How air conditioning works

Understanding how air conditioning works can help you find the most appropriate energy saving air conditioner, or make your current air conditioning unit more efficient.

Most air conditioners work on the same principle as your refrigerator – extracting heat from your home (or the refrigerator) and releasing it to the outside (or your kitchen). Air conditioners use a compressor to pump a refrigerant through a closed loop that includes an evaporator and a condenser. The evaporator usually operates inside the house; as the refrigerant evaporates (changes from liquid to gas) it captures indoor heat and stores it in the evaporated gas. The condenser operates outside; as the refrigerant condenses back to a liquid (by being compressed), its heat is released, and escapes to the outdoor air through the condenser coils.

Depending on the design of an air conditioner, and how suited it is for the characteristics of the space being cooled (e.g. surface area, insulation in house, air leaks, indoor and outdoor heat sources), a unit that appears to be an energy saving air conditioner in the product literature can turn out to be very inefficient for the job.

One special type of central air conditioner is a geothermal air conditioner, which can also be used to heat your home and heat your hot water. While a geothermal air conditioning system works on a similar principle to other compressor-based air conditioners, there are enough differences that geothermal cooling systems are covered separately on my Energy saving geothermal page.

More broadly, any air conditioner that is also able to provide heat is usually referred to as a heat pump rather than an air conditioner. If you live in an area with anything cooler than a very mild winter, I would recommend opting for a high efficiency heat pump over an air conditioner. The heat pump can provide most or all of the heat you’ll need in cool weather, and keep your house cool in hot weather.

Room and central air conditioners

Single-room energy saving air conditioners are usually window units. Most are designed for a sash window and have both the evaporator and condenser in one sealed unit shaped more or less like a box. Other room models may be energy saving air conditioners for casement windows, wall units (which are like window units but are built into a rectangular hole cut out of the wall), or even movable energy saving air conditioners on casters, known as portable air conditioners.

Central air conditioners typically consist of an indoor evaporator unit whose coils are integrated into ductwork, with a ventilation fan that blows warm air from the house against the coils to cool it, along with an outdoor condenser unit that releases the heat into a yard or garden. These typical central energy saving air conditioners are generally incorporated into a forced air heating system if one is installed. Other central units which are also integrated to the ventilation system have both the evaporator and condenser integrated into one unit, and are mounted on an outside wall; only the cooled refrigerant flows to the ventilation system to cool the forced air.

Ductless minisplit systems

Ductless minisplit systems are energy saving air conditioners for whole house use, with an evaporator unit located in each room being cooled. Each evaporator unit is connected to the single outside condenser unit by lines for the refrigerant, power, and water condensed out as humidity is removed from the room. A single, three-inch-diameter hole is required through the outside wall to bring the refrigerant lines and electrical controls of the ductless air conditioning system inside. From there, the refrigerant lines can be passed within wall interiors if you are prepared to have walls opened up, or on the wall surface, to up to four different indoor units.

Minisplit systems are suitable for houses where no ductwork already exists, or where you want to control temperature at an individual room or zone level. These ductless air conditioning systems are much more energy saving air conditioners than traditional central air conditioner systems (those with one evaporator unit connected to the forced air system) if you strive to control room temperature based on which rooms are occupied at any given time of day (for example, you can let bedrooms warm up during the day). Ductless air conditioning systems are also very much energy saving air conditioners in that the energy losses attributable to leaky or poorly insulated ductwork do not apply to a ductless system. While ductless air conditioner costs are about 30% higher than a comparable central forced air system, the energy savings usually justify this added expense, so zone control and the aesthetics of the indoor units high on room walls are the main factors you’ll want to think about when choosing between ductless split air conditioner and conventional forced air systems.

Evaporative cooler air conditioners

A less common type of energy saving air conditioner is an evaporative cooler. These energy saving air conditioners are much more efficient in terms of electricity use than refrigerant-based air conditioners. They use only 25% of the energy of a traditional air conditioner, and are less expensive to buy and install. However they require a hot, dry climate to operate, and they use water, so may not be suitable in areas where water is scarce and priced accordingly (such as hot, dry areas!). They are certainly not appropriate for areas where hot weather tends to be accompanied by moderate or high humidity. Speaking of humidity, they also increase humidity levels in your home because the air blown into your home has cooled by evaporating liquid water into water vapor. But if you live in a desert, a little extra indoor humidity might be welcome!

Evaporative coolers make use of the fact that evaporation is a cooling process, in order to cool intake air from the outdoors by using it to evaporate water inside the cooler unit. This air is then pumped into the house, and air already in the house is pushed out through windows left partially open.

Suitable areas for evaporative coolers would be desert or dry areas, such as parts of Australia or the southwestern US.