal agencies in selecting the proper sized and efficiency unit. You can make use of the same data.

Since sizing is one of the most critical elements in system design, ask your contractor to perform a Manual J calculation on your home. This comprehensive calculation determines the heat loss and load of your home and is used by the contractor to accurately size your system. Do not settle for an "eye ball" system sizing. You will end up will poorer comfort and higher bills.

If ductwork is part of your installation, insist on a Manual D calculation to go along with your Manual J. You will be far more satisfied with the results. For a bit more of what to look for with respect to ducts, check out our Weatherization article and browse to the Ducts section.