How To Install A Central Heat And Air Unit

Whether summers are really getting hotter or we've lost our tolerance for heat as we grow older, lots of us are tempted to yank out noisy, dripping window air conditioners and replace them with quiet, efficient whole-house central-air-conditioning systems.

But we don't take the idea any further. Those of you with forced-air furnace ducts in place worry that modifying the heating system will be costly and result in inefficiency. And if you don't have ducts, you can't imagine how they're going to be added without destroying the house and costing a bundle. But adding central air is easier and less costly than you might think.

How Much Does it Cost to Install Central Air?

When added to an existing forced-air heating system, central air for a 2,000-square-foot home costs $3,500 to $4,000 and can be done by two technicians in two to three days, often with little or no change to the ducting.

For a house that needs ducts, the costs and work time double. But a contractor experienced in retrofitting can cleverly hide ductwork behind walls, in the back of closets and up in the attic with minimal cuts into walls and the ceiling and very little mess.

How to Select the Best Central Air Unit

The first order of business — and maybe the most important decision in having central air conditioner installed in your home — is selecting the right equipment.

Heat-gain calculation

All cooling contractors should start their evaluation of your house by performing an Air Conditioning Contractors of America Manual J load calculation. This determines the heat gain your home is subject to. The calculation is relatively easy to perform and will reveal what size unit you need. Move on to another contractor if the one you're considering wants to determine the right unit for your house solely on experience. Though strongly influenced by where you live, several factors affect the size unit required, including the amount of wall and attic insulation you have; the types and placement of windows and doors; and the orientation of your home to the sun. The calculation can also alert you to the possible benefits of upgrading the insulation in your house. By making your home more energy efficient, you may be able to reduce the size of the air conditioner you need.

How Big of a Central Air Unit Is Needed?

Residential heating and air conditioner compressor units near suburban house.

Take these steps when determining how big of a central air unit is needed for your home's size.

Sizing the unit

Using the heat-gain calculation, your contractor will recommend an air conditioner size, expressed either in tonnage or Btu per hour (Btu/h). One ton equals 12,000 Btu per hour. Why is getting the proper size unit so important?

An undersize unit won't be able to cool rooms down completely on the hottest days and will cost more to operate because it has to run longer than a correctly sized unit. An oversize compressor also costs more to operate because it simply takes more electricity to run a bigger unit. And an oversize unit doesn't lower humidity effectively. That's because it cools the air so quickly that it shuts off before it has a chance to circulate the proper volume of air past the coils to extract the necessary moisture. The result is a room that doesn't seem as cool as the temperature indicates. In fact, the room can feel clammy and damp.

Unit efficiency

After determining the size of the unit, your contractor should talk to you about efficiency, which is expressed by the seasonal energy-efficiency rating (SEER). The SEER rates how many Btu an air conditioner will remove for each watt of electricity it consumes. The higher the SEER, the less it costs to operate.

Federal law requires that new A/C units have a SEER of at least 13. These units have lower operating costs, tend to be higher quality, have more safety features, better sound shields and lower voltage requirements. In general, the higher the SEER, the higher the initial cost for the unit.

Unit types

Next, you must decide what type of system to buy. There are two kinds. A "package system" gangs the condenser, which cools the refrigerant and exhausts warm air, with the fan-and-coil system, which cools and blows the air. The ducting connects directly to the unit. This is essentially a large wall air conditioner with ducts. But package systems are rare.

With a "split system," the condenser is outside the house and the fan-and-coil system are inside; they're connected to each other by pipes that carry refrigerant. If you have a forced-air furnace, the refrigerant pipes are connected to a cooling coil system fitted into the furnace air handler.

Sometimes it goes right into the existing plenum. If that won't work because of space limitations, the contractor will fabricate a separate sheet-metal plenum. If you don't have forced air, the fan-and-coil system is typically placed in the attic, where it will deliver cool air through ducts. The 20- to 30-foot-long pipes that carry the refrigerant are disguised to look like a downspout.

Proper placement

Even the quietest condensers make noise, so work with your contractor to find a location that's not near a bedroom or home-office window. Don't place the condenser under a deck or completely enclose it because it exhausts warm air out the top. Any airflow restriction will lower the unit's efficiency. You can, however, hide the condenser in the landscaping, as long as air can freely circulate around it.

No Ducts? No Problem

Most central air conditioners are split systems: The condenser is outside and the fan-and-coil system is inside, connected to the condenser by pipes that run up the outside of the house. The pipes can be disguised as part of the gutter-and-downspout system. Ductwork services second-floor rooms through ceiling registers. Ducts run through closets on their way to first-floor rooms.

Delivering the Air

Furnace ducts that deliver hot air in cold months can be used for air-conditioning.

If you have ducts

You should have your ducts inspected. Many times, modifications are required to accommodate the greater volume of air produced by air-conditioning. Ironically, older duct systems, which tend to be larger, work best when retrofitting central air-conditioning. (As the heating industry has learned more about the dynamics of airflow and sizing furnaces and delivery systems, ducting has gotten smaller.)

Here's a list of typical modifications that existing ducts require:

Upsizing the furnace blower (rated in cubic feet per minute, or cfm) to move the cubic feet of cool air required for your house. An undersize blower won't move enough air through the coils, and they can freeze.
Sealing the ducts to boost efficiency. This is a job best left to a pro, as an imbalanced system can lead to hazardous backdrafting during the heating season or pockets of warm air during the cooling season.
Swapping out older supply registers for ones that allow a greater volume of air to pass. Old register grilles with slits just 1/4 inch wide offer too much resistance to airflow; new grilles can improve airflow by 15 to 25 percent.

If You Need to Install Ducts

Because the fan-and-coil unit is mounted in the attic in most retrofit air-conditioning systems, the challenge is to get supply and return ducts to ceiling registers in the first- and second-floor rooms below.

Ducts that feed second-floor rooms are typically run across the attic floor and plunge down between the attic floor joists, where they are connected to ceiling registers. Ducts that feed first-floor rooms run down through second-floor closets wherever possible.

The first step your contractor will take to determine the position of ducts is to draw a floor plan of the second floor and lay it over a floor plan of the first floor. "In 99 percent of homes," says Frank Scarangello, of Staten Island, New York-based Scaran Heating and Air Conditioning, "second-floor closets will give you access to locations for first-floor ceiling registers."

Ducts running in closets take up less space than you might think. Because most ducts are 12x6 inches or 10x8 inches, even in a relatively small 2x4-foot closet, they take up just 4 of the 64 available cubic feet. Some cooling contractors might suggest using "flex duct" (a small, inexpensive flexible hose) instead of rigid square ducts. But avoid flex duct in active closet spaces — it doesn't stand up well to wear and tear and is easily punctured.

What kind of mess can you expect if you have to have ducts installed? In most cases, wall and ceiling surfaces are left largely untouched. Holes do have to be cut in first- and second-floor ceilings for registers, and some second-floor closet floors will undergo surgery to accommodate the new ducts. But these holes are relatively small, and the mess is easily contained.

Even if there's no ductwork in your home, if air-conditioning will make your house more pleasant in the summer months, contact HVAC contractors to review the cost of adding A/C and the methods they would use. Choosing the right pro and equipment will ensure you years of comfort, and it's often less expensive and less intrusive than you might imagine.

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