Central air conditioning units remove warm air from your home, cool the air and then send it back inside. This cycle repeats until your thermostat says enough is enough.
Three main components are responsible for the cool air : the evaporator, condenser and compresser. Cooling follows these steps:
1. A liquid refrigerant enters the evaporator coil and absorbs heat from inside your house, dropping the temperature and humidity. In the process, the liquid evaporates and turns to gas.
2. The cooled gas travels to the compressor, which reduces the volume of the gas and raises its temperature so it is hotter than the outside air.
3. The hot gas moves to the condenser coil where it is cooled by either outside air or water from your home, depending on your system.
4. The cooling process condenses the gas into a liquid, which passes through a pressure-reducing device — essentially a very small hole — causing its pressure and temperature to drop as it leaves the condenser.
5. The cooled liquid makes its way back to the evaporator where the process starts anew.
The cycle continues until the temperature reaches the setting on the thermostat. A sensor on the thermostat turns on and off the air conditioner to maintain that setting.
Systems
Central air conditioning units are often categorized as split or packaged systems, and those can be either water cooled or air cooled. All are connected to the furnace and distribute cooled air through the existing ducts.
Alternatives are the mini-split ductless systems and mini-duct stems, which are not connected to forced-air heating ducts.
Split and Packaged Systems
Split and packaged refers to the location of the evaporator, condenser and compressor. Here’s how they differ:
Split systems. The condenser and compressor are together in an outdoor cabinet; the evaporator is inside the house, probably installed alongside the furnace. Split systems are the most common type of residential central cooling.
Packaged systems. The evaporator, condenser and compressor are all in one cabinet, which usually sits near the foundation on a concrete slab, but could be on a roof or in a basement.
Air-Cooled and Water-Cooled Systems
Whether split or packaged, central air conditioners rely on the condenser to cool the refrigerant. Typical systems use air or water for that step.
Air-cooled. A fan in the condenser blows outdoor air across the condenser coil to cool the refrigerant. Air cooling is the most common method in residential air conditioning units.
Water-cooled. Water from the indoor plumbing system cools the refrigerant gas at the condenser. Because the compressor doesn’t need outside air, both the condenser and compressor normally can be installed in the basement. The waste water must go down a drain or can be used for watering a lawn or filling a swimming pool. The water is non-potable (non-drinkable) and cannot be re-introduced to the drinking water.
Water-cooled systems can be severely damaged if water flow stops, so at least one water supply valve always must be open.
Ductless Systems
Mini-duct and mini-split ductless systems are alternatives for homes without ducts — such as those with hot water heat or electric baseboards — or with inadequate ducts.
Mini-duct and ductless systems use the same cooling process as other central air conditioning units, but the location of components and ductwork, the size of the ducts and the air distribution differ.
Mini-Duct Systems
With mini-duct systems — also called high-velocity systems — a blower and evaporator coil are usually in the attic, and the standard condenser is usually outside, but instead might be in the basement.
A special fan coil and an air-handling unit pump cool air at high velocity through flexible, insulated plastic tubes that are only a few inches in diameter. The tubes usually feed through the ceiling or upper wall, but can come through the floor. A diffuser at the end of the tube spreads the cooled air through the room. The large return grille is mounted in the ceiling.
Attic-mounted systems have disadvantages. They can be noisy, expensive and harder to service because of their location. And condensation can leak into the living space below if the condensate pan overflows.
The ducts can support electronic air cleaners and humidifiers, although humidifiers can freeze in winter if in the attic.
Mini-Split Ductless Systems
With a mini-split ductless system, the compressor-condenser unit — which looks like a thin window-mounted room air conditioner — is mounted outside the house. The outside unit serves one or more evaporator coil fan units, which are installed on inside walls to distribute cool air. A hose running through the wall links the indoor and outdoor units.
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