How Air Conditioning Works

This is the cooling cycle.  If this were a Heat Pump the refrigerant flow is just reversed.

 


Condenser coil Evaporator Coil

Diagram of a typical air conditioner

 

 

This is how the evaporation cycle in an air conditioner works (See How Refrigerators Work for complete details on this cycle):

  1. The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas (red in the diagram above).
  2. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid.
  3. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure Freon gas (light blue in the diagram above).
  4. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside the building.

Mixed in with the Freon is a small amount of a lightweight oil. This oil lubricates the compressor.


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. 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:

4 mo. x 30 days/mo. x 6 hr/day = 720 hours

(720 hrs x 200 watts/hr) / (1000 watts/kW x $0.10/kWh) = $14.40

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).