Understanding Your Home Air Conditioning Unit

Know when to buy, upgrade and overhaul a home air conditioner, including AC basics, selection, and sizing the system.
By the MOTHER EARTH NEWS editors
June/July 1993
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Don't sweat and suffer all summer long. Let your air conditioner keep you cool and comfortable.

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Approximately 57 million U.S. households (roughly 60%) have air conditioners, with an energy consumption equivalent to the output of seven large coal power plants, and at a cost to homeowners of $9.8 billion. That amount of electric energy results in the release of about 100 million tons of carbon dioxide per year, or a little under two tons per year on average for every home with air-conditioning. Furthermore, air conditioner use in the United States has increased more than 20% since 1984. Seventy-five percent of new homes are being outfitted with central air-conditioning systems, reports Air Conditioning, Heating, and Refrigeration News, including over half the homes in the Northeast. A switch to high-efficiency air conditioners and measures to reduce cooling loads in homes can reduce this energy use by 20-50%

Air-Conditioning Basics  

Air-conditioning, or cooling, is more complicated than heating. Instead of using energy to create heat, air conditioners use energy to take heat away. The most common air-conditioning system uses a compressor cycle (similar to the one used by your refrigerator) to transfer heat from your house to the outdoors.

How is this done? A compressor is filled with a special fluid called a refrigerant (usually HCFC 22, one of the environmentally notorious chlorofluorocarbons). One of the chemical's properties is that it can change back and forth between liquid and gas. As it changes, it absorbs or releases heat. Thus, it is used to "carry" heat from one place to another, such as from the inside of a refrigerator to the outside, or-using an air conditioner from the inside of a house to the outside. Simple, right? Well, no. And the process gets quite a bit more complicated with all the controls and valves involved.

But its effect is quite remarkable. An air conditioner takes heat from a cooler place and dumps it in a warmer place, seemingly working against the laws of physics. What drives the process, of course, is electricity-quite a lot of it, in fact.

Types of Air Conditioners  

There are three common types of air conditioners: separate room air conditioners, central air conditioners, and electric heat pumps. Both central and room units work the same way, with the compressor located outside. Room air conditioners are sized to cool just that one room, so a number of them may be required for a whole house. Central air conditioners are designed to cool the entire house. The large compressor unit is located outside, and the inside coils cool air that is distributed throughout the house via ducts.

Often, the same duct system is used for forced, warm air heating systems and central air conditioners. Heat pumps are like central air conditioners, except that the cycle can be reversed and used for heating during the winter months. In addition to these common types of air conditioners, there are several other types used very successfully in certain parts of the country. Evaporative coolers are practical in very dry areas, such as the Southwest.

Sometimes called swamp coolers, they work by blowing house air over a damp pad or by spraying a fine mist of water into the house air. The dry air evaporates moisture and cools off. You've experienced this process-it's why a breeze makes you feel cold when you get out of a swimming pool. A direct evaporative cooler adds moisture to a house. An indirect evaporative cooler is a little different in that the evaporation of water takes place on one side of a heat exchanger.

House air is forced across the other side of the heat exchanger where it cools off but does not pick up moisture. Like the direct evaporative cooler, this system depends on very dry air to operate. If you live in a hot, arid region, such as the Southwest, look into evaporative coolers. For the rest of the country, compressor driven air-conditioning systems are about the only choice, other than natural cooling.  

Selecting A New Air Conditioner

The type of air conditioner you need depends in large part on your climate and cooling loads. In small homes and those with modest cooling needs, room air conditioners often make the most sense. In fact, in a small, highly insulated house, even the smallest central air conditioner may be too large.

If you are considering room air conditioners, you will need to decide between units that mount in the window and those that are built into the wall.

Wall-mounted units are often a better choice, both for aesthetic and practical reasons, though they will cost more to install. Usually, a hole has to be cut into the wall to accommodate a mounted unit.

But once in place, it is much easier to seal and insulate, blocks no natural light or ventilation from entering through the windows, and generally makes the exterior of the house look more attractive. Central air conditioners have their own set of advantages.

They are out of the way, quiet, and convenient. If you already have a forced air heating system, you may be able to tie into the existing duct work. Whether or not your existing ducting will work for air conditioning depends on its size and your relative heating and cooling loads.

Plus, central air conditioners are more efficient. Heat pumps, though more expensive, provide heat in addition to air-conditioning all in one unit. If you already have a satisfactory gas or oil heating system and have decided to add air-conditioning, it usually doesn't make sense to consider a heat pump because even a high-efficiency heat pump will be more expensive to operate than your gas or oil heating system.

If you currently have electric resistance heat, however, and you live in a relatively warm climate (winter temperatures seldom dropping below 30°F), a heat pump may be a good choice.

If you're unsure about which type of air conditioner makes the most sense for your house, ask for opinions and bids from several local air-conditioning installers.

Sizing the System

No matter what type of system you choose, make sure that it is sized properly. Most air conditioners are rated in BTU/hour (BTU is an acronym for British Thermal Unit. A BTU is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.). Central air conditioners and heat pumps may also list cooling capacity by tons. One ton is equivalent to 12,000 BTU/hour. With air conditioning systems, equipment cost is much more proportional to size than it is with heating equipment. Doubling the cooling output nearly doubles the cost, so it makes a lot of sense to be very careful with sizing.

Don't let a salesman convince you to buy a larger system than you need. In addition to the higher purchase cost, it will run for only short periods, cycling on and off. When any heating or cooling element is forced to turn on and off rather than run at a consistent level, energy consumption will increase and efficiency will decrease.

If it runs for just short periods of time, it also won't do as good a job dehumidifying the air. Find a qualified air-conditioning technician or energy auditor to determine your cooling load.

Do not rely on simple rules of thumb quoted by air conditioner salespeople, but insist on a thorough analysis, including local climate information and calculations of heat gain through windows and walls. With heat pumps, proper sizing can be especially difficult because the same unit is used for both cooling and heating.

A heat pump sized for heating loads in a cold climate will be considerably oversized when it comes to cooling, and a heat pump that is sized for cooling loads in a warm climate will tend to be oversized when it comes to heating. If the heating load is larger than the cooling load, some heat pump salespeople will recommend sizing the heat pump for cooling and then adding enough electric resistance heat to make up the difference in the winter.

In such a situation, it generally makes more sense to size the heat pump to provide all of the heating requirements in average winter conditions, even though it will mean a larger and somewhat more expensive model. A good heat pump technician should be able to help you choose the best compromise between cooling and heating capacity.

Air Conditioner Efficiency

Efficiency is just as important with air conditioning systems as it is with heating systems. Central air conditioners and heat pumps operating in the cooling mode are rated according to their seasonal energy-efficiency ratio (SEER), which is the seasonal cooling output in BTU divided by the seasonal energy input in watt-hours for an average U.S. climate. Many older central air conditioners have SEER ratings of only seven or eight.

The average new central air conditioner sold in 1988 had a SEER of about nine. Some models today are available with SEER values of 13-16. The national appliance efficiency standard for central air conditioners took effect in 1992, requiring a minimum SEER of 10.

The efficiency of room air conditioners is measured by the energy efficiency rating (EER), which is the ratio of the cooling output (in BTU) divided by the power consumption (in watt-hours). The EER does not factor in performance over the cooling season.

An average new room air conditioner has an EER between eight and nine. You can consider a room air conditioner with an EER greater than nine to be efficient, and a model with an EER over 9.5 to be very efficient. The national appliance efficiency standards for room air conditioners took effect in 1990. The required efficiency varies depending on the design and cooling capacity of each unit.

On average, the 1990 standard requires a minimum EER of about 8.6. When you're shopping for air conditioners, look for SEER ratings over 12 for central air conditioners and EER ratings over nine for room air conditioners. High-efficiency units generally cost more, but in hot climates, reduced electricity bills easily pay for that extra cost over a few years .

(This is not necessarily true in cooler climates, where the air conditioner runs fewer than 300 hours per year.) Central air conditioners are almost always more efficient than room air conditioners, and in general, larger capacity air conditioners have higher efficiency. However, don't buy a larger system than you need just because it has higher efficiency (see discussion on sizing above).

Other energy-saving features to look for include: a fan-only switch, which will enable you to use the unit for nighttime ventilation and substantially reduce air conditioning costs; a filter check light to remind you to check the filter after a pre-determined number of operating hours; an automatic-delay fan switch to turn off the fan a few minutes after the compressor turns off; and quiet operation. (Sound ratings are generally not listed by manufacturers, but with room air conditioners, you can ask to listen to the different models in operation.).


Air conditioners remove a certain amount of moisture from the air because the room air is forced past cold coils. Water vapor from the air condenses on the coils the same way moisture from the air condenses on a glass of ice water on a hot humid day. This water drains outside through a condensate drain.

Lowering the humidity in this way is both good and bad. You feel more comfortable at lower humidity levels, so the dehumidification contributes to cooling. But when water vapor condenses into liquid, it releases stored heat, reducing the efficiency of the air conditioner.

One of the ways manufacturers have boosted the efficiencies of air conditioners in recent years is by keeping the condenser coils somewhat warmer, thus reducing condensation. Some of the new high-efficiency air conditioners, therefore, do not dehumidify air as effectively.

This can be a problem, especially in the humid Southeast. High-efficiency air conditioning systems can get around the dehumidification problem by including variable-speed or multi-speed blowers. High-speed operation leads to high efficiencies but low dehumidification. Lower speeds reduce efficiency some what, but increase dehumidification. Although there is no industry standard for rating the effectiveness at removing moisture, most literature does list water removal in pints per hour, which will help you compare one model to another. Some manufacturers have variable-speed blowers controlled by a humidistat, automatically reducing fan speed at high humidity. These models are the best bets for a particularly humid climate.

Making Your Air Conditioner Last

Upgrading An Older Model: The compressor units of most air conditioners have an average lifetime of only about ten to twelve years. By carefully following proper maintenance procedures, a quality model may hold up twenty years, but don't expect the kind of lifetime you get with boilers and furnaces. If you have an old air conditioner—more than ten years old—chances are pretty good that it's also inefficient. A 10-year-old central air conditioner probably has a SEER rating between seven and eight, compared with the best new models that are up to twice as efficient. It will definitely pay to replace it, but you may not have to replace the entire air conditioner. In most circumstances, just the outdoor compressor component needs to be replaced, though it might well be difficult to find high-efficiency parts for low-efficiency models. If you're replacing just the compressor, though, make sure that the new outdoor unit is compatible with the indoor-blower coil. The highly efficient outdoor unit will not reach its capacity if it is not properly matched to a corresponding indoor unit. An air-conditioning service technician should be able to help you match units effectively.

Operation And Maintenance: Air conditioners and heat pumps need regular maintenance in order to perform at peak efficiency. Clean the air filters on room air conditioners monthly. They should not be allowed to get dirty enough to impede air flow because this could damage the unit. The condenser should be cleaned by a professional every other year, or even yearly in dusty conditions. Central air-conditioning units should be inspected, cleaned, and tuned by a professional once every two to three years. This will extend the life of the unit and reduce electricity consumption.

Check with your service technician about the proper maintenance schedule for your unit. During service of your unit, its refrigerant (HCFC-22) may need recharging. It needs to be charged correctly. A 10% under charged system can operate at 20% lower efficiency. However, an overcharged system can cause damage to the unit and reduce the lifetime of the system. Also, because refrigerants damage the ozone layer, it is important that the refrigerant not be leaked to the environment; it can and should be recycled. The power to a central unit should be shut off when the cooling season ends; otherwise the heating elements in the unit could consume energy all winter long. Flip the circuit breaker to turn it off if the unit doesn't have a separate switch. Turn the power back on at least one day before starting up the unit in order to prevent damage to the compressor.

Reducing The Need for Air-Conditioning

Even with the most efficient air conditioners, it makes a great deal of sense to do everything you can to reduce air conditioning loads. The following conservation measures are often so effective that houses in the northern third of the country and in mountainous regions can be maintained at a comfortable temperature without air conditioning on all but the very hottest days. If you're planning to buy a new system, reducing the cooling load and encouraging natural air circulation will save you a lot of money right away by letting you buy a smaller, less expensive system.

Human Comfort

In looking at how air-conditioning costs can be reduced, it helps to understand how temperature affects human comfort. The standard human comfort range for light clothing in the summer is between 72°F and 78°F and between 35% and 60% relative humidity, according to the American Society for Heating, Refrigeration, and air-conditioning Engineers (ASHRAE). The comfort range can be extended to 82°F with modest air movement, as might be provided by ceiling fans, for example. Often, the house can be kept within this range using little or no mechanical air-conditioning. To extend the comfort range to 82°F, you need a breeze of about 2.5 feet/second or 1.7 miles per hour. A slow-turning ceiling-mounted paddle fan can easily provide this air flow. Fans should have multiple speed settings so that air flow can be reduced at lower temperatures.

Additional Tips for Efficient Cooling  

1. If your air conditioner has an outside air option, use it sparingly. It is far more economical to recirculate and cool the indoor air than to cool down the hot outdoor air to comfortable temperatures.

2. Always keep all doors and windows closed when operating an air conditioner. Do not operate a whole-house fan or window fans while using the air conditioner.

3. You will save 3-5% on air conditioning costs for each degree that you raise the thermostat.

4. You can increase comfort at warmer temperatures by reducing humidity. Use a bathroom exhaust fan while you shower, don't dry firewood in your basement, don't vent air from your clothes dryer inside, and put house plants outside during the summer months.

5. In hot climates, plant shade trees around the house. Don't plant trees on the south side if you want to benefit from passive solar heating in the winter.

6. Make sure that compressors are well shaded, both for central and room air conditioners. Ideally, the compressor should be on the north side of the house. This may be difficult with room models, but try to do your best.

7. Try not to use a dehumidifier at the same time your air conditioner is operating. The dehumidifier will increase the cooling load and force the air conditioner to work harder.

8. Seal all air conditioner ducts with duct tape and insulate ducts that run through unheated basements, crawl spaces, and attics.

9. The use of window and whole-house fans can minimize very effectively the heat gain from the sun, lights used in the home, appliances, etc. Both types of fan are very inexpensive.

From Consumer's Guide to Home Energy Savings (1991),  published by the American Council for an Energy-Efficient Economy (ACEEE).

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