Vapor-compression refrigeration (compression cooling) systems are the most common type of cooling equipment used to cool residential and commercial buildings. Compression cooling is often referred to as air conditioning, although technically any system used to intentionally heat, cool, or ventilate the indoor air could be referred to as an air conditioning system. Residential compression cooling systems include any system that uses the refrigeration cycle for space cooling; this includes split-system air conditioners, unitary air conditioners, air-source heat pumps, and ground-source (or geothermal) heat pumps, in system sizes up to 65,000 Btuh with forced-air distribution systems. Noncompression cooling systems include evaporative cooling and absorption systems. However, the lion's share of the U.S. housing market utilizes a compression cooling system for space cooling. According to data from the U.S. Energy Information Administration (data collected in 2009 and released in 2013), 94 million out of 113.6 million households had some kind of cooling equipment – 69.7 million of those homes had central compression-based air conditioners (19% of these were heat pumps), 25.9 million had window or wall-unit air conditioners, and 2.8 million had an evaporative or swamp cooler (EIA 2013). Other cooling options available to consumers that are worth considering because of their energy-saving potential include passive cooling techniques such as shading with architectural and landscape features, night ventilation cooling systems, and ceiling fans. See the Passive and Low-Energy Cooling guide.

The compression cooling cycle, especially as it applies to split-system central air conditioners, is described here. Traditional split heat pumps, mini-split heat pumps, and ground-source heat pumps are described in more detail in other guides, as are evaporative cooling systems and absorption cooling.

Central air conditioners are typically installed with central furnaces and use the same blower and duct distribution system. Residential air conditioners are typically split systems, which refers to the fact that there is an outside unit and an inside unit: the condenser and compressor are part of an outside unit, and the evaporator and expansion valve are located within the air handler in the inside unit (Figure 1). Refrigerant is piped to the evaporator coil in the air handler unit where it cools the distribution air.

Window- or wall-mounted room air conditioners contain all of the components in one box. Larger single-unit air conditioners called packaged-unit air conditioners also contain all of the components (the compressor, condenser, evaporator, expansion device, and blower fan) in one unit that is located outside, typically mounted on the wall or on the roof (Figure 2). The conditioned air is vented inside either directly into a room or into a duct system for distribution throughout the building. Many small commercial buildings are equipped with packaged units but they are rarely used for single-family homes.

Packaged air conditioners and heat pumps come from the factory ready to go as soon as they are connected to a duct system, thermostat, and power source. Split systems have to be plumbed, i.e., the indoor unit must be connected to the outdoor unit via refrigerant piping, and electrical wiring must be connected to both units.

Air Conditioner Capacity and Sizing

Air conditioners are sized by their capacity in terms of tons. One ton equals 12,000 Btu/hour of cooling capacity. The capacity is often indicated in the model number. Look at the name plate on the outdoor condensing unit and locate the model number (not the serial number). Look for two digits in the model number that match the numbers below to indicate tons or Btus/hour. For example, a model SSX160241 is a 2-ton (24,000 Btu/hr) air conditioner.

18 = 1.5 Ton (18,000 Btu/hr)
24 = 2 Ton (24,000 Btu/hr)
30 = 2.5 Ton (30,000 Btu/hr)
36 = 3 Ton (36,000 Btu/hr)
42 = 3.5 Ton (42,000 Btu/hr)
48 = 4 Ton (48,000 Btu/hr)
60 = 5 Ton (60,000 Btu/hr)

Proper sizing of air conditioners has become more important in recent decades as homes have become more air-tight and better insulated. HVAC contractors can no longer rely on rules of thumb based on a rough estimate of square footage. Where an older two-story 3,000-ft2 home might have required two 3-ton units, a new 3,000-ft2 high-performance home might be adequately served by one 3-ton unit with zone dampers.

An oversized system will turn on and bring the air temperature down below the thermostat set point quickly then shut off before the system has had time to remove moisture from the air, which can cause high humidity problems in the home, especially in humid climates. The HVAC contractor should use ACCA’s Manual J: Residential Load Calculation to calculate the home’s cooling load and ACCA’s Manual S: Residential Equipment Selection to correctly size the central air conditioning system. ACCA makes available Excel-based spreadsheets to help contractors with these calculations. 

The HVAC equipment that is installed must be a matched system, as certified according to the Air-Conditioning, Heating, & Refrigeration Institute (AHRI). AHRI is an industry association that assigns a certification number and efficiency ratings to specific combinations of equipment (outdoor unit, indoor unit, indoor coil, fan type, etc.), which have been tested by the manufacturer according to AHRI test procedures using AHRI-specified test conditions (AHRI 2012). See the AHRI Directory of Certified Products. Proper matching of system components according to AHRI is important for equipment warranty and one of the items that will be confirmed by a rater if the home gets an energy assessment such as a Home ENERGY Rating System (HERS) score.

Many designers use the performance data listed on the AHRI certificate for selecting equipment to meet the home’s design cooling load. However, a more accurate method (and one required by ACCA Manual S) is to use the original equipment manufacturer (OEM's) expanded performance table to obtain performance data at design conditions. AHRI uses a specific set of conditions (95°F outdoor, 80°F indoor, and 67°F wet bulb) when determining the equipment performance data, such as heating and cooling capacity and SEER2 and EER2 cooling efficiencies; these performance data are then listed on the AHRI certificate. The OEM expanded performance tables list data for many more cooling conditions. An example expanded performance table is shown below for a 1.5-ton air conditioner. Linear interpolation can be used to find the performance data for conditions in between two data points.

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Example expanded performance table from a 1.5-ton air conditioner

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With a central air conditioning system, the cooled air will be distributed by ducts so it is important to design an efficient air distribution system with a compact layout in accord with ACCA Manual D. Good installation (with short straight runs and air-sealed and insulated ducts) is important for maximum airflow and efficiency. Consider the the ENERGY STAR Single-Family New Homes Checklist for prescriptive requirements for duct installation quality targets. For best performance, the ducts and air handler should be located within the home’s thermal boundary (this is a DOE Zero Energy Ready Home requirement).

Some new homes are so well air sealed and insulated, they can be considered low-load homes (over 1,000 square feet of floor space per each ton of cooling). While older, less air-tight, less well air sealed homes might require two or more cooling units or one large unit, for example a 5-ton unit, with well-insulated homes, one smaller unit, perhaps 2 or 2.5 tons, might do.

For larger homes with one unit, zone dampers are recommended. The dampers are located near the air handler unit at the base of each branch duct that will serve a zone. The dampers communicate electronically with a computer that communicates with thermostats located in each zone. Dampers are a good idea for several reasons. They save energy because different temperatures can be set for different zones, and cooling can be reduced to less-used areas of the home. They also help provide more airflow where needed. Systems with intelligent blower fans can also slow the fan down when one or more zone dampers are closed.

The HVAC system sizing should be based on the heating or the cooling system, whichever is more in demand in your climate zone. Both the DOE Zero Energy Home Program and ENERGY STAR will allow designers to oversize furnaces by up to 150% to satisfy the airflow requirements of the cooling system. Cooling systems should not be oversized.

The Refrigerant Cycle

The vapor-compression refrigeration system uses a circulating liquid refrigerant as the medium that absorbs heat from the indoor air and rejects the heat outside. Figure 4 shows the path of the refrigerant as it cycles through a typical, single-stage vapor-compression air conditioner’s indoor and outdoor components. This figure depicts an air conditioner only. If this unit were part of a furnace, the system would have the blower pulling air from the return and blowing the air across the furnace’s gas heat exchanger then the air conditioner’s evaporator coil.

All compression cooling systems have four main components: a compressor, a condenser, a metering device (known as a thermal expansion valve (TXV), fixed orifice, or electronic expansion valve (EEV)), and an evaporator coil. Circulating refrigerant moves through the suction line and enters the compressor as a low pressure vapor. In the compressor, it is compressed to a higher pressure and therefore a higher temperature. The now “superheated” vapor is routed through the condenser coil where it is cooled by flowing outdoor air. As a result, it condenses back into a liquid, releasing heat which is carried away by the flowing air.
The liquid refrigerant is carried back to the indoor unit where it passes through an expansion valve. The expansion valve (or metering device) causes the liquid refrigerant to experience an abrupt drop in pressure and temperature as it enters the evaporator coil. In the coil, the liquid absorbs heat from the circulating house air, thus cooling the house air that is passing through the air handler. The absorbed heat causes the refrigerant to turn to a low-pressure vapor again and the vapor is again routed outside to the compressor, beginning the cycle all over again.  

Measuring the Efficiency of Cooling Systems

The efficiency of compression cooling systems is measured in SEER2, EER2, and COP. 

Seasonal Energy Efficiency Ratio (SEER2) – the SEER rating of a unit is the cooling output during a typical cooling-season divided by the total electric energy input during the same period. The higher the unit's SEER2 rating the more energy efficient it is. In the United States, the SEER2 is the ratio of cooling in British thermal units (BTU) per hour to the energy consumed in watt-hours.

Energy Efficiency Ratio (EER2) – the EER2 of a particular cooling device is the ratio of output cooling (in Btu/h) to input electrical power (in watts) at a given operating point.

Coefficient of Performance (COP) - the COP (sometimes referred to as CP) of a heat pump is the ratio of the heating or cooling provided over the electrical energy consumed. The COP provides a measure of performance for heat pumps that is analogous to thermal efficiency for power cycles.

Technology improvements in recent years have made air conditioners much more efficient. These improvements include variable-speed fan motors, variable-speed compressors, electronic expansion valves, and micro-channel heat exchangers. These changes enable the air conditioner to vary the amount of refrigerant flow and thus modulate up or down rather than just turning on or off like a single-speed system would. By better matching the system’s cooling capacity to the cooling load, the newer models can improve efficiency, lower energy consumption, and increase comfort.

Since 2023, the federal government has required new air conditioners sold in the United States to have a SEER2 rating of 13.4 or higher when installed in the northern region. In the southeast and southwest regions, air conditioners must have a SEER2 rating of 14.3 for systems smaller than 45k BTUH and 13.8 for systems larger than or equal to 45k BTUH (SEER2 Guide). 

To receive an ENERGY STAR label under Version 6.0, a split system air conditioner must have a SEER2 of 15.3 or greater. The best available central air conditioning units can have SEER ratings of well over 20.

Installation Concerns

How the installation and connection of the copper tubing for the refrigerant lines is performed is critical to the life expectancy of the compressor. During new construction, the copper tubing is roughed in early on, before or during duct installation. The equipment (indoor unit and outdoor unit) is typically installed and connected to the copper tubing toward the end of construction. This means the copper lines can lay unconnected for quite some time.

Copper tubing used for refrigerant lines is sold in rolls (15, 25, 30, and 50-foot lengths are common). The tubing is dried out (dehydrated) and sealed at both ends before shipping. Water is the enemy of the refrigeration system. If water vapor is allowed to enter the refrigeration lines during construction, it will greatly reduce the life of the compressor and create havoc with metering devices and check valves. The oil used for lubricant in refrigeration systems is highly hygroscopic, which means that the oil wants to absorb moisture. If the lubricant mixes with water or water vapor, it creates an acidic sludge that eats away at compressor windings, causing burnouts; the sludge can also block orifices and valve openings inside the system.

During the rough-in installation, the open ends of the copper tubing should be kept sealed at all times. When the installation is completed, the lines should be charged with dry nitrogen and soldered closed. After connecting the indoor unit and the outdoor unit, the lines should be vacuumed to 500 microns to remove moisture and non-condensable gases, which can reduce heat transfer and cause erratic operation (Figure 5).

Proper refrigerant charging is critical for maximizing the performance of compression cooling equipment. Too much or too little refrigerant can reduce the efficiency of the equipment and lead to premature component failures. Use the charging method recommended by the manufacturer. There are three methods for refrigerant charging: the subcooling method (typically for units with a thermal expansion valve), the superheat method (typically for units with a fixed orifice), or the weigh-in method (using the refrigerant weight amount listed on the data plate on the outdoor unit). Verify that you are using the correct method for the specific air conditioning model to be installed. Refrigerant charging must be done by an EPA-certified technician.

How to Select and Install Compression Cooling Equipment

1. Choose the highest performing air conditioner that project costs will allow, in order to meet the design cooling load of the project. Consider variable-speed equipment that can modulate its output down to achieve comfort and energy efficiency during mild days that only require only a fraction of the cooling capacity of the design temperature condition. Variable-speed equipment is especially important if the design load is low (e.g., <14,000 Btu). In dry climates, consider direct/indirect evaporative coolers and ventilation or passive cooling. If you are participating in an energy-efficiency program, select cooling equipment that complies with the requirements for your climate zone, as described in the Compliance tab.
2. Confirm that the selected system is a matched system, as certified according to the Air-Conditioning, Heating, & Refrigeration Institute (AHRI). 
3. Properly size the cooling equipment for the design cooling load of the home. Use ACCA Manual J to calculate your cooling load and use ACCA Manual S to correctly size your system. This is especially important if you have done significant air sealing and insulating, which will reduce your heating and cooling load.
4. Design an efficient air distribution system with a compact layout in accord with ACCA Manual D. Install ducts properly for maximum airflow and efficiency in accord with ACCA Manual D. See also the Building America Solution Center guides on duct installation, insulation, and air sealing in the ENERGY STAR Single-Family New Homes Checklist.
5. Charge the copper tubing with dry nitrogen, seal the open ends with solder, and keep the tubing sealed at all times during the rough-in installation to prevent moisture from entering the lines.
6. Install in accordance with the manufacturers’ instructions and relevant standards including ACCA Standard 5: HVAC Quality Installation Specification, ACCA’s Technician's Guide for Quality Installations and ACCA Standard 9: HVAC Quality Installation Verification Protocols. Consider using tools such as the Quality Install Tool to document installation. 
7. After connecting the indoor unit and the outdoor unit, perform a temperature-compensated nitrogen pressure test. Check for leaks using a non-corrosive bubble solution. After that test, vacuum the lines to below 500 microns to remove moisture and non-condensables. Isolate the system from the vacuum pump and perform a vacuum decay test. The vacuum should not rise above 500 microns.
8. Follow the manufacturer’s recommendations for refrigerant charging. Verify that you are using the correct charging method for the specific air conditioning model to be installed. Refrigerant charging must be done by an EPA-certified technician.
9. Make sure the drain pans and condensate lines are correctly installed. If the condensate lines are running through an unconditioned attic, they must be insulated.
10. Check and set the installer settings in the thermostat or control board for the design conditions. In humid climates certain fan and compressor operation settings can be employed to prevent moisture on the evaporator coil from evaporating back into the air stream and contributing to indoor humidity.

11. Measure the total external static pressure to ensure it is below the limits of the equipment. Measure the air flow and adjust the fan speed settings as needed to achieve the target air flow. Measure duct leakage to ensure it is below the maximum allowed.

     

Multi-Family Homes

For multi-family homes, the equipment selection guidance depends on the situation.

For row homes and townhouses, consider a central air conditioner that is inverter driven. These units are quieter than single-stage equipment. Consider a heat pump instead of just an air conditioner since the cost difference is very small. For tight lot lines and installations where the outdoor unit will be placed on a balcony, consider a side-discharge unit as opposed to a top-discharge unit. The side-discharge units are slimmer and thus easier to install into tight spaces.

For multi-family units without access to an attic or crawlspace, and/or where a ducted distribution design is desired, consider a medium-static pressure, concealed, ducted ceiling cassette indoor unit. These units can be installed into a hallway ceiling and they use only 1 ft or less of ceiling height. The air can be ducted to individual rooms via a supply register installed above the door to interior rooms. Ducted systems also have a maintenance advantage: a filter slot or grille can be included in the duct design to accept disposable pleated air filters which can be high MERV filters. High-MERV air filters do a good job keeping the ducts, the equipment, and the indoor air very clean.

For condominiums and apartments with single-hung or double-hung windows, consider using U-shaped inverter window air conditioners or saddle bag-shaped packaged window heat pumps. These shaped air conditioners and heat pumps allow the window to be almost fully closed, reducing the air leakage around the equipment. The inverter electronics allow the equipment to use significantly less energy on start-up and during mild temperature days which is important since these units will often be plugged into shared circuits.