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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. To size an air conditioner for your specific needs, contact an HVAC contractor.)
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 unit
2.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.
See this page for a great explanation of seasonal energy efficiency rating (SEER).
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 split-system and a window air conditioner.
In warehouses, businesses, malls, large department stores, etc., the condensing unit normally lives on the roof and can be quite massive. Alternatively, there may be many smaller units on the roof, each attached inside to a small air handler that cools a specific zone in the building.
Let's take a look now at a chilled-water air conditioner.
Chilled-water System
In larger buildings and particularly in multi-story buildings, the split-system approach begins to run into problems. Either running the pipe between the condenser and the air handler exceeds distance limitations (runs that are too long start to cause lubrication difficulties in the compressor), or the amount of duct -work and the length of ducts become unmanageable. At this point, it is time to think about a chilled-water system.
In a chilled-water system, the entire air conditioner lives on the roof or behind the building. It cools water to between 40 and 45 F (4.4 and 7.2 C). This chilled water is then piped throughout the building and connected to air handlers as needed. There is no practical limit to the length of a chilled-water pipe if the system can be got well - insulated.
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