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How an Air Conditioner Conditions Air

A Little Chemistry, a Little Physics, and a Whole Bunch of Tubes and Wires. Here Are the Basics of Keeping Cool.

An air conditioner makes your home cooler, true. But in terms of how the system actually works, it's more accurate to say that an air conditioner makes your home less warm. What it's really doing is drawing heat energy out of the house and transferring that heat to the outdoors (where it's already so blasted hot that nobody notices the difference).

1. A cool gas (the refrigerant) flows into the compressor, where it is compressed into high temperature, high pressure gas. The compressor pumps this hot gas through the tubes to . . .

2. The condenser coil in the outdoor unit (also called the condenser.) As the hot gas passes through the condenser coil tubes, heat is transferred from the gas, through the metal fins surrounding the air. This causes the gas to cool and condense into liquid. A motorized fan forces air across the condenser metal fins to increase the rate at which heat is transferred. Meanwhile...

3. The liquid refrigerant flows through tubes to the evaporator inside your home. Once inside the evaporator, the refrigerant goes through a pressure reducing device that causes the refrigerant to quickly expand and to cool rapidly. Then the cold refrigerant travels through the tubes of the evaporator coil where it absorbs heat energy from the surrounding air and turns back into a gas (it evaporates). Also, humidity from warm moist indoor air condenses on the evaporator and drains away. Meanwhile...

4. A blower draws warm air from the house, moves it through the evaporator where heat energy is removed and blows this air on through the ductwork into your home -- cooler, dryer and altogether more pleasant. From there...

5. The cooler gaseous refrigerant travels through tubes back to the compressor where the cycle begins again.

So you see, that air blowing out the top of your outdoor unit is so hot because it contains heat energy that was inside your house just a couple of minutes before.

EPA "Energy Star" Identifies Highly Efficient
Central Air Conditioners

If you're in the market for a new heat pump or central air conditioner, add the government's new Energy Star symbol to your list of things to look for.

Why? Because the Energy Star logo means the unit is at least 20 percent more efficient than the products currently meeting minimum federal government standards.

Experts at the Trane Home Comfort Institute explain that, in terms of cooling, only heat pumps and central air conditioners that have a Seasonal Energy Efficiency Ratio (SEER) of 13 or higher qualify for the Energy Star label.

The SEER is a measure of a unit's cooling efficiency. The higher the SEER, the higher the efficiency. The higher the efficiency, the lower the energy bills.

The U.S. Environmental Protection Agency (EPA) created the Energy Star program to help increase the efficiency of heat pumps and central air conditioners being installed in homes. Doing so will dramatically reduce the nation's energy consumption.

It will also help utilities offset their peak loads and avoid constructing costly new power plants. Homeowners gain the added benefit of saving money because more efficient equipment costs less to operate.

To make the high efficiency models easier to purchase, EPA is offering financing assistance through special Energy Star loans from participating banks and other financial institutions. The loans have lower interest rates, longer payment periods or both.

It also stresses the importance of purchasing the properly sized system. A unit that's too small will lack the capacity to keep you cool on the hottest days. A unit that's too large will cost more to purchase, more to operate and may not remove enough humidity to keep you comfortable. Your air conditioning dealer can help you determine the correct size for your home.

The Truth Behind Those Efficiency Ratings

How Much Efficiency Is Enough? Depends on How Fast You Want your System to Pay for Itself.

12 SEER, 14 SEER, 80% AFUE, 90% AFUE - don't get too confused by trying to figure out where all the numbers come from. All you really need to know is that these are relative measures of fuel economy -- SEER (Seasonal Energy Efficiency Ratio) numbers for air conditioners, or AFUE (Annual Fuel Utilization Efficiency) for gas furnaces.

In simplest terms, the higher the number, the more heating or cooling you'll get for your energy dollar.

As you shop around, use the numbers, not vague terms like "high efficiency" or "super high efficiency," to really compare systems. Any air conditioner or furnace on the market today can be called "high efficiency" compared to the equipment of just a few years ago. What was called high efficiency then -- say 9 SEER for an air conditioner or 70% for a gas furnace -- wouldn't even be permitted on the market today!

Air Conditioners

  • 10 SEER -- the minimum efficiency allowed by law for new central air conditioning systems.
  • 12 or 13 SEER -- trade up to this level from your old system and you'll probably be delighted at how much lower your electric bills are.
  • 14 SEER plus -- pushing the upper limits of what's possible with today's technology.

Gas Furnaces

  • 78% -- the legal minimum for new furnaces on the market today.
  • 80% -- another once-impossible degree of efficiency that means drastically lower gas bills than you probably have with an old furnace.
  • 90-plus % -- currently the highest efficiency you'll find (but we're working on it)

Reliability Is Most Desired Quality in Central Air Conditioning System

Imagine entertaining guests at your home on a hot, humid day, and the central air conditioning system breaks down. Imagine that, and you easily can understand why reliability is what homeowners want most from their central air conditioning system.

A study by the Trane Home Comfort Institute, a consumer information service on heating and cooling, shows that nearly seven out of ten (69 percent) rate reliability as the most important attribute to consider when choosing a central air conditioning system. Homeowners in the study said they wanted a unit that could be counted on not to break down when it was needed most.

According to the homeowners, durability, the second most frequent response (63 percent), meant that the unit should last a long time. As one respondent put it, "It's not something I should have to purchase often. I want it to last."

The study shows that efficiency is another attribute high on homeowners' minds (59 percent). Respondents said a key question to ask concerning this feature is, "With this new system, will my utility bill be less than it was in the past?"

Completing the first tier of attributes was overall quality (53 percent). In the respondents' eyes, this meant the quality of the manufacturing, ranging from the construction of the compressor to the composition of the paint finish.

Heading up the second tier of considerations was weather resistance (37 percent). This attribute deals with how the outdoor unit is protected from the elements. Here, respondents felt cabinets with louvered full-side panels provide the most protection from rain, hail, falling leaves, dirt and children.

In homeowners' minds, affordability (36 percent) referred primarily to purchase price, since operating costs of newer units were expected to drop due to higher efficiencies. Respondents said a key question to ask is, "Is the value and comfort this system provides worth the price?"

In one of the more interesting findings, Trane Home Comfort Institute researchers found that even though respondents admitted to not knowing the inner workings of their air conditioning systems, technology was mentioned enough as a highly desirable attribute to make the list (29 percent).

Finally, when it came to sound level (23 percent), homeowners said they wanted the air conditioner to perform without making its presence known. They also felt that way about their neighbor's unit. As one respondent said, "When your neighbor's outdoor unit sits near your bedroom window, you hope he selected a quiet system."

Central Air Conditioner Efficiency Ratings:
What a SEER Really Means to You

When purchasing a car, smart buyers usually check the miles-per-gallon rating.

When purchasing gasoline, they check the octane rating.

And, when purchasing a central air conditioning system, they check the SEER rating.

The what?

The SEER, or Seasonal Energy Efficiency Ratio, rating. Like its "mpg" counterpart in the automotive industry, the SEER gives an indication of the performance efficiency of the system. The higher the SEER, the more efficient the unit. And, the more efficient the unit, the lower the operating costs.

The Trane Home Comfort Institute explains that the air conditioning industry originally used an Energy Efficiency Ratio or EER to rate efficiency. This was a simple mathematical ratio of cooling output versus electrical power input.

Recently, however, the U.S. Department of Energy developed a more sophisticated test method that rates the performance of a unit over a wide range of operating conditions. The result -- the SEER -- is indicative of the unit's operation throughout an entire cooling season.

As of 1992, federal regulations require all new split system central air conditioners have a SEER of at least 10. A split system means part of the system is located outdoors and part indoors.

Bear in mind, however, that SEER ratings compare equipment only. Many other factors, including how you use your home and the condition of ductwork, affect energy use. That's why it's important to have a competent air conditioning dealer visit your home and evaluate your entire system.

It's also important to remember that the rating of an outdoor unit is based on a matching indoor component. As a result, if you replace an outdoor unit without replacing the indoor unit, you're not likely to get the efficiency you expect.

Experts at the Trane Home Comfort Institute say that by purchasing a system with a high SEER, you'll use less energy to cool your house, resulting in lower electric bills. In many cases, these savings are enough to partially or fully offset the cost of the new equipment within a few years.

They also recommend checking with your local utility about a possible rebate. Many utilities offer such programs based on the unit's SEER rating and cooling capacity. In most cases, the higher the SEER rating, the higher the rebate.

Finally, they point out that there's more to purchasing central air conditioning than just the SEER rating. You should also look into the reliability and durability of the system. After all, a "clunker" with good gas mileage is still a "clunker."

Increase Your Indoor Comfort
by 'Zoning' Your Home's Interior

If you're designing, building or buying a new home, or upgrading the heating and cooling system in your existing home, there's an increasingly popular option that can help you get the most indoor comfort and efficiency. Its name: zoning.

According to the Trane Home Comfort Institute, the easiest way to describe zoning is to compare it with lighting. Each room in your home has its own light switch. With individual switches in each room, you don't waste electricity lighting the entire house if you only need to light one or two rooms.

In a similar way, zoning provides individual comfort control within specific, preplanned areas of your home. If you only need to heat or cool one or two rooms, zoning lets you do it more economically and with greater comfort.

In a typical zoning system, thermostats in each room or grouping of rooms -- zones -- monitor the temperature. Whenever a thermostat senses the need for heating or cooling, it signals a central control panel which turns on the furnace, air conditioner or heat pump. At the same time, the control panel also activates motorized dampers in the ductwork to open or close. This directs the heated or cooled air only to the zone or zones calling for it.

Experts at the Trane Home Comfort Institute say zoning offers three significant benefits:


A zoned comfort system quickly adapts to changing conditions within an area without affecting other areas.

For example, many two-story houses are zoned by floor. Because heat rises, the second floor usually requires more cooling in the summer and less heating in the winter than the first floor. A nonzoned system can't completely accommodate this seasonal variation. Zoning, however, can eliminate wide variations in temperature between floors by supplying heating or cooling only to the space that needs it.

Demand for heating and cooling can also vary from one zone to the other during the course of a day. A well-controlled zoned system can quickly respond to these changes, eliminating hot and cold spots and allowing more consistent temperatures throughout the house.


A properly zoned system can be up to 30% more efficient than a nonzoned system.

Why? Because a zoned system supplies warm or cool air only to those areas that require it. It is usually not supplying the whole house all the time. Energy is not wasted heating and cooling spaces that aren't being used.

In addition, a zoned system can sometimes allow you to install smaller capacity equipment without compromising comfort. This reduces energy consumption by reducing wasted capacity.


A zoned system allows more control over the indoor environment because you decide what areas to heat or cool and when.

Consider, for example, cases where children have moved out and some rooms have only occasional use. Or, rooms originally intended for one purpose are converted to other uses. Zoning lends itself well to these situations because it easily adapts to heating and cooling load changes caused by increased or decreased usage, room additions or a change in occupants or lifestyle.

Experts at the Trane Home Comfort Institute say that, applied correctly, zoning systems can provide a very high level of comfort at a very reasonable cost. However, they also say that there is no single best solution to fit all homes. Choosing the right zoning solution depends on the size and layout of the home and the desired degree of control.

Test Your Central Air Conditioning Knowledge

If you have a quizzical look on your face whenever someone mentions central air conditioning, it's time to brush up on your knowledge of condensers and coils.

Start by taking the following quiz developed by the Trane Home Comfort Institute, a consumer information service on heating and cooling.

Answer true or false to the following statements:

1. Most central air systems are "split" systems.

2. A ton of air conditioning refers to the weight of the system.

3. Heat pumps can cool your home.

4. The higher the "SEER" of a central air conditioner, the cheaper it is to operate.

5. Price is the most important consideration when choosing a central air system.

6. The compressor is considered "the heart of the system"

When it comes to the subject of cooling, you're "hot" if you've given the following answers:

1. Most central air systems are "split" systems:
True. A split system means it consists of two parts: an indoor unit or coil that pulls heat and humidity from inside air, and an outdoor unit or condenser that releases that heat to the outside air. When the coil and condenser are both housed in the same unit, the system is called a packaged system.

2. A ton of air conditioning refers to the weight of the system:
False. It refers to the capacity of the system. One ton of air conditioning removes 12,000 Btu's of heat energy per hour from a home. Residential units usually range from 1 to 5 tons.

3. Heat pumps can cool your home:
True. Don't be fooled by their name. Heat pumps do pump heat into your home in winter like a furnace. However, they also pump heat out of your home in summer like a central air conditioner. This ability to both heat and cool makes them very economical and efficient home comfort systems.

4. The higher the "SEER" of a central air conditioner, the cheaper it is to operate:
True. The SEER, or Seasonal Energy Efficiency Ratio, is an indication of the performance efficiency of the system. The higher the SEER, the more efficient the unit. The more efficient the unit, the lower the operating costs.

5. Price is the most important consideration when choosing a central air system:
False. A recent study by the Trane Home Comfort Institute shows that homeowners rate reliability as the most important attribute to consider when selecting a central air conditioner. Purchase price ranked fifth on their list after reliability, dependability, efficiency and overall quality.

6. The compressor is considered "the heart of the system":
True. It is so called because it is the pump that circulates the refrigerant from the indoor coil to the outdoor condenser and back to the coil again. Because of its importance, look carefully at a compressor's reputation and warranty when selecting a central air conditioning system.

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