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|HVAC & Water
Air Conditioning > Central Systems
Central AC Basics
Central AC Types
|If you are looking for an air conditioning product which can provide cooling to your entire home, in most cases that means that you are in the market for a central AC system. These are larger, more powerful, and more expensive than their portable counterparts, such as those you may see protruding from a window or embedded in the wall. They also offer greater functional complexity and a more diverse feature set than portable units.|
Air conditioning systems that cool the entire home are referred to as “central AC” because cold air is generated by a central system and then transported throughout the home, most commonly via a network of ducts. This is different from portable air conditioners, such as floor, window or wall units, which provide localized cooling to a single room or individual enclosed area and do not utilize ducts.
However, central AC systems are also significantly more expensive than portable air conditioners and use more energy. A central AC system is typically comprised of an air handling unit and a system of supply and return ducts which runs through the entire home. When the temperature in the home rises above the thermostat setting, the central AC system begins to operate, drawing the warm air out of the rooms of the home via the return ducts and sending cool air into the rooms via the supply ducts. This air cycling process continues until the temperature falls below the thermostat level once more.
A central air conditioner can be either a packaged system, which is comprised of a single unit housing all of the necessary cooling components, or a split system, which is comprised of an indoor unit housing the air handler and an outdoor unit housing the compressor and condenser. A split system is the more common type found in homes throughout North America. We explore the two types and their respective benefits and drawbacks subsequently. It also bears noting that central air conditioners work on the principle of utilizing a power source to run a process that reduces the temperature of the surrounding air. The process used is known as a refrigeration cycle.
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Central AC Basics
A central air conditioner is a major appliance that is permanently installed inside or outside the home with the purpose of providing cooling to either all, or a majority of the interior spaces. A central AC system transfers heat from inside the home to outside the home via a three-step process that involves an evaporator, a condenser, and a compressor.
The actual process is not as complicated as these terms may suggest. The evaporator function takes place within coils that are placed inside the home. Heat is absorbed from the interior air as a result of an evaporation process. This heat is captured into a liquid called a refrigerant flowing through the evaporator coils. Once it picks up this heat, the refrigerant is compressed in the compressor and pushed into the condenser, where the heat is released into the outside air.
This process is referred to as the “refrigeration cycle”, which makes sense considering that it is also the way a refrigerator cools its interior compartments. Heat is removed from the inside of a refrigerator using evaporation, compression, and condensation, with the warm air being pumped out of the interior compartment and deposited into the kitchen.
Some of you may remember the concept of “phases” from high school chemistry. There are three phases: solid, liquid, and gas. When a substance goes from one phase to another, such as ice turning to water, or water turning to vapor, this is known as a “phase change”. During a phase change, heat is either taken from the environment or released into the environment. One way to effect a phase change is by heating or cooling a substance. Another way to effect a phase change is to apply pressure to a substance.
In a refrigeration cycle, the refrigerant is first moved into evaporator coils, which are located in the compartment that needs to be cooled. The pressure in the evaporator coils is kept low, which causes the refrigerant to turn from a liquid into a gas, a phase change known as evaporation (which is why the coils are called “evaporator” coils). Evaporation is a phase change which removes heat from the environment. As a result, the refrigerant being pumped through the coils and turning from liquid to gas has the effect of continually removing heat from the area, thereby cooling the ambient temperature.
After passing through the evaporator coils, the refrigerant is moved into a compressor and condenser, which cause the gas to turn back into liquid form. This phase change back to liquid has the opposite effect, returning heat back to the environment. However, this heat is not returned to the interior, but rather expelled to the outside. The net effect is that heat is taken out of one area (the interior of a home) and transferred to another area (the outside).
Reading this, you may have realized that if the evaporator and the condenser were switched around, the interior of the home could be heated rather than cooled. This is indeed the case and this type of heating is exactly the principle behind the operation of a heat pump. In fact, a heat pump may be used both to heat and to cool the home, bringing heat in from the outside during the winter and taking heat out from the inside during the summer. Take a look at our Home Heating section for more information about heat pumps.
Air that passes through the evaporator coils of an air conditioning system has the moisture literally condensed out of it, akin to wringing out a wet towel. The result is that air conditioned air is not only cooled, but also dehumidified. The net effect is that air conditioned interior spaces provide not only a less warm, but also a less humid environment.
There are several types of central AC systems that are widely available and popular throughout North America, but the majority of these operate on the basic idea of the refrigeration cycle. We discuss the different types and their advantages and disadvantages in greater detail in the next section.
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Central AC Types
The two primary types of central AC are split systems and packaged systems. In a split system, as the name suggests, there are two separate units. The indoor unit contains the evaporator, which does the work of cooling the indoor air by phasing the refrigerant from liquid to gas. The outdoor unit contains the compressor and the condenser, which do the work of releasing the heat captured from the interior spaces by the evaporator into the outside air.
In a packaged system, the evaporator, the compressor, and the condenser are all contained within a single unit that is placed either atop or next to the house. Air ducts run to and from this single outdoor unit, looping throughout the interior of the home in order to supply cooled air from the unit while recycling hot air back to the unit.
Both types of central AC systems can be coupled with a house heating system, although in different ways. In a situation where there is already a furnace in place, along with ductwork that is used to heat the home, a split system AC will be the easiest installation. The evaporator coil is simply placed in the primary supply duct of the existing heating system, and the condenser and compressor are installed outdoors.
In the case of a packaged AC system, the unit will often come packaged together with an electric heater or gas furnace, thereby providing both cooling in the summer and heating in the winter. Thus, a packaged AC system is generally more appropriate for a home with a non-existent or outdated heating system, while a split AC system is generally better for a home which already has a quality central heating system in place.
The traditional split and packaged AC systems both require ductwork in order to circulate cool air into and warm air out of the various interior spaces of the home. In cases where there is no existing ductwork, a system called a “ductless mini split” can be installed. Generally rated somewhere between a central system and a portable system, the ductless mini split consists of an outdoor compressor and condenser unit, and anywhere from one to four indoor air handling units which may be placed in different rooms, or “zones”, of the home. Rather than ductwork, the indoor units and the outdoor unit are linked by a specialized tube which can be fairly narrow, making it easy to install through the wall of a home.
The benefits of ductless mini split systems are that they can be more efficient, as there are no energy losses due to poorly insulated ductwork and indoor units in individual zones can be turned off when there is nobody using those interior spaces. In addition, these systems are fairly easy to install, obviously do not require ductwork, and provide a lot of options as far as the placement of the air handlers. There are also disadvantages, however. The main drawback is that a ductless mini split system is more obtrusive than a ducted central AC system, with the air handlers clearly visible in each room where they are installed. In addition, a ductless mini split may be less powerful than a central AC system and may not be appropriate for a larger home.
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In selecting a central AC system, it is important to not only choose the right type but also to select a system that provides all of the features that are necessary for convenient and safe operation on an on-going basis. An important feature is the relative loudness or quietness of the air conditioner unit’s operation. If at all possible, you should ask to see the air conditioner at full workload and listen to the noise to avoid any surprises down the line. This is a comfort issue not only for the residents of your household, but also for the neighbors, as in most instances, the unit will be placed outside of the home. If actually seeing an air conditioner in operation is not possible, you can ask for the decibel rating of the particular model. Generally, a system that is rated 76 decibels or less is considered to be fairly quiet.
Another important feature, particularly from the standpoint of energy efficiency, is a fan only switch, which enables the air conditioner to work in a ventilation only mode. In many parts of the country, the night-time temperature drops significantly, meaning that interior spaces can be cooled effectively by simply running the fan rather than turning on the entire refrigeration cycle. This has the added benefit of saving energy, reducing utility bills, and minimizing the your environmental footprint.
Certain central AC systems use a two-stage design which incorporates two compressors and multiple fans rather than a single compressor and a single fan. Although such systems are somewhat more expensive, they provide more efficient, more comfortable, and more consistent cooling, offering a longer cycle and maintaining the temperature within a narrower band throughout the day.
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