Central Air Conditioning: Understand How It Works
Central air conditioning is one of the most convenient and energy-efficient ways to get relief from the hot, humid summer weather throughout Connecticut. Whether you’re planning to install an A/C system for the first time or looking to upgrade your current system, it’s important to understand your HVAC options.
What Is Central Air Conditioning Really?
The “central” in central air conditioning comes from the fact that the system conditions the entire building from one large central location in your home. This system cools and dehumidifies the air, then blows the cool air into your duct system and out through the supply air registers located in each room.
In stark contrast, window A/C units cool only one room or area. Trying to cool an entire home with a window unit in each room would require not only the inconvenience of managing each separate unit, but it would also require more energy and be excessively noisy.
Central air conditioning systems are typically combined with the furnace system, if your home has one (or use a dedicated air handler if it doesn't), and both use the same electrical system and duct-work.
A central AC’s main components are:
- Fans that pull air through the system
- Evaporator coils that change refrigerant from liquid to gas
- A compressor that pressurizes refrigerant gas
- Condenser coils that turn refrigerant gas into a liquid
- Refrigerant lines that carry refrigerant between the coils
- An expansion device that regulates refrigerant flowing to the evaporator
How Does an Air Conditioner Work?
When a liquid turns into a gas or “evaporates” it absorbs heat. This is why pouring water over your skin makes you feel cooler in hot weather. Air conditioners use a chemical compound known as refrigerant that turns from liquid to gas at low temperatures. Refrigerant moves through the air conditioning system, absorbing heat from your house and transferring it outdoors in a continuous cycle.
- Cold refrigerant is pumped through the evaporator coil. A fan blows air over the coil, and the refrigerant in the coil absorbs heat from the air. The now-cool air is blown into your ducts to be distributed through your home. As the refrigerant absorbs heat, it evaporates into a low-pressure gas.
- Hot, low-pressure refrigerant gas moves to the compressor, which increases the pressure and temperature of the gas.
- Hot, high-pressure refrigerant gas moves to the condenser. The gas releases heat and, as it does so, it condenses back into a liquid.
- The liquid refrigerant flows to the expansion valve, which regulates how much refrigerant gets through to the evaporator. From here, the refrigerant flows to the evaporator to start the cooling cycle over again.
Split or Packaged: Which is right for Your Home?
Central air conditioning systems come in two basic designs: split and packaged. Split systems are the most commonly used type, and are split between an outdoor and an indoor unit, as the name suggests. In a packaged/ductless system, all the components are housed in one unit that is installed outside your home. Split and packaged systems both contain the same components and can provide the same amount of cooling, but there are some differences.
Split Air Conditioner
The two parts of a split system are:
- Indoor unit – A metal cabinet that contains the evaporator coil. This is installed inside your home, usually in a utility closet or the attic. It’s usually the same cabinet that houses your furnace. If your home doesn’t have a furnace, the evaporator coil works as part of an air handler.
- Outdoor unit – A metal cabinet that contains the condenser, compressor and expansion valve. This is located outside your home, usually on a concrete slab.
These two units are connected by refrigerant lines that run underground and conduct refrigerant back and forth between the units.
Packaged Air Conditioner
Packaged air conditioners are similar to window air conditioners since all the components are housed in one single unit. Unlike window A/Cs, however, packaged air conditioners are installed outside your home, usually on the roof, and can cool the whole building. Some even contain a heating system, such as a furnace, eliminating the need for a separate furnace indoors.
Two types of condensers are used in packaged systems:
- Air-cooled – These condensers are cooled by the surrounding air. These are more common for residential use.
- Water-cooled – These condensers require a cooling tower, so they’re used primarily for large commercial applications where water is plentiful.
If you’re not sure whether a split or packaged system will meet your needs better, consider these points:
Energy efficiency – In general, split systems are more energy efficient than packaged systems. Split systems are available with Seasonal Energy Efficiency Ratio (SEER) numbers of up to 24, whereas packaged systems don’t often achieve SEER numbers above 18. SEER 18 is still considered high efficiency.
Labor costs – Installation costs for a split system are typically higher than for a packaged system. That’s because installing a split system involves working both outside and inside the house, as well as connecting the two units. Installing a packaged system requires only outdoor work with no need to connect different pieces of equipment. Split systems also need to be charged with refrigerant whereas packaged systems may come pre-charged.
Space requirements – If the open areas (attic and/or basement) in your home are limited to conveniently accommodate an A/C indoor unit, a packaged system (mini split) may be a better option. These systems are well suited to homes without attics or basements.
Ease of maintenance – Because a packaged system has all of its components in one unit, maintenance is somewhat more convenient than for split systems.
Energy Efficiency: Know What Factors Matter
With heating and cooling accounting for around 40 percent of the average household’s energy bills, it’s well worth your time to look for an energy-efficient central air conditioning system. To do that, the first thing you’ll want to understand is the Seasonal Energy Efficiency Ratio (SEER).
SEER is a ratio of a system’s total cooling capacity throughout the cooling season over the amount of electricity it uses in that time. The higher the system’s SEER, the less electricity it requires to cool your home. SEER 13 is the minimum allowed by the U.S. Department of Energy.
Energy Efficiency Ratio (EER) is another number used to indicate an air conditioner’s efficiency. This number is less accurate than SEER because it doesn't account for fluctuations in temperature over the season.
Looking for an Energy Star-qualified system can also help you find an efficient system. These systems are 15 percent more efficient than non-Energy Star-qualified models. To gain Energy Star status, a split-system A/C must have a SEER of at least 14.5 and a packaged system must be at least 14 SEER.
For a split air conditioner, a properly matched system is another important factor. This is a consideration if you’re planning to upgrade either your indoor or outdoor unit, but not both. With incorrectly matched units, your system will be less efficient and wear out faster. It may not work at all if one system is much older than the other.
Your technician can assure you that the units are matched by providing you with an AHRI Certificate of Product Ratings or an AHRI Certified Reference Number. Buying a complete system, with indoor and outdoor units designed to work together, will also help you avoid mismatching.
Features to Consider for Better Performance
When you’re shopping around for a central air conditioning system, there are a few features you’ll want to look for to find a high-efficiency, high-performance model.
Scroll compressor – These compressors use two spiral-shaped scrolls rather than a piston and cylinder like the conventional reciprocating compressor. Scroll compressors are more efficient, quieter, and last longer.
Two-stage compressor – A conventional single-stage compressor runs at the same capacity no matter what your cooling demand is. A two-stage compressor can operate at high and low capacity. For low and average cooling demands, it runs on low to save energy. For high demand on hot days, it will kick into high gear.
Variable speed fan – Unlike one- and two-speed blower motor fans, variable speed fans can run at any speed within a wide range. This allows them to select the exact speed that’s most efficient for any given cooling or heating demand. These fans also provide more even temperatures and minimize drafts.
Thermal expansion valve – Commonly called a TX valve, this device can regulate the flow of refrigerant to the evaporator coil more precisely than other types of valves, improving the system’s energy efficiency.
Programmable thermostat – With a programmable thermostat, you can set your air conditioner and furnace to automatically run at previously selected energy saving temperatures when you’re away from home or in bed. You stand to save more energy because there’s no risk of forgetting to re-set the thermostat as needed.
Zoning system – A zoned heating and cooling system uses programmable thermostats and dampers in the ducts to allow different rooms or groups of rooms (zones) to be heated and cooled separately from one another. This way you won’t waste energy cooling or heating unused rooms.
Quick Reference Guide for 2009 IECC Residential Energy Efficiency
Let’s say you’d like to have air conditioning but your home or new room lacks the necessary duct-work. Or maybe you have a room that’s a little too hot or too cold and you’d like to make it more comfortable. Perhaps you own a business that has supplemental heating or cooling needs. Regardless of your residential or commercial challenge, ductless systems potentially may offer the elegant solution you’re looking for.
**As a side note, if the weather conditions do not warrant operation (65 degrees or greater ambient temperature), the central air conditioning cannot be tested.
Most manufacturers have this or a similar statement in their operating instructions
**Do Not Operate Below 55°F/12.78°C.
Your outdoor unit is not designed to operate when outdoor temperatures are lower than 55 degrees F/12.78 degrees C without modification. “When the temperature is below 55 degrees the gases in the air conditioning system can turn to liquid. Turning on the Air Conditioner, when the gas is a liquid, can cause the liquid to slug the compressor.”
(Think of throwing a bird into a jet engine)
What you should know about Refrigerants when purchasing or repairing a Residential A/C System
http://www.epa.gov/ozone/title6/phaseout/22phaseout.html
When you’re shopping or are in need of repairs for a central air conditioning system, there is some important information that needs to be known with regards to the types of refrigerants being used.
Background of R-22 – HCFC-22 (also known as R-22) has been the refrigerant of choice for residential heat pump and air-conditioning systems for more than four decades.
Ozone – Depleting Properties – Unfortunately for the environment, releases of R-22, such as those from leaks, contributes to ozone depletion. In addition, R-22 is a greenhouse gas and the manufacture of R-22 results in a by-product (HFC-23) that contributes significantly to global warming. As the manufacture of R-22 is phased out over the coming years as part of the agreement to end production of HCFCs, manufacturers of residential air conditioning systems are offering equipment that uses ozone-friendly refrigerants. Many homeowners may be misinformed about how much longer R-22 will be available to service their central A/C systems and heat pumps.
R-22 Phase-out Schedule & Obligations – The U.S. agreed to meet certain obligations by specific dates that will affect the residential heat pump and air-conditioning industry;
- January 1, 2004: The Montreal Protocol required the U.S. to reduce its consumption of HCFCs by 35% below the U.S. baseline cap. As of January 1, 2003, EPA banned production and import of HCFC-141b, the most ozone-destructive HCFC. This action allowed the United States to meet its obligations under the Montreal Protocol. EPA was able to issue 100% of company baseline allowances for production and import of HCFC-22 and HCFC-142b.
- January 1, 2010: The Montreal Protocol requires the U.S. to reduce its consumption of HCFCs by 75% below the U.S. baseline.Allowance holders may only produce or import HCFC-22 to service existing equipment. Virgin R-22 may not be used in new equipment. As a result, heating, ventilation and air-conditioning (HVAC) system manufacturers may not produce new air conditioners and heat pumps containing R-22.
- January 1, 2015: The Montreal Protocol requires the U.S. to reduce its consumption of HCFCs by 90% below the U.S. baseline.
- January 1, 2020: The Montreal Protocol requires the U.S. to reduce its consumption of HCFCs by 99.5% below the U.S. baseline. Refrigerant that has been recovered and recycled/reclaimed will be allowed beyond 2020 to service existing systems, but chemical manufacturers will no longer be able to produce R-22 to service existing air conditioners and heat pumps.
Availability of R-22 – The Clean Air Act does not allow any refrigerant to be vented into the atmosphere during installation, service, or retirement of equipment. Therefore, R-22 must be recovered and recycled (for reuse in the same system), reclaimed (reprocessed to the same purity standard as new R-22), or destroyed. After 2020, the servicing of R-22-based systems will rely solely on recycled or reclaimed refrigerants. It is expected that reclamation and recycling will ensure that existing supplies of R-22 will last longer and be available to service a greater number of systems. As noted above, chemical manufacturers will no longer be able to produce, and companies will no longer be able to import, R-22 for use in new A/C equipment after 2010, but they can continue production and import of R-22 until 2020 for use in servicing existing equipment. Given this schedule, which was established in 1993, the transition away from R-22 to the use of ozone-friendly refrigerants should be smooth. For the next 10 years or more, R-22 should continue to be available for all systems that require R-22 for servicing.
Alternatives to R-22 in Residential Air Conditioning – The EPA has reviewed several alternatives to R-22 for household and light commercial air conditioning and has compiled a list of substitutes that EPA has determined are acceptable. One of these substitutes is R-410A, a blend of hydrofluorocarbons (HFCs) that does not contribute to depletion of the ozone layer, but, like R-22, contributes to global warming. R-410A is manufactured and sold under various trade names, including GENETRON AZ-20®, SUVA 410A®, Forane® 410A, and Puron®. An additional refrigerant on the list of acceptable substitutes for R-22 in residential air conditioners and other products is R-407C. Residential air conditioners and heat pumps using R-407C are not available in the U.S., but are commonly found in Europe. EPA will continue to review new non-ozone-depleting refrigerants as they are developed.
Servicing Existing Units – Existing units using R-22 can continue to be serviced with R-22. There is no EPA requirement to change or convert R-22 units for use with a non-ozone-depleting substitute refrigerant. Such changes, called "retrofits," are allowed if the alternative has been found acceptable for that type of use. R-407C is allowed for retrofits but R-410A is not allowed in retrofits due to its higher working pressures. In addition, the new substitute refrigerants would not work well without making some changes to system components. As a result, service technicians who repair leaks to the system will most often continue to charge R-22 into the system as part of that repair.
Installing New Units – The transition away from ozone-depleting R-22 to systems that rely on replacement refrigerants like R-410A has required redesign of heat pump and air conditioning systems. New systems incorporate compressors and other components specifically designed for use with specific replacement refrigerants. For instance, if a new outdoor unit (typically called a "condensing unit," containing the condenser and compressor) is installed, it is likely that a new indoor unit (typically called an "evaporator") will also be required. With these significant product and production process changes, testing and training must also change. Consumers should be aware that dealers of systems that use substitute refrigerants should be schooled in installation and service techniques required for use of that substitute refrigerant.
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