Sizing a Room Air Conditioner

Before buying a room air conditioner, check its BTU rating to make sure you buy the proper size. Room air conditioners perform best when they are sized properly. Pick a unit with a cooling capacity that is right for the room where it will be installed. A too-small unit won’t cool a room adequately, but an over-sized air conditioner will cycle on and off, wasting energy, increasing electric bills, straining the unit, and doing a poor job of dehumidifying the air.

Before shopping, measure the dimensions of the room you want to cool. Multiply the length by the width to get the square footage of living space. Then, match the room size with the BTU rating, typically printed on the packing box. As a rule-of-thumb, figure 20 BTU for each square foot, though your climate, window sizes, house shading, and the room’s height will affect this calculation.

Small, medium or large?

Room air conditioners operate at 5,000 to 24,000 BTU per hour; 12,500-BTU units are considered large. Expect the cost of the air conditioner to increase as the BTU rating goes up.

Small room air conditioners generally operate at 5,000 to 7,000 BTU per hour and can adequately cool 100 to 300 square feet.

Mid-sized models run at 8,000 to 10,000 BTU per hour. They can cool a room up to 450 square feet. A large unit rated between 10,000 and 12,500 BTU will cool a room sized 400 to 650 square feet.

Cooling Capacity Considerations

Several variables can affect cooling capacity:

• If you live in a very warm climate, you may need an AC unit that outputs more BTU per hour than recommended.

• Add 10% capacity for particularly sunny rooms or subtract 10% capacity for shady rooms.

• If the room you are cooling is permanently open to an adjoining space, figure the square footage of both rooms when calculating the size air conditioner you will need.

• Portable air conditioners generally are not as efficient as window air conditioners, so it is a good idea to get a more powerful unit than the square footage indicates.

AC Unit Buying Advice

Before you choose a new AC unit for your home, be sure to consider these issues:

Draining Condensations

Air conditioners remove water from the air as they cool it. Window and in-the-wall air conditioners have drainage tubes that carry condensation outside. Portable units have reservoirs that need to be periodically emptied, though some also come with optional hookups for a drainage hose. Some new models evaporate much of the moisture and exhaust it, greatly reducing the frequency of need to empty a container.

Electrical Requirements

Before buying, be sure your home’s electrical system can handle the appliance’s power needs. If a dedicated or 230-volt circuit is required, you’ll need to have an electrical contractor install this.

Delivering the Cool

Keep your room’s layout in mind when purchasing a particular model—make sure it will be able to direct cooled air where you want it. If you’ll be installing the air conditioner at the end of a long room, consider buying a model that has a “Super Thrust” or “Power Thrust” fan control that pushes air further into the room—just be aware that this type of unit will be noisier than standard models.