Cooling systems are normally replaced every 15 years; therefore, it is critical to purchase the most efficient and cost-saving new system. Selection of the most cost-effective system will depend on price of the equipment and electricity. Higher initial investments are often justified by reduced utility costs over the lifetime of the unit. The additional cost for a more efficient system must be compared to the energy savings. Annual cost of cooling is affected by cost of electricity, efficiency of the cooling system, cooling requirements of the home, and the lifestyle of the occupants.
Central cooling systems include traditional air conditioners and both air-and-water source heat pumps. Distribution of cooling is almost always by way of cooled air through ducts. Studies throughout the country indicate duct leakage significantly reduces delivered efficiency of heating and cooling systems.
Two types of compressors are commonly used in central air conditioners and heat pumps. For years, all compressors were of the reciprocating (to move forward and backward alternately) type. Improvements in manufacturing technologies have allowed the inexpensive manufacture of scroll compressors. A scroll compressor uses two interleaved axles to compressfluids. Often, one of the axles is fixed, while the other axle orbits, thereby compressing pockets of fluid between the scrolls. Scroll compressors are more rugged, tolerant of liquid refrigerant slugs, and quieter than typical reciprocating compressors. Most manufacturers now offer scroll compressors on mid to high performance central cooling equipment.
Air conditioners with two stages of capacity are available. These units operate at low capacity when the weather is mild and at full capacity when it is hot outside. This two-stage operation offers distinct advantages. The overall energy performance is high, and humidity control is excellent because the unit operates more hours.
Air-source heat pumps deliver cooling as efficiently as traditional central air conditioners. An air-source heat pump works like a standard air conditioner in the summer delivering cool air to the home, but is capable of reversing the flow of heat in the winter so it delivers heat to the home. During the summer, heat is gathered from the house and rejected to the outside air. During the winter, heat is gathered from outside air and rejected inside the home.
Ground-source heat pumps work in a fashion similar to air-source heat pumps except they transfer heat from or to the ground rather than the air. Two general approaches are used for coupling the heat pump to the ground. The oldest method pumps water from a well or other water source, and transfers heat from the water in the winter and to the water during the summer. The water is discharged back to the ground. The second and newer approach buries a long loop of plastic pipe, which circulates water and a glycol mixture, into the ground. The heat pump transfers heat to and from the water mixture.
Most central cooling equipment is charged with Freon® 22, a refrigerant used for air conditioning. However, production of Freon® 22 is scheduled for phase out in 2020. Some alternative refrigerants are available at a premium price, but there is no improvement in system performance as a result of the refrigerant. Decisions on the type of refrigerant used should be based on a relationship between the anticipated life of the equipment and the comparison of the refrigerant cost.
Some cooling systems provide cooling to only a portion of a home. These include window, through-the-wall, and ductless split-system air conditioners and heat pumps. Through-the-wall air conditioners and heat pumps are often called packaged terminal air conditioners. Unitary systems are frequently used where ductwork is not present, installation difficulties exist, or where cooling is desired in only a portion of the home.
Accurate sizing of cooling systems is important if the unit is to both cool and properly dehumidify. If the unit is oversized, it cannot control humidity properly. Over-sizing also increases the cost paid for the equipment; money spent on purchasing more efficient equipment.
Some national surveys have determined that well over half of all air conditioning contractors do not size heating and cooling systems correctly when it's time for a new replacement. The most common sizing mistake is in over-sizing. This not only makes the new system more expensive to install, but also forces it to operate inefficiently, break down more often, and cost more to operate. Oversized heating equipment also often creates uncomfortable and large temperature swings in the house. Oversized air conditioners (and heat pumps) do not run long enough to dehumidify the air, which results in the "clammy" feeling and unhealthy mold growth in many air-conditioned houses.
It is the installer/contractor's job to perform the correct sizing calculation for the building. However, many installers only check the "nameplate" (the label on the unit that has the Btu per hour output among other things) of the existing system and sell you one similar, or even worse, one that's larger. Before the era of tightly constructed homes, it was not uncommon to install furnaces and air conditioners that had two to four times the necessary capacity. Since many people have added new windows, caulking, weather-stripping, and insulation to their homes, going by the nameplate is likely to result in an oversized system. Other methods include simple "rules of thumb" based on the size of your home or using a chart that accounts for a variety of factors. While these methods might provide a first estimate, they should not be used to size your system. These are not correct sizing methods and are not in your best interests!
Many factors affect a home's heating or cooling requirement, or "load." A good estimator will measure walls, ceilings, floor space, and windows to determine the room volumes, and will assess the home's insulation, windows, and building materials. A close estimate of the home's air leakage is, also, necessary. A blower door test is the best way to measure air leakage. A good estimate will, also, include an inspection of the size, condition of seals on joints and insulation, and location of the distribution ducts in forced air systems. The placement of supply and return registers should be appropriate for the system type and size. The orientation of the house, also, affects heat gain and loss through windows. Overhangs can reduce solar gain through windows. Make sure the contractor uses the correct design for the outdoor temperature and humidity in your area; using a higher summer design temperature results in over-sizing air conditioners.
Homeowners should insist that contractors use a correct sizing calculation before signing a contract. This service is often offered at little or no cost to homeowners by gas and electric utilities, major heating equipment manufacturers, and conscientious heating and air conditioning contractors. Manual J, "Residential Load Calculation," published by the Air Conditioning Contractors of America (ACCA), is the recommended method for use in the United States. If ducts are part of the installation, they should be sized using the ACCA's Manual D, "Residential Duct Design." The ACCA also offers a comprehensive guide for choosing home heating and cooling systems, called Manual S, "Residential Equipment Selection."
There are also many user-friendly computer software packages or worksheets that can simplify the calculation procedure.
At the time you contact the contractor, you should be provided an estimated cost of his initial service call. The contractor will conduct all necessary tests and evaluations to estimate the cost of your new cooling system, or his or her repair work requirement. The contractor will prepare a cost sheet with the prices listed according to the respective part or work requirement. You will be provided the cost sheet, accompanied with a written contract, listing the terms and conditions, with a warranty on equipment and workmanship. The contractor will sign the first page, near the bottom. After you review and accept the contract, and the attachments, you will sign it. The contractor and you now have a binding contract.
