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    a complete air conditioner in a small space. The
    units are made small enough to fit into a
    standard window frame. You close the window
    down on the unit, plug the unit in and turn it on
    to get cool air. If you take the cover off of an
    unplugged window unit, you will find that it
    contains:
    A compressor
    An expansion valve
    A hot coil (on the outside)
    A chilled coil (on the inside)
    A control unit
    The fans blow air over the coils to improve
    their ability to dissipate heat (to the outside air)
    and cold (to the room being cooled).
    BTU and EER
    Most air conditioners have their capacity
    rated in British thermal units (BTU). Generally
    speaking, a BTU is the amount of heat required
    to raise the temperature of one pound (0.45 kg)
    of water 1 degree Fahrenheit (0.56 degrees
    Celsius). Specifically, 1 BTU equals 1,055
    joules. In heating and cooling terms, 1 "ton"
    equals 12,000 BTU.
    A typical window air conditioner might be
    rated at 10,000 BTU. For comparison, a typical
    2,000-square-foot (185.8 m2) house might have
    a 5-ton (60,000-BTU) air conditioning system,
    implying that you might need perhaps 30 BTU
    per square foot. (Keep in mind that these are
    rough estimates. )
    The energy efficiency rating (EER) of an
    air conditioner is its BTU rating over its
    wattage. For example, if a 10,000-BTU air
    conditioner consumes 1,200 watts, its EER is
    8.3 (10,000 BTU/1,200 watts). Obviously, you
    would like the EER to be as high as possible,
    but normally a higher EER is accompanied by a
    higher price.
    Is the higher EER is worth it?
    Let's say that you have a choice between
    two 10,000-BTU units. One has an EER of 8.3
    and consumes 1,200 watts, and the other has an
    EER of 10 and consumes 1,000 watts. Let's
    also say that the price difference is $100. To
    understand what the payback period is on the
    more expensive unit, you need to know:
    1.Approximately how many hours per year
    you will be operating the unit2.How much a kilowatt-hour (kWh) costs
    in your area
    Let's say that you plan to use the air
    conditioner in the summer (four months a year)
    and it will be operating about six hours a day.
    Let's also imagine that the cost in your area is
    $0.10/kWh. The difference in energy
    consumption between the two units is 200
    watts, which means that every five hours the
    less expensive unit will consume 1 additional
    kWh (and therefore $0.10 more) than the more
    expensive unit.
    Assuming that there are 30 days in a month,
    you find that during the summer you are
    operating the air conditioner:
    Since the more expensive unit costs $100
    more that means that it will take about seven
    years for the more expensive unit to break
    even.
    Split-system Units
    A split-system air conditioner splits the hot
    side from the cold side of the system 。
    The cold side, consisting of the expansion
    valve and the cold coil, is generally placed into
    a furnace or some other air handler. The air
    handler blows air through ducts. The hot side
    known as the condensing unit, lives outside the
    building. The unit consists of a long, spiral coil
    shaped like a cylinder. Inside the coil is a fan,
    to blow air through the coil, along with a
    weather-resistant compressor and some control
    logic. This approach has evolved over the years
    because it is low-cost, and also because it
    normally results in reduced noise inside the
    house (at the expense of increased noise outside
    the house). Besides the fact that the hot and
    cold sides are split apart and the capacity is
    higher (making the coils and compressor
    larger), there is no difference between a
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