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Central Air Conditioning

5. Central Air Conditioners

Central air conditioners are designed to cool the entire house. The large compressor and outdoor coil are located outdoors and are connected by refrigerant lines to an indoor coil mounted in the furnace (Figure 6). The same duct system is used for both heating and cooling air distribution.

Figure 6 Installed central air conditioner

Installed central air conditioner

How does a central air conditioner work?

A central air conditioner uses energy to take heat away. The most common type uses a compressor cycle (like a refrigerator), illustrated in Figure7, to transfer heat from the house to the outdoors. Using a special fluid called a refrigerant, heat is absorbed and released when the refrigerant changes back and forth between a liquid and gas state. As it changes from liquid to gas, it absorbs heat; in changing back to a liquid from a gas, it releases heat.

The compressor cycle passes liquid refrigerant through an expansion device, changing the liquid to a low-pressure liquid/gas mixture. In the indoor coil or evaporator, the remaining liquid absorbs heat from household air and becomes a low-temperature gas.

Figure 7 The operation of a central air conditioner

The operation of a central air conditioner

The low-temperature gas is compressed by a compressor that reduces its volume and increases its temperature, causing it to become a high-pressure, high-temperature vapour. This vapour is sent to the outdoor coil or condenser where its heat is transferred to the outdoor air, causing the refrigerant to condense into a liquid. The liquid returns to the expansion device and the cycle is repeated.

Household air is cooled and dehumidified as it passes over the indoor coil. The moisture removed from the air, when it contacts the indoor coil, is collected in a pan at the bottom of the coil and sent to a house drain.

Energy efficiency considerations

Select a central air conditioner with as high a SEER as is practical within your budget. The annual cooling efficiency of a central air conditioner is affected by the manufacturer's choice of features and components. The SEER of central air conditioners ranges from a minimum of 10.0 to a maximum of about 17.0.


An ENERGY STAR® qualified central air conditioner must have a SEER rating of at least 12 or greater for a single-package unit and 13.0 or greater for a split system.

More efficient compressors, larger and more effective heat exchanger surfaces, improved refrigerant flow and other features are largely responsible for recent improvements in the efficiency of central air conditioners.

Figure 8 Efficiency of a central air conditioner

Efficiency of a central air conditioner  

Advanced reciprocating, scroll and variable-speed or two-speed compressors, when combined with the current best heat exchangers and controls, permit SEERs as high as 17.0 (Figure 8). Central air conditioners with the highest SEERs always use variable-speed or two-speed high-efficiency compressors.

Sound considerations

Select a central air conditioner with an outdoor sound rating of about 7.6 B or lower, if possible. The sound rating is expressed in bels. The lower the sound rating, the lower the sound power emitted by the outdoor unit. New, energy-efficient designs often have low sound ratings. The ratings are published by the Air-Conditioning and Refrigeration Institute (ARI), 4301 North Fairfax Drive, Arlington, Virginia, 22203 U.S.A.

Sizing considerations

Cooling loads should be determined by a qualified air-conditioning contractor, using a recognized sizing method such as that specified in CSA-F280-M90: Determining the Required Capacity of Residential Space Heating and Cooling Appliances. Do not rely on simple rules of thumb for sizing, but insist on a thorough analysis from the sales representative.

Select a central air conditioner size or capacity to just meet the design cooling-load calculated. Over sizing the unit will result in short operating cycles, which will not adequately remove humidity, resulting in an unpleasantly cold and damp home. Under sizing the unit will result in an inability to attain a comfortable temperature on the hottest days.

Also, with a central air conditioning system, the equipment cost is much more proportional to size than it is with heating equipment. Unnecessary over sizing will increase the purchase price and increase on-and-off cycling, which will decrease the unit's overall efficiency.

Installation considerations

When installing a central air conditioner, it is important that the contractor follow the manufacturer's instructions carefully.

The outside compressor for a central air conditioner should be shaded from direct sun.

The outside compressor for a central air conditioner should be shaded from direct sun.

The following general guidelines should be considered when installing a central air conditioner:

  • Locate the outdoor unit or condenser in a cool, shaded place where the waste heat can be readily rejected.
  • Locate the outdoor unit where its noise will not be a problem for you or your neighbour. This generally means away from bedroom windows or patios and not between houses.
  • In new construction, consider installing the central air conditioner outdoor unit on a frame mounted to the house. This avoids problems due to settlement of backfill around the foundation, which causes the outdoor unit to lose its level.
  • The central air conditioner will generally require more airflow than the furnace needs for heating. Consider a two-speed fan motor with the correct speed automatically selected depending on whether cooling or heating is called for.
  • Keep refrigerant lines as short as possible, and where the lines pass through the outside wall, ensure that the surrounding space between the lines and the wall is packed with a resilient material, such as plumber's putty. This will prevent noise or vibration problems and air leaks.

The cost of installing a central air conditioner will vary depending on the nature of the existing furnace, whether or not the existing ductwork needs to be modified, and whether there is a need to upgrade the electrical service to deal with the increased electrical load of the central air conditioner.

Where an existing central air conditioner is being replaced, ensure that the existing indoor coil is replaced by one matched to the new outdoor unit. If the existing indoor coil is not replaced, the new unit will not deliver its rated efficiency.

Operation considerations

In the interest of energy efficiency, use central air conditioning only when ventilation is inadequate to ensure comfortable conditions. Natural ventilation of the house at night, when it is relatively cool, combined with closing up the house during hot days and running the central air conditioner can be an effective strategy.

The indoor thermostat should be set somewhere in the range of 22–25°C, depending on your comfort requirements. A setting at the higher end of the range will result in lower air-conditioning costs. If the humidity level is lower, temperature settings can be at the higher end. Humidity levels can be reduced by using a bathroom exhaust fan when you bathe or shower and by using a rangehood fan, if it is vented outside, when cooking on the range top.

Continuous indoor fan operation can keep the temperature more uniform throughout the house by eliminating temperature differences due to stratification. It can also help keep the home cleaner, especially if there is an electronic air cleaner installed.

However, continuous indoor fan operation can increase operating costs compared with on-off or automatic fan operation. In more humid climates, the moisture removed during compressor operation is re-evaporated by the fan operation when the compressor is off. This can increase humidity levels and cause discomfort.

As in winter, adjusting the thermostat when the house is unoccupied can reduce operating costs. If the house will be empty during the day, you can raise the thermostat a few degrees before you leave and reset it to the preferred temperature when you return. An automatic programmable thermostat will reliably adjust the temperature for you to help you save money on cooling costs.

The power to the central unit should be shut off when the cooling season ends. Most central air conditioners have a small electric heater on the compressor to keep refrigerant out of the lubricating oil. Flip the circuit breaker to turn this heater off. To prevent damage to the compressor, remember to turn the power back on a day or two before you need to operate the central air conditioner.


Proper maintenance is critical in ensuring that your central air conditioner will operate efficiently and have a long service life. You can do some of the simple maintenance yourself, but you may also want to have a competent service contractor do a periodic inspection of your unit. The best time to service a central air conditioner is just prior to the cooling season.

Filter and coil maintenance can have a dramatic impact on system performance and service life. Dirty filters and dirty indoor and outdoor coils and fans reduce airflow through the system. This reduction in airflow decreases system efficiency and capacity and can lead to expensive compressor damage if left for an extended period of time.

Furnace filters should be inspected and cleaned or replaced, depending on the type of furnace and the furnace manufacturer's instructions. The outdoor coil should be vacuumed or brushed clean to keep it clear of dirt, leaves and grass clippings. It can be carefully cleaned with a garden hose after debris is vacuumed off. Consider a professional cleaning if the outdoor coil becomes badly plugged.

Both the furnace fan and outdoor unit fan should be cleaned and lubricated where applicable and following manufacturers' instructions. The furnace-fan speed can be checked and adjusted at the same time, to ensure peak performance.

Ductwork can be professionally cleaned if needed, but the need for cleaning can be reduced by a proper filter replacement and cleaning routine. To ensure that all ducts are airtight, seal the joints with a special duct mastic (sealant). This should reduce or eliminate air leaks. High temperature duct tape may work, although it tends to degrade or permit air leaks over time. Be sure that vents and registers are not blocked by furniture, carpets or other items that can resist airflow. Extended periods of inadequate airflow can lead to compressor damage. For professional cleaning or supplies, look in the Yellow Pages™ under “Furnaces – Heating” or “Furnaces – Supplies and Parts. ”

Using a high-efficiency air cleaner on a central cooling/heating system is one way of ensuring a clean indoor coil and a cleaner indoor environment.

If, after attending to filter maintenance and coil cleaning, your central air conditioner does not appear to be doing its job, you will need to hire a competent service contractor to undertake more difficult maintenance or service, such as checking the refrigerant level or making electrical or mechanical checks and adjustments.

Operating costs

The operating cost of a central air conditioner is influenced by a number of factors, such as how much you use your air conditioner and how efficient it is, the amount of insulation and glazing in your home, and the frequency and duration of door and window openings when the system is operating.

It also depends on the activities in your home and the use of other equipment and appliances that increase the load on the air conditioner. Finally, it depends on the local climate and electricity costs.

The section of his guide entitled “Air-Conditioning Operating Costs, ” provides estimates of the cost of operating a central air conditioner in different regions of Canada.

Life expectancy and warranties

The life expectancy of a central air conditioner is 15 years or longer. When the air conditioner starts giving more problems than seem cost-effective to fix – particularly when major components, such as a compressor, require replacement – it may be time to replace the central air conditioner. New units offer greater efficiency and lower operating costs; it may be more cost-effective in the long run to replace rather than repair.

The warranty on your equipment will vary according to the manufacturer. Air conditioner warranties range from one year for complete parts and labour to five years for the compressor. Some manufacturers are now offering 10-year warranties on their compressors. Make sure you fully understand the terms of a warranty. Ask the contractor or manufacturer for an explanation, if necessary.

Replacing an existing central air conditioner

If your existing air conditioner needs replacement or is more than 10 years old, chances are good that it is also inefficient. A 10-year-old air conditioner probably has a SEER rating between 7.0 and 8.0, compared with some new models that are twice as efficient. The more efficient unit should pay for itself through decreased utility bills and offer improved reliability and warranty protection.

If you have an electric or oil furnace or a conventional gas furnace and your space heating costs $1,000 or more per year, you should consider installing an efficient air source or ground-source heat pump instead of a central air conditioner. Find out from two or three contractors how much more it would cost to add a heat pump to your furnace. Heat pump equipment SEER ratings are competitive with hose of central air conditioners, but heat pumps have the added advantage of providing savings in heating costs during the winter.

Heat pump savings range from a low of about 20 percent, where gas is the primary heat source, to as high as 60 percent, where an electric furnace is the main source of heating. If the additional capital cost divided by the estimated savings is five years or less, consider installing a heat pump instead. They are good for the environment and are an efficient way of using electricity for home heating.

For more information about heat pumps, read the companion NRCan booklet Heating and Cooling with a Heat Pump. See “Need More Information?” to find out how to order a copy.

Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency