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Window air conditioners cool, circulate, filter and dehumidify air in the house. They range in size from small units with a cooling capacity of 5,000 BTUs (enough to cool a small room) to as high as 32,000 BTUs (sufficient to cool a small house with adequate airflow).

It is crucial that you purchase the right size air conditioner. An oversize unit will cool but leave a damp and clammy feeling because of high relative humidity. An undersized unit will not operate effectively on very hot days.

An air conditioner should always be selected by BTU rating, not horsepower-horsepower is just the size of the motor driving the compressor; BTU is the actual cooling capacity of the unit.

An effective rule of thumb to determine the BTUs needed to cool a room is to multiply the square feet of the room by 25; add 1,000 for every window and 400 for each occupant.

Example: A room measures 12′ x 15′ with two windows and two occupants. The formula works out to 180 sq. ft. x 25 = 4,500 + 2,000 for the windows + 800 for the occupants = 7,300, or a 7,000-8,000 BTU air conditioner.

To check the efficiency of an air conditioner, divide the watt rating into BTU output. A factor of 10 means it is an efficient unit, and the factor should not be less than 5.

Units with 115 volts having less than a 9,000 BTU capacity will not overload average house wiring; for larger capacity units and those requiring 220V wiring (high BTU capacity), an electrician should be consulted.

Features vary with manufacturers’ lines, but things to look for include variable air direction to prevent drafts, easy to remove and easy to clean air filters for reducing dirt and airborne irritants, silent motor, air exchanger to exhaust stale or smoke-filled air, multi-adjustable speed, and thermostatic temperature controls. Mounting kits are included with most models, including sill brackets for extra support of the unit and side panels to ensure an airtight fit in the window.

For maximum cooling efficiency, a window unit should be placed in a window that will afford maximum air circulation into the room or rooms to be cooled and is shaded most of the time.

The filter should be cleaned periodically, or cooling efficiency will be sharply reduced.

Most units are permanently lubricated and require no oiling; if you encounter this, follow the manufacturer’s instructions.

Homes in which air conditioning is used year-round should be closed year-end-leaving the storm windows or insulated glass in place all year. However, if the home is located in a seasonal climate, the air conditioning should be shut off at night, if the outside air cools down and the windows opened, or used in conjunction with a whole-house fan.

You can reduce heat from the sun by closing drapes and shades. Also use a kitchen ventilator fan when cooking to draw out stove heat. All air conditioners will operate more efficiently when the area to be cooled is well insulated.

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the North American Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.