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How an Air Conditioner works

Curious to know how your air conditioner works? It all starts with
the refrigeration cycle, but before we begin, there are a few things you
you need to understand.

  1. Heat is a form of energy. “Cold” is not a form of energy; it is simply the lack of heat.
  2. Heat always travels from hot to cold. The greater the temperature difference, the faster it travels.
  3. Liquids absorb heat when they boil off to a gas
  4. Gasses give off heat when they condense to a liquid.
  5. The temperature at which a liquid boils is controlled by its pressure. High pressure = high temperature; low pressure = low temperature.

Most people tend to think that air conditioners make cold air and the temperature at which they set their thermostat should be the temperature of the air that is coming out the supply vents. In fact, an air conditioner simply moves heat from one area to another. What is left behind is “cold”. The thermostat is simply a switch to turn on/off the unit when desired temperature is needed or met.

So how does my air conditioner remove heat energy?

Your air conditioner removes heat energy by means of the refrigeration cycle. All air conditioners have the same basic components .  A Compressor, Condenser, Evaporator, and Metering Device. These four items are required to maintain the refrigeration cycle.

It all starts with the Compressor. The compressor  “compresses” the refrigerant to a high temperature; high pressure superheated gas. The superheated gas then moves to the condenser where it “condenses” to a high pressure subcooled liquid. Remember, gasses give off heat when they condense to a liquid. So, the Condenser rejects heat.

The now subcooled liquid now makes it way to the evaporator, but before entering the evaporator the liquid refrigerant passes through a Metering Device. This device restricts the flow of refrigerant causing a drop in pressure. What passes through is now a low pressure; low temperature saturated mix of liquid and gas. This saturated mix travels through the evaporator and because of its low temperature (around 40 degrees F), heat energy from the home is absorbed through the coils turning the low temperature; low pressure saturated mix to a low temperature; low pressure superheated gas. The superheated gas then makes its way to the Compressor and the whole cycle starts over.

There are other components that aid in the refrigeration cycle, such as fans to push or pull air across the evaporator and condenser coils. These components are also very important in aiding your air conditioner in removing heat from your home.



Heat Pump Insights

Not all air conditioners are heat pumps, but ALL heat pumps are air conditioners. Heat pumps can be air-source or water-source and are generally more efficient than it’s straight-cooled counter part.

So what’s the difference?  In short, a straight-cool AC can only cool a structure while a heat pump can cool and heat a structure in moderate  temperatures. This is why they are popular in southern regions of the country where the winters are mild. Basically, a heat pump “pumps” heat energy from one location to another. It can pull heat energy from the outside and put it inside a structure for heat or pull heat from inside a structure to the outside. A straight-cooled air conditioner requires a furnace or electric heat to heat a structure.

Air to Air (air-source) heat pumps are the most common heat pumps used today. These heat pumps are generally less expensive and are easier to install than other types of heat pumps. They are available in package units or split systems and can use auxiliary heat when the outdoor temperatures fall below the balance point.


Like air conditioners, heat pumps have two coils (evaporator & condenser), the difference is  the role that each coil plays changes depending on if the unit is calling for heat or cool. For this reason, technicians identify these coils as “indoor coil” or “outdoor coil” instead of evaporator or condenser.

In cooling, a heat pump runs much like a straight-cool ac. The evaporator absorbs heat from a structure and the condenser rejects the heat to the outside. In heat mode, the condenser takes on the role of the evaporator and absorbs heat energy from the outside while the evaporator becomes the condenser.  This is done by reversing the flow of refrigerant by means of a reversing valve.

Balance Point

The lowest temperature at which a heat pump can heat a structure is called the “balance point”. When the balanced point is reached, auxiliary heat (2nd stage heating) is required. Auxiliary heat consists of electric heat strips which are more expensive to operate. In practice, the auxiliary heat is energized 4-5 degrees above the balance point. Some units have an outdoor thermostat that is set to 5 degrees above the balance point. This is to keep the heat strips from energizing until outdoor temperatures reach the set point. It should be noted that if you set your thermostat 4 degrees above room temperature, the heat strips will energize to meet the demand. Also, when purchasing a heat pump, auxiliary heat strips are generally not included and must be purchased separately.