Every year, stores sell out of their air conditioners during the first heat wave. People desperate for relief from the heat settle for anything they can find and afford, with little thought for how well the air conditioner will suit their needs. Many people don't know the square footage of their homes, let alone how powerful an air conditioner needs to be to cool them effectively. It's easy to wind up with a unit too weak to cool off the space effectively, and it's just as bad to have a unit that is too powerful, since it will cycle on and off constantly, wasting energy and losing effectiveness. With a little advance planning and knowledge, it is possible to choose an air conditioner specifically suited to the space it will be used in, whether that is a bedroom, a studio apartment, or a whole house.

Window Unit

The most familiar type of air conditioner for most users is the simple window unit. These are designed to fit into standard windows that slide up and down. Because there is considerable variation in the size of the windows in different houses, it is essential to measure the width and height of the space the air conditioner will fit in. By measuring first, the buyer can avoid the annoyance of buying a unit they can't actually use. Installation generally involves little more than opening the window and propping the air conditioner in the space, with the window closed on top to help hold it in place. In some cases, installation may also involve drilling holes in the window frame to screw the edges of the air conditioner in to, or preparing additional support for especially heavy units. More powerful air conditioners are also larger and heavier, so it's good to have a friend to help with lifting and moving the unit.

Also referred to as room air conditioners, window air conditioners generally have a cooling capacity of between 5,000 BTUs and 30,000 BTUs, though 5,000 to 10,000 is more common. Prices start at around $100 for lower power units and range up to $400 for 10,000 BTUs and can go over $1,000 for higher BTU ratings on a window air conditioner. There are often sales at the beginning or end of the cooling season, as stores compete for business or try to clear out unsold inventory.

Casement Unit Although it can be a little more difficult to find them, air conditioners to fit casement or side to side sliding windows are also available. BTU ratings are similar to regular room air conditioners, as is the installation process. The price range, however, is a bit narrower because of the more limited selection. In general, a casement air conditioner may cost between about $300 and $500.

Through-Wall Unit

Through-wall air conditioners require more complicated installation, since they are installed into an outside wall of the house. A metal sheath lines the hole through the wall and provides support and a close fit for the air conditioner. Installing a through-wall air conditioner without the sheath can lead to degradation of the wall and gaps through which cooled air can escape to the outside. Many through-wall air conditioners can double as window air conditioners if the homeowner doesn't want to install it through the wall. The main advantage of using a through- wall air conditioner is that it does not take up window space.

For insulation during the winter months, it is advisable to also purchase an insulated air conditioner cover to fit the measurements of the opening. Like window units, through-wall air conditioners are available with BTU ratings between 5,000 and 30,000, and prices range from around $100 up to about $1,500.

Central Air

While a central air conditioner can cool the whole house effectively, they cannot be used in all houses. Central air conditioning units can only be installed if there is ductwork to allow the air to flow throughout the house. They cannot be installed with heating systems that do not use air ducts, such as electric or hot water baseboards or radiators. It is possible to have the ductwork for a central air conditioning unit installed separately, but is generally prohibitively expensive. Even with pre-existing ductwork, a central air conditioner generally requires professional installation. Prices and power levels vary widely.

Ductless Split Unit A split system air conditioner is an elegant solution to higher powered cooling without extra noise or the need for ducts. They split system places the air compressor outside the house, connected through a small (about 3") hole through the wall to the interior cooling unit. Keeping the air compressor outside means that most of the noise of the system is also outside rather than inside. These units are typically more expensive than window units, and generally require professional installation. The BTU range is from around 10,000 to 30,000, and prices range from about $500 to $3000.

Portable Units Portable air conditioners are a fairly new development, but a very welcome one. They are perfect for rooms without windows, or with windows that cannot support the weight or size of an air conditioner, and they are easily moved from room to room as needed. These unique units use an exhaust system to vent the heat produced by the air compressor. The exhaust is vented through attached tubes that can be set up venting through a nearby window. Most portable units have between 7000 and 30,000 BTUs of cooling power, although some units are much more powerful (and expensive). Prices range from about $300 to over $10,000.

Portable Air Coolers Portable air coolers are often grouped together with air conditioners, but they actually use a completely different method of cooling the air. They do not generally have specific BTU ratings because the technology has varying efficiency, depending on the conditions. Rather than using an air compressor and condenser like an air conditioner, air coolers use a water tank and evaporation as their cooling mechanism. This method is most effective in areas of low relative humidity. They also act as a humidifier, whereas air conditioners have a dehumidifying effect. This type of mechanism uses much less electricity and is generally much quieter than an air conditioner, which can be great advantages. Prices tend to range between about $80 and $150.

Air conditioners use the same technology as refrigerators. Instead of cooling an insulated box with heat from the coils diffused into the room, the cooling half of the mechanism blows cool air into the room and the hot portion of the mechanism vents heated air to the outside. This same principle applies to window units, through-wall units, split units and central air conditioners. The main difference between the types of air conditioner is how the components are arranged.

The basic components of an air conditioner are:

1. A compressor, which compresses the coolant and raises the pressure and temperature.
2. Hot condenser coils, which the compressed, hot coolant runs through in order to dissipate heat and condense into a liquid.
3. An expansion valve, which causes the coolant to expand and evaporate into a cool gas.
4. Cold evaporator coils, which the low temperature coolant runs through in order to cool the surrounding air.

Fans are also used in air conditioners to constantly circulate inside air over the cool coils and to blow outside air over the hot coils to dissipate the heat more effectively.

The capacity of an air conditioner is generally rated in BTUs, or British Thermal Units. Most room air conditioners fall between 5,000 and 30,000 BTUs, but what does that mean in terms of cooling? The real effectiveness of a unit can vary depending on how many people use the space being cooled, what equipment or electronics are being used in the space, and what the outside temperature is. In general, however, a 5,000 BTU air conditioner will cool a room of 100-150 square feet, 6,000 BTUs will cool a space of 150-250 square feet, 7,000 BTUs will cool a space of 250 to 300 square feet, and so on. One general rule of thumb for estimating the necessary BTUs for a space is to find the square footage and multiply it by 35. It's not an exact method, but should provide a workable ballpark figure.

There are some additional factors that will adjust the recommended BTU rating to effectively cool a space. If the air conditioner will be in a room shaded from the outside, reduce the BTUs by 10%, but if it is very sunny, increase them by 10%. For a room that is regularly used by more than two people, add about 600 BTUs per person. For cooling a kitchen, add 4,000 BTUs to counteract the effect of the oven, stove and microwave.

The EER, or energy efficiency rating, of an air conditioner is found by dividing the BTU rating by the wattage. A higher EER means better energy efficiency, but also often means a more expensive air conditioner. Many air conditioners qualify for the Energy Star label, which can also be helpful in choosing between units.

There are several mistakes that are easy to make when buying an air conditioner, but with a little advance planning they can also be easily avoided.

Too Strong or Too Weak

Getting an air conditioner with either too high or too low a BTU rating for the space it needs to cool can be a problem. If it has too low a BTU rating, the unit will run continuously but still not cool the room adequately. This is noisy and consumes a lot of energy without getting much benefit because of the weak cooling.

On the other hand, getting too strong an air conditioner can also be a problem. An air conditioner with too high a BTU rating will cool the space rapidly, but it will also tend to cycle on and off rapidly. This can waste energy, and it also lessens the effectiveness with which the air conditioner removes humidity from the air, since the space doesn't achieve optimum air circulation.

Size Problems

In order to avoid problems with the size of the air conditioner, it's important to take into account the space it will be installed in. For through-wall and split air conditioners, be sure to know where in the house they will be installed before making the purchase, and look for units that will fit that space appropriately.

For window air conditioners, take down the inside dimensions of the window before picking out an air conditioner and make sure that any unit will fit in the intended window before completing the purchase. Also take into account the weight of the unit, since the homeowner needs to be able to lift the air conditioner into the window and set it up, as well as taking it out of the window for winter storage. Unless the air conditioner will be used year-round, it can also be useful to consider where it will be stored. If it needs to be carried down to the basement for the winter, make sure it is a weight that can be managed by one or two of the people available to carry it.

Some features to look for include:

Warranty

It's generally advisable to look for an air conditioner with a one or two year warranty on parts or labor. This will cover repairs or replacements for manufacturer defects. Some stores also offer an extended warranty for an additional fee, although opinions vary on whether such plans are worth the extra expense.

Temperature Range and Settings

While many air conditioners still use a dial to set the relative cooling strength, it is becoming more common to find digital displays that allow the user to set the actual desired temperature. This allows for greater control over the extent to which the space is cooled and can prevent the excessive nighttime chilliness that simple "low to high" dials may cause. Before buying, check what increments the temperature can be adjusted by, and what the temperature range is. The smaller the increments and larger the range, the easier it will be to set the cooling level with precision.

Energy Saver/Sleep Setting

Another option that can be useful is an energy saver or sleep setting. This can be used at night or when the room is unused, setting the cooling process at minimum so that the user saves money without needing to turn the unit off completely. This keeps the room at a manageable temperature that can quickly be cooled to a comfortable level.

Fan Settings

Fan speed can affect noise levels, cooling speed, and air circulation. Because the best level of fan speed can be different depending on the time of day or relative temperature, it's useful to choose an air conditioner with several fan settings to choose from.

Filters

Air conditioners come with a filter to screen out dust and other particles as air is processed through the unit and blown back into the room. These need to be cleaned or replaced periodically, so it is useful to look for a unit with a slide-out filter, rather than one enclosed by the frame. Washable filters can be a money-saving option, since they can simply be cleaned rather than replaced.

Additional Functions

Air conditioners naturally act as dehumidifiers because of how the mechanism works, but some manufacturers also include extra functions such as an air purifier, or the ability to run the unit only as a dehumidifier without cooling the room. Some units are even designed to be used as either an air conditioner or a heater, as the situation requires.