Everything You Need to Know About How Central AC Works

Everything You Need to Know About How Central AC Works
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Sweltering through another summer parked in front of a window AC unit? Here’s what you need to know before investing in a whole-house system

Illustrations by Ian Worpole

Central AC Today

More than any other technology, air-conditioning has transformed how we live and work—and where we live and work. Yet as recently as 1993, only 68 percent of houses in the United States had some cooling equipment, either window air conditioners or central AC systems. Now, more than 87 percent of houses are so equipped. While the number with window AC units has shrunk overall, these appliances are still cooling significant numbers of homes, particularly in the Northeast, despite being noisy and obtrusive, and having limited efficiency and cooling capacity. Not to mention the twice-yearly struggle of putting them in and taking them out.

By comparison, central AC is a giant leap forward in convenience, quiet, and, most of all, comfort. Properly sized and installed, a split system—one with indoor and outdoor equipment—can keep an entire house nicely chilled on the hottest days, and do so far more efficiently than a battalion of window units.

On the following pages, we walk you through the factors that affect how these systems function. So whether you’re planning to deep-six the window ACs and install central air from scratch, or you’re investing in an HVAC upgrade, you’ll find loads of useful information to help you make smart decisions.

The basicsAnswers to frequently asked questions

Who does the install?You need a pro familiar with these complicated systems. Get quotes from at least three local HVAC contractors with good reputations and stellar online reviews. The company you choose could very well be maintaining your system for years to come. Affiliation with the Air Conditioning Contractors of America is a plus; an ACCA certificate in residential HVAC design is even better. Also make sure you’re satisfied with the equipment a contractor sells; most have affiliations with particular manufacturers.

What does it cost?Many variables affect the system price, including the local climate, existing insulation, labor costs, and equipment size and efficiency. Here’s a rough estimate of the cost to upgrade a 3-ton residential system in Des Moines, IA, to one with an Energy Star certification and a SEER 16 efficiency rating: $5,000, including installation.

How long does it last?Typical equipment warranties last for 10 years. If properly maintained, these systems should function for about 15 years before needing replacement.

When is it time to replace?If a system is about 15 years old when an expensive component, like an evaporator coil or a compressor, fails, it’s probably time for new equipment indoors and out. Replacing one but not the other will likely hobble system performance.

How It Works: Ducts

Simply put, split cooling systems pull heat out of a house with refrigerant that circulates between the condenser and the air handler

Supply ducts distribute cool air from the air handler to the rooms in the house. Return ducts carry warm air back to the air handler to be filtered and cooled. When properly sized, ducts deliver conditioned air evenly and quietly throughout the house.

How It Works: Register

Each room needs at least two of these grilles: one connected to a supply duct, and one to a return. Ideally, supply registers should be located on or near the ceiling, and return registers should go near or on the floor.

How It Works: Air Handler

This indoor equipment has an expansion valve that turns refrigerant into a cold liquid that flows through the evaporator coils. A blower pushes air over the coils, which warm the refrigerant into a gas that goes back to the condenser.

How It Works: Condenser

This outdoor equipment takes warm gaseous refrigerant from the house, pressurizes it with a compressor, and condenses it back into a liquid as a fan cools the coils.

For more information about thermostats, go to thisoldhouse.com

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