clock menu more-arrow no yes mobile

Want Central Air Conditioning? Read This Before You Install

Tired of your window AC unit? Here’s what you need to know about how central AC systems work before investing in cooling your entire house.

an air conditioning unit iStock

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.

Central Air Conditioning System Diagram Ian Worpole

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.

Read this guide to walk 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.

Central Air Conditioning: Common Questions to Consider

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.

How much 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 for a replacement?

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 Does Central Air Conditioning Work?


Central Air Conditioning System Diagram: Ducts Illustration by Ian Worpole

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.


Central Air Conditioning System Diagram: Registers Illustration by Ian Worpole

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.

Air Handler

Central Air Conditioning System Diagram: Air Handler Illustration by Ian Worpole

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.


Central Air Conditioning System Diagram: Condenser Illustration by Ian Worpole

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.

Better Efficiency & Greater Comfort

Temperature Control Line Graph

Recent advances in compressor- and blower-motor technology have achieved unprecedented improvements in whole-house comfort.

Electronically Commutated Motor

A blower with an electronically commutated motor (ECM) gradually ramps up to speed, reducing strain on the motor and consuming less power. In zoned systems, it dials back airflow to eliminate worries about frozen coils or a buildup of static pressure in the ducts. When paired with an inverter-controlled compressor (below), it helps to smooth out temperature fluctuations and reduce humidity.

Inverter-Controlled Compressor

Not so long ago, all AC compressors operated at one speed, no matter how much cooling was needed. The result: high operating costs, big temperature swings, and clammy indoor air due to poor humidity removal.

Two-speed compressors did a better job, but the real performance breakthrough came with the arrival of the inverter-controlled compressor, which varies its speed to meet the changing demand for cooling. The temperature stays closer to the thermostat setting, and the system wrings more humidity out of the air.

Size Your Air Conditioning System Right

“Beware of installers who specify your new system’s cooling capacity based on a rule of thumb, or ‘what you had before.’ What they should do is perform a Manual J load calculation, which determines system size based on the local climate, window orientation, and insulation levels, among other factors. Without a Manual J, you could end up with a bigger, more expensive system than you need, one that’s costly to operate and cycles on and off so often that it can’t bring down the humidity to a comfortable level.”Richard Trethewey, TOH HVAC expert

Heat Pump: Cold + Heat

Heat Pump System Illustration Illustration by Ian Worpole

Despite its name, a heat pump is able to cool houses as well as warm them up. The equipment looks like a straight AC system, and works the same way in summer. In winter, the refrigerant flow reverses and carries heat indoors. Heat pumps are best suited to climates with hot summers and cold (not frigid) winters. Highly efficient, they can slash annual heating and cooling costs dramatically.

Air Conditioning Terms to Know

Energy Guide Sticker


The Seasonal Energy Efficiency Ratio indicates how well a system converts electricity (watts) into cooling (Btus) during a typical cooling season. The higher the number, the better the efficiency. Now, units sold in northern states have a minimum SEER of 13; in southern states, 14. Currently, the top SEER for a split system is 26.


The Energy Efficiency Ratio is the amount of cooling a system delivers for each watt, minus seasonal averaging. In hot, dry climates, 12.2 is the minimum EER for units of 3.5 tons or smaller.

Energy Star

Identifies equipment with a SEER rating of 15 or higher. This designation is often required in order for a system to qualify for tax and utility rebates.


Measures a system’s cooling capacity: the number of Btus removed per hour (12,000) to keep a ton of ice from melting in a day. Most residential units are rated for 3 tons.


The Minimum Efficiency Reporting Value rates how effectively a filter captures dust, germs, and odors. The higher the number, the more airborne particles a filter can remove.

Zoned Air Conditioning Systems

Family Eats In Kitchen That Has Zoned System Air Photo by Hero Images/Getty Images

Zoned systems channel cool air to individual rooms and floors, offering greater comfort and efficiency than a single-zone system can deliver.

In most homes, one thermostat controls the temperature everywhere in the house. But such single-zone systems, as they’re called, can’t respond if far-flung rooms are broiling hot, freezing cold, or not occupied. These problems can be corrected by creating separate zones in which additional thermostats work with mechanical dampers to direct air where it’s needed most.

Done right, zoning provides unparalleled room-by-room comfort and can reduce electric bills by as much as 30 percent. Done wrong, it can lead to whistling registers, frozen evaporator coils, shorter equipment life, and higher electricity use. Using specialized pieces of equipment like the ones below helps to avoid those problems.

Zoned Systems: In Duct Damper And In Room Retrofit

In-Duct Damper

On command from a thermostat, a motor opens and closes a metal flap—fully or partway—to control how much air goes into a zone. A central hub monitors airflow throughout the system to prevent a buildup of static pressure.


In-Room Retrofit

This battery-powered register opens and closes in response to smartphone-enabled sensors that set the temperature in each room. The sensors and registers work together to maintain optimum system airflow.


Best Smart Thermostats


These little control modules play a crucial role in ensuring that HVAC equipment performs at its best.

(Not So) Smart

Programmable thermostats automatically turn the temperature up or down at preset times. Setbacks of 10 degrees for 8 hours can achieve savings of up to 10 percent. Good for households with predictable schedules, but many people don’t bother programming them anyway.

Shown: Focuspro 6000, $49


Thanks to occupancy sensors and geofencing features that detect our comings and goings, these devices reset temperatures automatically, saving up to 15 percent when cooling. They’re smartphone enabled, so you can monitor and adjust them remotely.

Shown: Nest 3rd generation, $249


They have many of the features of a smart thermostat, but these proprietary devices also communicate with a company’s equipment to maximize humidity removal, air quality, and system efficiency, tasks that run-of-the-mill smart ’stats can’t handle.

Shown: iComfort S30, starting at $450

Need New Ducts?

Duct Board, Flex Duct, Sheet Metal Duct Photo by Meg Reinhardt

Adding AC to a house that already has ducts is relatively straightforward. But when you need new ducts, you have three types to choose from.

Good: Duct Board

These 10-foot-long, foil-wrapped fiberglass panels are cut, folded, and taped to form four-sided ducts.

Pros: Needs no added insulation. Lightweight and doesn’t sag. Quiet.

Cons: Requires an interior liner to stop bits of fiberglass from coming loose during cleaning or as its binders break down. Even so, the liners trap dust and allow microbes and moisture into the fiberglass. Costs more than flex duct, less than sheet metal.

Better: Flex Duct

This big version of a dryer duct has a vinyl skin supported by a spiral wire. Available in 25-foot lengths.

Pros: Bends with ease. Comes insulated up to R-8. The only joints are at the ends, so installation is fast. Connections are made with zip ties and duct sealant. Inexpensive.

Cons: Kinks and sags easily, choking off airflow if not installed properly. The ridges made by the wire slow and disrupt airflow. Runs should be confined to 10 feet or less.

Best: Sheet Metal

Galvanized steel, bent and fastened into its final shape, is the gold standard of ducting. Comes in 5-foot lengths.

Pros: Its smooth, rigid walls won’t sag and don’t interfere with airflow. Will last as long as the house remains standing. Easy to clean.

Cons: A good installation depends on skilled sheet-metal workers, who are in short supply. All runs have to be wrapped in insulation to minimize condensation and heat gain. Expensive.

Installation Issues

Man Uses Mastic Adhesive On Air Duct

Leaking ducts, or ones in the wrong place, rob you of comfort, clean air, and optimal system performance.

An average home loses 25 percent of its conditioned air through duct leaks. Take these steps to get rid of them.


Hire an HVAC technician to hook a pressure-monitoring blower to your ducts. The blower can also help pinpoint where the leaks are.


Apply a thick coat of mastic over the joint (left) and reinforce it with fiberglass mesh tape. Foil-backed butyl tape is a less messy option. (Despite its name, duct tape should never be used on ducts.) For hard-to-reach leaks, there’s Aeroseal. This treatment blows a polymer aerosol into the ducts; the polymer collects around small gaps and holes, plugging them up.

Test again

A leakage rate of 10 percent is acceptable. If higher than that, go back to Step 2.

Pro Advice: Keep ’em inside

“When ducts run through an uninsulated attic or crawl space, all that expensive conditioned air inside then warms up, and the likelihood of condensation collecting on the duct walls skyrockets.” —Richard Trethewey

Retrofit Options


Mini Duct In Room Photo by iStock

Flexible, 31⁄2-inch-diameter tubes slip easily through stud and joist bays, delivering high-velocity air through unobtrusive 2-inch openings. The special air handler that generates this high-velocity air dehumidifies and mixes room air so effectively there’s no need for return ducts in every room; a single, centrally located return is sufficient. Yet the system can still be zoned.

The Unico System


Mini Split AC System Photo by

There’s no central air handler with these systems (also known as mini-splits). Instead, refrigerant travels from an outdoor unit through bundles of skinny tubing to an indoor room unit, like the one above. Each unit has a fan, an air filter, a condensate drain, and separate controls, so each room becomes its own zone. Mini-splits can be superefficient; some have SEER ratings as high as 31.

Mitsubishi Electric

Home AC Filters

Types Of AC Filters Courtesy of (left to right) Purolator, Honeywell, Trane CleanEffects, Lennox

Inefficient filters won’t catch much, but too high an efficiency can choke airflow and strain the blower. If you upgrade, make sure the new one is system-compatible.

1-inch Filter

Standard fiberglass-mesh filters stop little more than dust bunnies. Filters with pleated fabric, left, do a better job, up to MERV 13. Replace every month.

Shown: PAF 11 Purolator

Deep-pleated Media Filter

Ranging from 4 to 6 inches thick, they have MERV ratings from 8 to 16. Their greater surface area allows more air to pass and reduces replacement frequency to every six months.

Shown: FC200E Honeywell

Electronic Air Cleaner

They give particles an electrical charge, then trap them on a plate with the opposite charge. Just vacuum or wash the plate twice a year. These cleaners do produce ozone, a lung irritant. Look for ones below the FDA’s indoor ozone limit of 50 parts per billion.

Shown: Trane CleanEffects

Photocatalytic Air Cleaner

A deep-pleated media filter captures particles, while a UV light shining on a titanium dioxide catalyst destroys mold, viruses, and bacteria as it neutralizes odors and VOCs. Replace both the filter and light once a year.

Shown: Lennox PureAir

Air Conditioning Service

HVAC Technician Inspects AC System Photo by Keller & Keller

To get optimal AC performance and longevity, have an HVAC technician inspect the following once a year.

Refrigerant lines

A pressure test will indicate whether more refrigerant needs to be added. If so, it means there’s a leak, which will have to be plugged first.

Condensate pan and drain

They have to be cleaned and blown out to prevent clogs and spills.


To ensure enough airflow to cool the coils, the fins should be vacuumed, and shrubs or trees closer than 3 feet away should be trimmed back.