In HVAC systems an air conditioner is a system or a machine that treats air in a defined, usually enclosed area via a refrigeration cycle in which warm air is removed and replaced with cooler air.

In construction, a complete system of heating, ventilation, and air conditioning is referred to as HVAC. Whether in homes, offices or vehicles, its purpose is to provide comfort by altering the properties of the air, usually by cooling the air inside.The main function of air conditioner is to change adverse temperature.

In the refrigeration cycle, a pump transfers heat from a lower temperature source into a higher temperature heat sink. Heat will naturally flow in the opposite direction. This is the most common type of air conditioning. A refrigerated air conditioning system works in much the same way pumping heat out of the room in which it stands.

The most common HVAC refrigeration cycle uses an electric motor to drive a compressor. In an automobile the compressor is driven by a pulley on the engine’s crankshaft, with both using electric motors for air circulation. Since evaporation occurs when heat is absorbed, and condensation occurs when heat is released, air conditioners are designed to use a compressor to cause pressure changes between two compartments, and actively pump a coolant around an enclosed system. The cooling liquid, or refrigerant is pumped into the cooled compartment (the evaporator coil). Low pressure then causes the refrigerant to evaporate taking the heat with it. In the other compartment (the condenser), the refrigerant vapour is compressed and forced through another heat exchange coil, condensed into a liquid which then rejects the heat previously absorbed from the cooled space.

The best air conditioner is the one you don’t have to think about.  It comes on the moment the indoor temperature set on the thermostat requires cooling performance, and then runs quietly and efficiently when needed.  But when it’s time to perform routine maintenance, make repairs or replace your system, it’s helpful to understand how an air conditioning system works.

The Refrigeration Cycle

  1. Using electricity as its power source, the refrigerant flows through a closed system of refrigeration lines between the indoor unit and the outside unit.
  2. Warm air from the inside of your house is pulled into ductwork by a motorized fan.
  3. The refrigerant is pumped from the exterior compressor coil to the interior evaporator coil, where it absorbs the heat from the air.
  4. This cooled air is then pushed through connecting ducts to vents throughout the home, lowering the interior temperature.
  5. The refrigeration cycle continues again, providing a consistent method to keep you cool.

Parts of a Central AC System

To get a better sense of how your air is cooled, it helps to know a little bit about the parts that make up the air conditioning system. A typical central air conditioning system is a two-part or split system that includes:

  • The outdoor unit contains the condenser coil, compressor, electrical components and a fan.
  • The evaporator coil, which is usually installed on top of the gas furnace inside the home.
  • A series of pipes, or refrigeration lines, connecting the inside and outside equipment.
  • Refrigerant, the substance in the refrigeration lines that circulates through the indoor and outdoor unit.
  • Ducts that serve as air tunnels to the various spaces inside your home.
  • A thermostat or control system to set your desired temperature.

Benefits

  • Indoor comfort during warm weather – Central air conditioning helps keep your home cool and reduces humidity levels.
  • Cleaner air – As your central air conditioning system draws air out of various rooms in the house through return air ducts, the air is pulled through an air filter, which removes airborne particles such as dust and lint. Sophisticated filters may remove microscopic pollutants, as well. The filtered air is then routed to air supply duct-work that carries it back to rooms.
  • Quieter operation – Because the compressor-bearing unit is located outside the home, the indoor noise level from its operation is much lower than that of a free-standing air conditioning.

 

Types of Central Air Conditioners

A central air conditioner is either a split-system unit or a packaged unit.

In a split-system central air conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air conditioner’s evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air conditioner, a split-system is the most economical central air conditioner to install.

In a packaged central air conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house’s foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home’s exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors.

Choosing or Upgrading Your Central Air Conditioner

Central air conditioners are more efficient than room air conditioners. In addition, they are out of the way, quiet, and convenient to operate. To save energy and money, you should try to buy an energy-efficient air conditioner and reduce your central air conditioner’s energy use. In an average air-conditioned home, air conditioning consumes more than 2,000 kilowatt-hours of electricity per year, causing power plants to emit about 3,500 pounds of carbon dioxide and 31 pounds of sulfur dioxide.

If you are considering adding central air conditioning to your home, the deciding factor may be the need for ductwork.

If you have an older central air conditioner, you might choose to replace the outdoor compressor with a modern, high-efficiency unit. If you do so, consult a local heating and cooling contractor to assure that the new compressor is properly matched to the indoor unit. However, considering recent changes in refrigerants and air conditioning designs, it might be wiser to replace the entire system.

Today’s best air conditioners use 30% to 50% less energy to produce the same amount of cooling as air conditioners made in the mid 1970s. Even if your air conditioner is only 10 years old, you may save 20% to 40% of your cooling energy costs by replacing it with a newer, more efficient model.

Proper sizing and installation are key elements in determining air conditioner efficiency. Too large a unit will not adequately remove humidity. Too small a unit will not be able to attain a comfortable temperature on the hottest days. Improper unit location, lack of insulation, and improper duct installation can greatly diminish efficiency.

When buying an air conditioner, look for a model with a high efficiency. Central air conditioners are rated according to their seasonal energy efficiency ratio (SEER). SEER indicates the relative amount of energy needed to provide a specific cooling output. Many older systems have SEER ratings of 6 or less.

If your air conditioner is old, consider buying an energy-efficient model. Look for the ENERGY STAR® and EnergyGuide labels — qualified central units are about 15% more efficient than standard models. New residential central air conditioner standards went into effect on January 1, 2015; see the efficiency standards for central air conditioners for details, and consider purchasing a system with a higher SEER than the minimum for greater savings.

The standards do not require you to change your existing central air conditioning units, and replacement parts and services should still be available for your home’s systems. The “lifespan” of a central air conditioner is about 15 to 20 years. Manufacturers typically continue to support existing equipment by making replacement parts available and honoring maintenance contracts after the new standard goes into effect.

Other features to look for when buying an air conditioner include:

  • A thermal expansion valve and a high-temperature rating (EER) greater than 11.6, for high-efficiency operation when the weather is at its hottest
  • A variable speed air handler for new ventilation systems
  • A unit that operates quietly
  • A fan-only switch, so you can use the unit for nighttime ventilation to substantially reduce air-conditioning costs
  • A filter check light to remind you to check the filter after a predetermined number of operating hours
  • An automatic-delay fan switch to turn off the fan a few minutes after the compressor turns off.

Installation and Location of Air Conditioners

If your air conditioner is installed correctly, or if major installation problems are found and fixed, it will perform efficiently for years with only minor routine maintenance. However, many air conditioners are not installed correctly. As an unfortunate result, modern energy-efficient air conditioners can perform almost as poorly as older inefficient models.

When installing a new central air conditioning system, be sure that your contractor:

  • Allows adequate indoor space for the installation, maintenance, and repair of the new system, and installs an access door in the furnace or duct to provide a way to clean the evaporator coil
  • Uses a duct-sizing methodology such as the Air Conditioning Contractors of America (ACCA) Manual D
  • Ensures there are enough supply registers to deliver cool air and enough return air registers to carry warm house air back to the air conditioner
  • Installs duct work within the conditioned space, not in the attic, wherever possible
  • Seals all ducts with duct mastic and heavily insulates attic ducts
  • Locates the condensing unit where its noise will not keep you or your neighbors awake at night, if possible
  • Locates the condensing unit where no nearby objects will block airflow to it
  • Verifies that the newly installed air conditioner has the exact refrigerant charge and airflow rate specified by the manufacturer
  • Locates the thermostat away from heat sources, such as windows or supply registers.

If you are replacing an older or failed split system, be sure that the evaporator coil is replaced with a new one that exactly matches the condenser coil in the new condensing unit. (The air conditioner’s efficiency will likely not improve if the existing evaporator coil is left in place; in fact, the old coil could cause the new compressor to fail prematurely.

Central Air Conditioning Buying Guide

If summertime temperatures leave you hot under the collar, you’re not alone. More than 75 percent of U.S. homes use air conditioning, and 90 percent of new homes are equipped with central air. And eco-conscious consumers will be gratified to know that today’s air conditioners are more energy-efficient, which means they cost less to run while keeping you cool and comfortable.

Installing or replacing central air can be a huge expense, so you’ll want to get it right. To ensure that you have the best advice, we surveyed almost 22,000 Consumer Reports members about new central air-conditioning systems they bought and installed between 2007 and 2019. We learned how satisfied they were overall with their purchase, the cost of repairs, how many systems break, and which parts break most often.

Important Factors for Choosing Central AC

Size
A synonym for the air conditioner’s cooling capacity, size is measured in British thermal units per hour (Btu/hr.) or in “tons.” One ton of cooling equals 12,000 Btu/hr.For sizing guidance, check the Energy Star website.

Efficiency
This describes how much cooling the unit delivers for each watt of electricity. Efficiency is expressed as the seasonal energy-efficiency rating, or SEER. The minimum SEER for a split system central air conditioner allowed today is 14, so look for units with SEER ratings of 15 or greater. The higher the SEER, the more you can lower your energy costs.

Maintenance
A service plan that combines regular inspections with discounts on repairs and a labor warranty is worth negotiating into the overall price. Prices for such a service vary widely.

Programmable thermostats
Proper use of a programmable thermostat can reduce your cooling costs by about 10 percent. And using a box or ceiling fan, which costs little to run, can make you feel 3° F to 4° F cooler.

Upgrading an existing system
If you’re upgrading your central air, don’t assume you should buy the same-sized system. Any changes you’ve made to improve your home’s energy efficiency, such as upgrading your windows or adding insulation, can reduce your cooling needs. On the other hand, if you’ve added rooms, you might need more cooling.

Have your contractor do a load calculation based on a recognized method, such as one in Manual J from the ACCA. The contractor’s evaluation should include whether your ducts need to be resized, sealed and insulated, or replaced. Remember that an indoor evaporator coil and outdoor condenser must be a matched set from the same brand, or the performance, efficiency, and capacity might not meet expectations.

New systems are 20 to 40 percent more efficient than minimum-efficiency models made even 10 years ago. Costs will vary and can depend on whether you need ductwork installed, and the particular size and configuration of your home.

Installation: Find the Right Contractor

Whether you’re replacing an older air conditioner or installing one for the first time, finding a trustworthy contractor to install and service an air-conditioning system matters the most. Here’s what to do.

Ask around. Seek referrals from neighbors, family, or business associates. It’s wise to get price quotes from at least three contractors.

Check their background. Contractors who bid on your installation should show you verification of bonding and insurance, plus any required contractor’s licenses. Check with your local Better Business Bureau and consumer affairs office for complaint records. It’s a plus if technicians are certified by a trade organization, such as North American Technician Excellence or HVAC Excellence, to service residential heating and cooling equipment. Those and other similar programs assess the technician’s knowledge of specific types of equipment and their proper service methods.

Get specifics. Contractors who bid on your job should calculate required cooling capacity by using a recognized method such as one found in the the ACCA’s Residential Load Calculation Manual, also called Manual J. An additional reference for assessing ductwork needs is Manual D. The calculations produce a detailed, room-by-room analysis of cooling needs. Ask for a printout of all calculations and assumptions, including ductwork design. Be leery of a contractor who bases estimates merely on house size or vague rules of thumb.

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