How air conditioners work: Components and functions of AC units

This section focuses on central air conditioning and factors related to cooling of homes and businesses. We also cover ductless air conditioners that can be used in single rooms or smaller spaces but are fixed in place instead of being portable. To find resources on portable room air conditioners, visit our links page. To help understand central air conditioning systems, we will first sub-divide them by how they function.

How Air Conditioners Work (watch video)
The majority of home and smaller commercial air conditioning systems circulate a compressed gas refrigerant in a closed “split” system to cool and condition inside air. The refrigerant has to be re-cooled and condensed, and outside air is the medium most often used to accomplish this. The term “split” simply means that components are divided into inside and outside portions as opposed to being located together in a “package” unit.

The refrigerants, widely recognized by the trademark “freon” (which is a registered trademark of the DuPont company for refrigerants), helps cool and dehumidify the inside air. In a “forced air” system, an internal blower circulates the conditioned air through ducts to the rooms where the cooler air is needed. The air ducts generally run either below the ceiling and inside the rooms (conditioned air) or in the attic (unconditioned air). An outside fan pulls air across the external parts of the system to cool and condense the refrigerant.

The major parts and functions in a split air conditioning system
Compressor – outdoors: The electric pump, or heart of the system, that circulates the refrigerant in a closed loop between the condenser and evaporator coils. Compressors come in more than one variety. According to Consumer Reports™ “A reciprocating compressor is more trouble-prone than a scroll-type one, they say. While pricier, scroll-type compressors do tend to be higher in efficiency and quieter than reciprocating compressors. Most manufacturers offer both types of compressor.”

Condenser coil – outdoors: A network of tubes filled with refrigerant that remove heat from the heated gas refrigerant and convert the refrigerant into a liquid form again. The excess heat escapes into the outside air.

Fan – outdoors: Pulls air through the condenser coil for heat dispersal.

Evaporator coil – indoors: A network of tubes filled with refrigerant that remove heat and moisture from the air as the refrigerant evaporates into a gas again.

Air handling unit – indoors: the blower and related portion of the central air conditioning system that moves air through the air ducts.

Air filters – indoors: Air filter elements trap dust, pollen, and other airborne particles as air moves through the air conditioning system. Air filters contribute to both reliable air conditioner operation and health, so we dedicated a page to them.

Drainage system and pan – During the normal condensation process, an air conditioner produces a significant amount of water as a by-product. In a central A/C system, there is a primary system of pipes, often made of PVC, that carry this condensate water to the outside of the building. This piping needs periodic flushing to prevent it from getting stopped up with the algae and similar growth. At a minimum, this maintenance should be done by your service company during your annual system tune-up. Your inside A/C system should have an emergency drain pan in case the primary drain lines stop up. This pan usually comes equipped with an automatic cut-off switch that turns your air conditioning system off when then pan fills up with water. Otherwise, water will run out of the pan onto you ceiling or whatever is located below it. The need to flush the drain lines is a prime example of how a little preventive maintenance can prevent a major repair.

Air conditioning with a heat pump, often shortened to “heat pump” is a combination central air conditioning and heating system. In one mode, it functions as an air conditioner. In the reverse mode, it becomes a heater. Due to their unique design and special considerations, we have given heat pumps their own page on this website.

Today’s air conditioning systems are more efficient and cost more upfront
Today’s central air conditioning systems are much more efficient than their predecessors. The industry uses a rating called SEER for central systems, which is an acronym for Seasonal Energy Efficiency Ratio. Essentially, a higher SEER rating means the air conditioner uses energy more efficiently. When other factors such as thermostat settings are kept equal, a higher SEER results in lower monthly utility bills for the owner or occupant. A central air conditioning unit rated at 13 SEER uses almost a third less electricity than a 10 SEER system.

Some outside A/C units are rated at a range, such as 14/15 SEER, depending on what type of indoor equipment they are paired with. For example, if paired with a manufacturer recommended evaporator coil and a variable speed furnace or variable speed air handler, an outside unit could be rated as a 15 SEER system. Otherwise, the rating would be 14 SEER.

As you might imagine, higher efficiency A/C equipment costs more to build. Why? For one reason, more efficient condensers and evaporators contain more metal in their extra coils. Additionally, to gain higher efficiency, the systems may have more complex technology such as motor speeds and electronics.

If you are buying a new air conditioner, make sure you clearly understand the relationship between higher upfront costs and lower monthly utility bills of the more efficient equipment. The Energy Guide label clearly displays the SEER rating of all new A/C equipment.

Air conditioner maintenance(watch video)
A consensus of our research and interviews clearly encourages maintenance to prevent air conditioning repairs. Why? Yearly maintenance costs are mostly labor. If you wait until your air conditioner breaks, you will likely have to pay for labor and parts. Additionally, most A/C failures happen at the same time for everyone–during the first hot weather or times of the most intense use, such as the hottest day of the year. Since the A/C repair companies are more likely to be overloaded with work then, you will be more likely to pay for an after hours repair or an emergency trip charge on those days.

Small adjustments to your HVAC system can mean big savings in your bills. For example, an air conditioner that runs only one pound low on refrigerant can add 15 percent on your summer cooling bills. Since many A/C units are erroneously installed oversized, you might not notice a slight deficiency in cooling capacity, but you would probably would pay attention to a 15% reduction in your highest electric bills.

In addition to annual maintenance from your air conditioning contractor, be sure to change all consumer serviceable air filters in your A/C system according to the manufacturer’s directions. Also have your air ducts checked periodically for leaks. Make sure your ductwork is correctly insulated, especially if it runs through unconditioned space.

If you skimp on regular maintenance and adjustment of your air conditioning system, you will likely pay more each month on your utility bills due to wasted electricity. To top all that off, if you neglect the maintenance on water removal lines and pans, they will sometimes clog up, overflow, and cause water damage to your ceilings, floors and belongings.

Air conditioner repairs
As an air conditioning system ages, it naturally requires more repairs due to ordinary wear. Other than age, repairs usually stem from inadequate maintenance and can be divided into one of the following categories: refrigerant leaks, control or electrical component failure, drainage problems, and major component failure.

The following list shows some specific symptoms that lead to air conditioner service calls

  • Air comes out the vents, but will not cool down to the thermostat setting
  • No air is blowing out of the vents and your indoor thermometer reading is higher than the thermostat setting
  • Unit will not run and ice is visible on the coils.
  • It is cool in one part of house or building, but not the other.
  • Water leaks from drain line or pan onto floor, ceiling, etc.

If your system shows one of more of these symptoms, turn the air conditioning setting to “Off” at the thermostat. Call a your contractor right away for service and interim advice. Turning the system off could be especially important if parts of the A/C unit are frozen up or making unusual noises or odors.

When an A/C repair company visits your location, they will usually have charges in some or all the following areas: minimum trip charge, mileage costs, parts/supplies, hourly labor, and taxes. The repair company should be able to quote standard items such as a minimum trip charge and possibly a price per pound of refrigerant, etc. However, it is not realistic to expect them to diagnose the problem or estimate a total cost over the telephone. Air conditioning contractors will often give on-site price quotes for new systems for no charge, but trips for a repair almost always involve a minimum cost.

It pays in many ways to plan ahead. You can interview several air conditioning contractors at the time you choose one to do your maintenance. It’s much easier to have planned maintenance done yearly at your convenience than to call for repairs at random times of extreme temperature.

Due to rising electricity costs, a well-maintained air conditioning system can easily outlive its economic life. If your system is more than about ten years old you should compare the costs and benefits of buying a new system before pumping a lot of repair money into an old one. If you are on the fence” regarding repair versus replacing your air conditioning system, you will want to talk about this fact while setting the appointment with your local HVAC company. Internal Link to Repair vs. replacement.

Size matters a great deal when purchasing a new air conditioning system.
Contrary to the assumption often made, bigger is not better—when it comes to the sizing air conditioners, it’s optimum for your situation that you want to achieve. The cooling capacity of an air conditioner is measured in British thermal units per hour (Btu/hr.) or in “tons”. One ton of cooling equals 12,000 Btu/hr. If you get a system with too low a tonnage rating, the system will work too hard and use too much electricity attempting to cool the air. Conversely, if you get a system with too much tonnage, the system will cycle on and off too quickly to “condition” the air properly. This results in “clammy” uncomfortable feeling cold rooms plus extra wear on certain parts of the system.

Air conditioning contractors use a complex process and formula to calculate the size of equipment and design each system. Three of the major factors that affect A/C load calculations are location and climate, amount of area within in the structure to be cooled, and relationship of the structure’s “envelope” to the outside air.

The cooling and heating needs of modern home can be challenging. For example, a high capacity kitchen vent hood can remove so much air from a home that it adds substantially to the cooling or heating load. Homeowners can mitigate this somewhat by minimizing use of the hood on its higher settings during the hottest or coldest months.

Within the U.S., demand for cooling and on air conditioning equipment varies greatly with location. At the extremes, the southernmost zone of the U.S. has approximately 4.8 times as many cooling hours as the northern, or lowest demand zone. Factors that affect the envelope include the R Value of insulation in attic, walls, and (if applicable) under the floors, proper ventilation in the attic, whether or not radiant barriers are present, the type of windows and doors, weather stripping, caulking, and more.

Ductless and windowless air conditioners
Some situations call for air conditioners that do not require ductwork. In the online search related portion of our research we found a substantial interest in basic information on this topic.

Examples of suitable applications include one-room additions, offices, or garage apartments, and in a commercial setting, motel rooms. Although the equipment costs more than window air conditioners and needs professional installation, ductless, windowless air conditioners offer some distinct advantages over window units. Because of their design, ductless windowless air conditioners:

  • Allow a better view from and more light to a room
  • Preserve the option to open a window for ventilation
  • Allow a window to be cleaned on both sides much easier
  • Don’t block emergency escape from the window

To fill these needs, ductless air conditioners can be found in many configurations, including split, mini-split, slender, heat pump, and others. The features of these various systems determine where the equipment has to be located, what equipment is inside the room or outside the wall, what equipment has to penetrate the wall, and more.

Except for the absence of ductwork, the main components of these air conditioners work very much like the other units we described. Because of the variables and technical nature of the equipment, it needs professional installation. The same contractors and companies that sell the ducted variety usually stock or can order ductless equipment.

Some of the manufacturers of ductless fixed air conditioners include Mitsubishi, Sanyo, Fujitsu, and Carrier, and Panasonic, Friedrich, and Goodman. Although some of these companies also make portable room or window air conditioners, the portable and non-portable (fixed) models should not be confused.

Alternative Types Of Air Conditioning

  • Water Source Cooling (evaporative tower cooling to assist a conventional. system with a fan)
  • Water Source Cooling (from/to a body of water) use circulating cool water as a chiller to remove heat from the high temperature gas in the compressor/condenser unit.
  • Gas-Fired Air Conditioning