Systems and Components
Central Air Conditioner
All air conditioning systems reduce the temperature and adjust the humidity of the air in the home to levels that provide a degree of comfort.The most common type of system used in the USA is the basic, air-cooled, central air conditioning system. Another type of system, found primarily in hotter and arid areas like the southwestern USA, is known as an evaporator cooler or “swamp cooler”. Other, less common types include water-cooled, gas chillers and geothermal systems.
How They Work
The common, air-cooled central air conditioner removes heat from the air in the home and moves the heat to the air at the exterior. Air conditioners accomplish this by taking advantage of the basic physical law that heat moves to areas that are cooler. Heat from the interior of the home is transferred to a refrigerant that carries the heat to the exterior of the home. The most common refrigerant is freon, although it's makeup is continually being reworked due to it's ozone depleting nature.The warm air in the home is blown over an evaporator coil that contains cold (approximately 20 degrees F) freon liquid. The freon absorbs heat from the air in the house, which cools the air. The warmer (approximately 50 degrees F) freon, which has boiled and turned into a gas, is then moved to the exterior of the home where it disperses the heat.
The question is, "How is the heat dispersed when it is hot outside?" A gas or liquid, when compressed, will have a higher temperature. A compressor is used to compress the freon gas, increasing its temperature by approximately 100 degrees F and causing the freon to become much hotter than the air outside. The air outside (approximately 85 degrees F) can then be blown over the condensing coil that contains the hot freon (approximately 150 degrees F) . The air absorbs the heat from the compressed freon, in effect cooling it. The cooled, but still compressed freon (approximately 100 degrees F), is then returned to the house. To lower the temperature of the freon liquid even more, the freon is expanded or decompressed (to approximately 20 degrees F). The freon is then once again ready to absorb the heat from the air in the home.
The coil containing the cold freon liquid over which the warm air from the interior of the home is passed. The heat from the air is absorbed by the freon liquid in the coil cooling the air but causes the freon to boil and turn into a gas. The coil gets its name because the freon in the coil evaporates or turns into a gas when it absorbs the heat from the air passing over the coil. The location and use of the coil may indicate the type of air conditioning system. An evaporator coil that is incorporated into a warm air furnace is called a split system. An evaporator coil that is used to warm air is called a heat pump. An evaporator coil that is not part of a heating system is an independent system.
Compresses the freon gas to increase its temperature. The compressor is needed to make the temperature of the gas much higher than the temperature of the air outside the home. The compressor is the main device that moves the freon through the system and is usually located next to the condensing coil.
The coil containing the high pressure and high temperature freon over which air on the exterior of the home is passed. The air on the exterior of the home absorbs the heat from the freon in the coil. As the high pressure freon gas loses heat, it turns back into a liquid. The coil gets its name from the freon condensing or turning back into a liquid. Condensing coils are generally cooled by air, however, some systems use water to absorb the heat from the coil.
A device designed to reduce the pressure of the freon liquid coming from the condensing coil. Reducing the pressure greatly reduces the temperature of the freon. (Opposite action of the compressor.) The device reduces the pressure of the freon by regulating the amount leaving the device. The device is usually located before the evaporator coil. It captures the moisture that develops on the evaporator coil as the air in the home is cooled. The line directs the water to a plumbing drain.
What is the best way to determine if the heat exchanger of a standard furnace has a failure?
The difficult part of evaluating heat exchangers is the fact that only 15–20% of a standard heat exchanger is visible. The best way to determine if you have a failed heat exchanger is to do the following:
Turn the unit on by raising the thermostat. This should cause the atmospheric burners to come on. Look at the flame pattern before the fan comes on. There should be bluish flames with an occassional amount of orange/yellow at the tips, and they should be erect or vertical.
Look at the burner flames again when the fan comes on. There should be no difference in the flame pattern. If there is, then it suggests that the fan is forcing air from the house side into the path of the flame, and forcing exhaust through a crack or hole in the heat exchanger. This occurs because the fan will pressurize the heat exchanger from the house air side.
Although there are other methods to determine a failure, watching for a distorted flame pattern, while not perfect, is a very dependable method.
How can you conclusively determine if a steam boiler is good or bad?
You must first develop pressure in the system before steam will be forced through a crack. Steam boilers do not develop pressure until air is evacuated from all of the radiators and they are filled with steam. When the radiators are filled, pressure builds up in the system. (From a cold start, this usually takes 40–70 minutes to develop, depending on the size of the system and size of the burner).
Residential units typically operate with 2–5 psi as the limit. Commercial steam boilers may operate with 80–100 pounds or more, depending on the application. The limit control and the pressure gauge are usually located at the top of the boiler.
How do I determine the tonnage of my air conditioing unit?
A general rule of thumb is to locate the MODEL number of your air conditioning condensing unit. This usually is located at the rear of the exterior unit. Within the middle or near the end of the model #, look for a two digit number ranging from 18-60 and divisible by 6. Example: 18, 24, 30, etc.
- Adding three zero's to this number would indicate BTU's. Example: 36 = 36,000 BTU's.
- The industry standard is 12,000 BTU's per ton. Thus, a 48,000 BTU model would be considered a 4.0 ton unit.
- Calculate 20 BTU's per square foot of coverage desired. A 3000 square foot area to be cooled would require a minimum of 60,000 BTU's or a 5.0 ton unit.
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It's important to remember the square footage of your basement is usually not included in the calculation due to it's cool nature by virture of it's location. Also, understand many contractors when dealing with residential units will tend to shy away from the term "tonnage", but rather speak of BTU's per square foot. Tonnage is a term usually reserved for commercial work.
Size of System
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Bigger is not definitely not better. Oversized systems are prone to short cycling or turn on and off frequently and will reduce the life expectancy of the equipment. In addition, the ideal comfort level of the home will not be obtained because of dramatic changes in the temperature of the house as the house cools and heats rapidly. Finally, an oversized system will not properly dehumidify the air due to the short cycling, which can cause swamp-like conditions (cool but humid air).
Air conditioning systems should be serviced every year at the beginning of the cooling season. The exterior compressor/condenser units should be kept clean and free of all leaves and debris; they should never be covered or otherwise boxed-in, and shrubs and hedges should be cut back. All of the above restricts the free flow of air, which reduces the gas-to-liquid process and overall efficiency of the system. In extreme cases, this may cause damage to the compressor. Window units should be removed during the heating system, because they allow an excessive amount of heat loss when they are left in.
Though usually made of copper, these coils can corrode as the system ages.
The compressor is the main mechanical component of a central air conditioning system, which both compresses the freon and moves the freon through the system. In general, air conditioning compressors should not be operated in temperatures below 60 degrees F, or damage may result.
In addition to removing the heat from the air in the home, the basic central air condition system also lowers the moisture level in the air in the home. When warm, moist air comes in contact with a cooler surface, drops of water start to form on the cooler surface. A simple example of this is illustrated by a glass filled with an ice cold drink on a hot summer day. Water drops develop on the exterior of the cold cup. This is moisture in the air that is condensing on the cooler surface. The same occurs when the warm air from the house is passed over the evaporator coil. Moisture in the air turns to water on the evaporator coil. As the moisture accumulates on the coil, it will drop into a pan called the condensate tray, which is located under the evaporator coil. The water will collect here and eventually flow down a pipe into a drain. The old adage, "It's not the heat, its the humidity" rings true here. Dryer air allows the sweat produced by a human body to be evaporated from the skin, cooling the body and making the body more comfortable.
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