Central system buying guide

The following sections are designed to briefly inform you in each aspect of purchasing your new home comfort system. Please read all available information to help you make an informed purchasing decision on your new central heating and/or air conditioning unit.

Getting started

When you install central air conditioning size matters. Underestimate your cooling needs, and you could be sweating. Buy more power than you need and your living space may become cold and clammy. Any contractor you hire should calculate the size of the cooling equipment you need by using such recognized methods as the Air Conditioning Contractors of America (ACCA) Manual J. If you already have ductwork for your heating, adding a central system can cost less. But keep in mind that ducts used for heating might not be the right size or in the right location for optimal cooling.

Your contractor should use a duct-sizing method such as the ACCA Manual D. The pros should make sure that all duct sections are properly sized and that there are enough supply registers to deliver sufficient air to the right spots. Not only is the proper size ductwork essential for meeting each room’s cooling needs but also because undersized ductwork can make for noisy operation. Leaky or uninsulated ducts can reduce system efficiency considerably. All joints and seams must be sealed–and not with duct tape that can dry and fall off.

If your home doesn’t have ducts, adding them can be expensive, though if you plan to cool your entire home, a split system or packaged unit used for central air is typically the best choice. If you are not planning to cool the entire home, you might want to consider a Ductless mini split unit. Unlike central systems, split-ductless systems need no ductwork (though they require connections for electrical, refrigerant and condensate drains), making them easier to add to homes with designs that aren’t conducive to installing ductwork.


The most common type of central air conditioning is the split system which features a condenser outside the home and fan-and-coil system inside connected by pipes carrying refrigerant. However, not every home can accommodate the ductwork needed to install central air. For such residences, a split ductless system is an option.

Central air conditioning

Central air-conditioning systems use ducts to distribute cooled air throughout the house. In a “split system,” the typical design, refrigerant circulates between an indoor coil and a matching outdoor condenser with compressor. The refrigerant cools the air, dehumidifying it in the process; a blower circulates air through ducts throughout the house. A variation is the “heat pump,” a type of system that functions as heater and cooler. When used as an air conditioner, a heat pump discharges heat from the house either into the air or deep into the ground. In the winter, a heat pump extracts heat from the ground or the air to warm the house.

Split ductless systems

Split ductless systems are similar to central air. They have an outside condenser and one to four indoor units with blowers mounted high on the wall. Tubing connects the parts and circulates refrigerant. The tubing, along with an electric and drain line, is run through a 3-inch hole hidden behind the indoor unit. Each indoor unit cools the room it’s installed in and has its own remote control. Unlike central systems, split ductless systems need no ductwork, making them easier to add to homes without existing ducts. Split ductless systems are more expensive than window air conditioners, and professional installation is recommended, but it’s a way to add cooling without tearing up walls to install ducts.

Ducts or Pipes

Some systems use pipes instead of ducts, which distribute chilled water to heat exchangers in more than one room.


When considering central air conditioning features, think about the size, design and efficiency of the unit. In a “split system,” the typical design, refrigerant circulates between an indoor fan-and-coil and a matching outdoor condenser with compressor. The refrigerant cools the air, dehumidifying it in the process; a blower circulates air through ducts throughout the house.

A variation is the “heat pump,” a type of system that functions as heater and cooler. When used as an air conditioner, a heat pump discharges heat from the house either into the air or deep into the ground. In the winter, a heat pump extracts heat from the ground or the air to warm the house.


This describes how much cooling the unit delivers for each watt of electricity. Efficiency is expressed as the Seasonal Energy Efficiency Rating, or SEER. The minimum SEER for a split system central air conditioner is 13.


A synonym for the air conditioner’s cooling capacity, size is measured in British thermal units per hour (Btu/hr.) or in “tons.” One ton of cooling equals 12,000 Btu/hr.


What’s the best way to ensure that the central air-conditioning system you choose is installed properly, and will provide the most efficient and reliable cooling for your home?

The pointers below can help you to find the right hardware and the right technician to install your system, whether you’re replacing an older air conditioner or installing one for the first time.

Get the right contractor

Finding a trustworthy contractor to install and service an air-conditioning system matters the most. Here’s what to do.

Ask around. Seek referrals from neighbors, family, or business associates. It’s wise to get price quotes from at least three contractors.

Check the background.

Contractors who bid on your installation should show you verification of bonding and insurance, plus any required contractor’s licenses. Check with your local Better Business Bureau (BBB) and consumer affairs office for complaint records. It’s a plus if technicians are certified by a trade organization, such as North American Technician Excellence or HVAC Excellence, to service residential heating and cooling equipment. Those and other similar programs assess the technician’s knowledge of specific types of equipment and their proper service methods. We believe that a contractor who has made the effort to be certified and has practiced this trade and learned from several years of service and installation experience will be a better service provider.


A service plan that combines regular inspections with discounts on repairs and a labor warranty is worth negotiating into the overall price. Prices for such service vary widely. Follow these maintenance tips.

Call in a pro

Have a licensed professional perform important maintenance tasks, including changing all filters, cleaning and flushing the coils, draining the pan and drainage system, and vacuuming the blower compartments. The technician should also check that the system is properly charged with refrigerant, that there are no leaks, and that all mechanical components are working properly. As with a room air conditioner, replace disposable filters regularly. Check them monthly and replace if dust and debris have completely coated the filter.

Insulate ductwork

Ensure that ducts throughout the system are sealed and insulated–up to 30 to 40 percent of your cooling (and heating) energy can be lost through leaks or when uninsulated ducts pass through uncooled spaces such as attics and garages.

Perform seasonal checks

During the season, keep vegetation at least two feet away from the unit. Clean the grills and filters monthly and replace the filters as needed. Clear debris and dirt from condenser coils and check for blockages in the pipe that drains condensed water from the indoor unit.

Use a programmable thermostat

Proper use of a programmable thermostat can reduce your cooling costs by up to 20 percent. Also consider adding a ceiling fan, which will allow you to set your thermostat to a higher temperature.


If you’re upgrading your central air, don’t automatically buy the same-sized system. Any changes you’ve made to improve your home’s energy efficiency, such as replacing windows or adding insulation, can reduce your cooling needs. On the other hand, if you’ve added rooms, you might need more cooling. So have your contractor do a load calculation based on a recognized method, such as Manual J from the ACCA. The contractor’s evaluation should include whether your ducts need to be resized, sealed and insulated, or replaced. Remember that an indoor evaporator coil and outdoor condenser must be a matched set, or the performance, efficiency, and capacity claims might not be accurate.

New systems are 20 to 40 percent more efficient than minimum-efficiency models made even 10 years ago. If you’re replacing an old central-air system, you can expect to pay a few thousand dollars for the cooling equipment with a capacity of about three tons (36,000 Btu/hr.). If you need ductwork installed because you’re starting completely from scratch or are upgrading a forced-air heating system, expect to pay double that. Of course, the cost depends on the size and configuration of your home. Improving the system’s air-filtration capabilities is also easiest to do as part of a general upgrade.


Here are some factors that may affect the reliability of your system.

Matching new equipment with old

If you replace only the condenser, you have a “field matched” system that can be less efficient than advertised and that may require more repairs because of undetected incompatibilities between the indoor coil and the condenser.

Damper-zoned cooling

A large or multistory house is often divided into several heating and cooling zones to improve temperature control. But this type of system is complex and has many more moving parts and controls and so may require more repairs.

Adding Insulation

Insulation keeps your home warm in the winter and cool in the summer. There are several common types of insulation — fiberglass (in both batt and blown forms), cellulose, rigid foam board, and spray foam. Reflective insulation (or radiant barrier) is another insulating product which can help save energy in hot, sunny climates. When correctly installed with air sealing, each type of insulation can deliver comfort and lower energy bills during the hottest and coldest times of the year.


Insulation performance is measured by R-value — its ability to resist heat flow. Higher R-values mean more insulating power. Different R-values are recommended for walls, attics, basements and crawlspaces, depending on your area of the country. Insulation works best when air is not moving through or around it. So it is very important to seal air leaks before installing insulation to ensure that you get the best performance from the insulation.

  • For more comprehensive information, check the Department of Energy’s online Insulation Guide .

To get the biggest savings, the easiest place to add insulation is usually in the attic. A quick way to see if you need more insulation is to look across your uncovered attic floor. If your insulation is level with or below the attic floor joists, you probably need to add more insulation. The recommended insulation level for most attics is R-38 (or about 12–15 inches, depending on the insulation type). In the coldest climates, insulating up to R-49 is recommended.


Recommended Levels of Insulation

Insulation level are specified by R-Value. R-Value is a measure of insulation’s ability to resist heat traveling through it. The higher the R-Value the better the thermal performance of the insulation. The table below shows what levels of insulation are cost-effective for different climates and locations in the home.


Recommended insulation levels for retrofitting existing wood-framed buildings



Zone Add Insulation to Attic Floor
Uninsulated Attic Existing 3–4 Inches of Insulation
1 R30 to R49 R25 to R30 R13
2 R30 to R60 R25 to R38 R13 to R19
3 R30 to R60 R25 to R38 R19 to R25
4 R38 to R60 R38 R25 to R30
5 to 8 R49 to R60 R38 to R49 R25 to R30
Wall Insulation: Whenever exterior siding is removed on an

Uninsulated wood-frame wall:

·         Drill holes in the sheathing and blow insulation into the empty wall cavity before installing the new siding, and

·         Zones 3–4: Add R5 insulative wall sheathing beneath the new siding

·         Zones 5–8: Add R5 to R6 insulative wall sheathing beneath the new siding.



Insulation in your home provides resistance to heat flow. The more heat flow resistance your insulation provides, the lower your heating and cooling costs. Properly insulating your home not only reduces heating and cooling costs, but also improves comfort.


To understand how insulation works it helps to understand heat flow, which involves three basic mechanisms — conduction, convection, and radiation. Conduction is the way heat moves through materials, such as when a spoon placed in a hot cup of coffee conducts heat through its handle to your hand. Convection is the way heat circulates through liquids and gases, and is why lighter, warmer air rises, and cooler, denser air sinks in your home. Radiant heat travels in a straight line and heats anything solid in its path that absorbs its energy.

Most common insulation materials work by slowing conductive heat flow and — to a lesser extent — convective heat flow. Radiant barriers and reflective insulation systems work by reducing radiant heat gain. To be effective, the reflective surface must face an air space.

Regardless of the mechanism, heat flows from warmer to cooler until there is no longer a temperature difference. In your home, this means that in winter, heat flows directly from all heated living spaces to adjacent unheated attics, garages, basements, and even to the outdoors. Heat flow can also move indirectly through interior ceilings, walls, and floors — wherever there is a difference in temperature. During the cooling season, heat flows from the outdoors to the interior of a house.

To maintain comfort, the heat lost in the winter must be replaced by your heating system and the heat gained in the summer must be removed by your cooling system. Properly insulating your home will decrease this heat flow by providing an effective resistance to the flow of heat.


An insulating material’s resistance to conductive heat flow is measured or rated in terms of its thermal resistance or R-value — the higher the R-value, the greater the insulating effectiveness. The R-value depends on the type of insulation, its thickness, and its density. When calculating the R-value of a multilayered installation, add the R-values of the individual layers. Installing more insulation in your home increases the R-value and the resistance to heat flow. To determine how much insulation you need for your climate, use an insulation calculator or consult a local insulation contractor.

The effectiveness of an insulation material’s resistance to heat flow also depends on how and where the insulation is installed. For example, insulation that is compressed will not provide its full rated R-value. The overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because heat flows more readily through studs, joists, and other building materials, in a phenomenon known as thermal bridging. In addition, insulation that fills building cavities densely enough to reduce airflow can also reduce convective heat loss.

Unlike traditional insulation materials, radiant barriers are highly reflective materials that re-emit radiant heat rather than absorbing it, reducing cooling loads. As such, a radiant barrier has no inherent R-value. Although it is possible to calculate an R-value for a specific radiant barrier or reflective insulation installation, the effectiveness of these systems lies in their ability to reduce heat gain by reflecting heat away from the living space.

The amount of insulation or R-value you’ll need depends on your climate, type of heating and cooling system, and the part of the house you plan to insulate. To learn more, see our information on adding insulation to an existing house or insulating a new house. Also, remember that air sealing and moisture control are important to home energy efficiency, health, and comfort.


To choose the best insulation for your home from the many types of insulation on the market, you’ll need to know where you want or need to install the insulation, and what R-value you want the installation to achieve. Other considerations may include indoor air quality impacts, life cycle costs, recycled content, embodied energy, and ease of installation, especially if you plan to do the installation yourself. Some insulation strategies require professional installation, while homeowners can easily handle others.


Insulation materials run the gamut from bulky fiber materials such as fiberglass, rock and slag wool, cellulose, and natural fibers to rigid foam boards to sleek foils. Bulky materials resist conductive and — to a lesser degree — convective heat flow in a building cavity. Rigid foam boards trap air or another gas to resist conductive heat flow. Highly reflective foils in radiant barriers and reflective insulation systems reflect radiant heat away from living spaces, making them particularly useful in cooling climates. Other less common materials such as cementitious and phenolic foams and vermiculite and perlite are also available.


Choosing a Well-Insulated Window

Windows that have good insulating values will make your home more comfortable, particularly in winter. This is partly because they allow less heat to pass through, but there’s another reason – the inside surface of a better insulated window will be warmer. When you stand or sit by it, your body won’t lose as much heat to the window as it would to a colder surface. Sometimes the draftiness that people feel from windows isn’t due to air movement at all, but rather to the fact that we radiate body heat to the cold window surface.

 (picture here) This vivid example demonstrates the effectiveness of well-insulated windows against condensation problems. The condensation visible on the single-paned right-hand window is a result of the warm, humid, inside air coming in contact with the cold glass and frame. The inside of the well-insulated window to the left stays warmer, so condensation is less likely.

In addition to improving comfort, windows with high insulating values are less likely to have problems with condensation. Condensation occurs when warm, moist indoor air comes in contact with a cold surface, such as a poorly insulated window. Better insulating values are most valuable in cold climates. The bigger the difference in temperature between outside and inside, the faster heat will move through the window. You’ll notice the difference if you replace an old window with a better-insulating one. For instance, when it is 0°F outside, the inside surface temperature of a double-pane glass window is about 44°F, but for a high-performance window it jumps to about 56°F. High-performance windows will help in the summer as well, particularly if you are trying to cool the house to 78°F as the outside temperature climbs toward 100°F.

Understanding U-Factor

You can tell how much heat a window allows through by its U-factor, which measures thermal conductivity. A lower U-factor means a better-insulating window. The more common term R-value refers to the resistance of the window to heat conduction, and it is the inverse of the U-factor (that is, R-value = 1/U-factor). Better windows have high R-values and low U-factors (see Table 1).

Since the different parts of a window all have different U-factors, you should look at the U-factor for the whole window. The frame and the edge of the glass usually have higher U-factors than the center of the glass. If they don’t specify-and they often do not-manufacturers or dealers may refer to a window’s center-of-glass U-factor, which is almost always lower than the U-factor for the window as a whole.

Fortunately, many new windows are labeled with an energy information sticker from the National Fenestration Rating Council (NFRC). The U-factor on the NFRC label always refers to the whole window. To make sure you are comparing apples to apples, ask for the NFRC ratings even when there is no label on the window. Also, be sure to use the same size windows for comparison, as the ratio of glass to framing affects the result.

Table 1 – Whole Window U-factors of Sample Windows
Aluminum frame w/o thermal break Aluminum frame with thermal break Wood or Vinyl Frame
Single Glass 1.30 1.07 n/a
Double Glass, ½” air space 0.81 0.62 0.48
Double glass, low-e, (E*=0.2), ½” air space 0.70 0.52 0.39
Double glass, low-e, (E*=0.1), ½” air space 0.67 0.49 0.37
Double glass, low-e, (E*=0.2), ½” space with argon 0.64 0.46 0.34
Triple glass, low-e, on two panes, ½” paces with argon 0.53 0.36 0.23
Quadruple glass, low-e (E=.01) on two panes, ¼” spaces with krypton n/a n/a 0.22
*E is the emittance of the low-e coated surface.
Source: 1993 ASHRAE Handbook: Fundamentals, (Atlanta, GA:American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Incorporated, 1993).

Note: These are example of whole window U-factors of 3 ft x 5 ft windows. U-factors vary somewhat with window size. Ask the dealer for the specific values for the window you are looking at.

Double Panes

The first step to improving a window is usually to add a second pane of glass. This traps a layer of still air, a good insulator, between the panes. Double-pane windows insulate about twice as well as single-pane windows, so only half as much heat passes through the window.

Gas Fill

The space between the two panes can also be filled with argon or, less often, krypton gas, which insulate better than air. Krypton is somewhat more effective in windows with less space between the panes (1/4 inch to 3/8 inch), so it is often used in windows with multiple air spaces (such as triple-pane windows) to keep the thickness down. Windows filled with air or argon work best when the space is about 1/2 inch. Windows with krypton are usually more expensive, both because krypton itself is expensive and because the designs tend to be upper scale. Argon is nearly as effective and does not add much to the cost of a double-pane window.

Energy-efficient features in this Superglass window include two low-e coatings, gas fills, and an insulating spacer. Low-Emittance Coatings
Wouldn’t it be nice in winter if you could let in heat from the sun, but keep the home’s warmth inside? This is essentially what a low-emittance (low-e) coating does. A clear microscopic metal oxide layer installed on a surface of one of the panes of glass allows short-wavelength sunlight to pass through it, but reflects long-wavelength infrared radiation. The heat energy from the home (and people) that radiates toward windows is long-wavelength radiation, which is reflected back into the home.

Improved Frames and Spacers

Window frames are made from aluminum, wood, vinyl (polyvinyl chloride), or fiberglass. There are also composites of two materials (for instance, vinyl and wood) mixed together and formed or extruded like plastic. To achieve a certain look, manufacturers also offer vinyl or fiberglass frames with a thin veneer of wood on the inside (wood-clad vinyl). Others offer wood frames with a cladding of vinyl or aluminum on the outside for increased durability. The frame can account for about 15% of the energy loss through a window. Aluminum frames have high U-factors, unless they include a thermal break-a strip of urethane that interrupts the transfer of heat through the metal.

Wood, vinyl, and fiberglass are much better insulators than standard aluminum frames (without the thermal break). Of these, fiberglass performs slightly better than the rest and is also the most durable. You’ll find that vinyl and wood frames generally have similar U-factors. Some very expensive vinyl frames are filled with urethane foam insulation.

In double- and triple-pane windows, the panes of glass are separated by spacers. The spacers are traditionally made of aluminum, even in wood, vinyl, or fiberglass frames, creating greater conductivity around the window edges. This makes the windows colder at the edges in winter, and water vapor may condense there as it hits the cold surface. New warm-edge spacers are made from better-insulating materials, and are recommended for cold climates. The biggest advantage to warm edge spacers is that they reduce condensation around the edge of the window.


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