Central Air Conditioning: How It Works and What Breaks

Central air conditioning is one of those systems you don't think about until it stops working on the hottest day of the year. Understanding how it works makes it easier to spot problems early, describe them to a technician and know which repairs are minor and which are major.

How central AC moves heat

An air conditioner doesn't create cold air. It moves heat from inside your house to the outside. It does this with refrigerant, a fluid that absorbs heat as it evaporates and releases heat as it condenses.

A typical split system has two main parts:

• The indoor unit, which includes the evaporator coil, usually sitting on top of your furnace or air handler. Warm indoor air blows across the cold coil, the refrigerant absorbs the heat and cooled air flows back through your ducts.
• The outdoor unit, or condenser, which holds the compressor, the condenser coil and a large fan. The compressor pressurizes the refrigerant, and the fan blows outdoor air across the coil to release the heat.

Copper lines carry refrigerant between the two units. A thermostat tells the system when to run, and a condensate drain carries away the water the indoor coil pulls out of the air.

Parts that commonly fail

Some failures are small and quick to fix. Others are big-ticket repairs.

Smaller repairs, often:

1. Capacitors, which help start the compressor and fan motors. A failed capacitor is a common reason the outdoor unit hums but won't start.
2. Contactors, the electrical switches that turn the outdoor unit on and off.
3. Clogged condensate drains, which can cause water leaks or shut the system off.
4. Thermostat problems, including dead batteries or wiring issues.

Larger repairs, often:

5. Refrigerant leaks, which reduce cooling and must be found and repaired before recharging.
6. Fan motors, indoor or outdoor.
7. Evaporator or condenser coils, which can leak from corrosion.
8. Compressors, the heart of the system and usually the most expensive part to replace.

Warning signs

Call for service if you notice:

• Warm or weak airflow from vents.
• The system running constantly without reaching the set temperature.
• Ice on the refrigerant lines or indoor coil.
• Water pooling near the indoor unit.
• Buzzing, clicking or grinding from the outdoor unit.
• Breakers tripping when the AC starts.
• Higher electric bills without a clear reason.

If you see ice, turn the system off and switch the fan to "on" to help it thaw before the technician arrives.

Simple maintenance that helps

Most AC problems are made worse by neglect. A few habits go a long way:

• Change or clean your air filter regularly. A clogged filter restricts airflow and can cause freezing.
• Keep two feet or so of clear space around the outdoor unit. Trim plants and clear leaves.
• Gently rinse the outdoor coil with a hose if it's dirty. Avoid high pressure that can bend the fins.
• Pour a little water or a cleaner recommended by your technician into the condensate drain to check that it flows.
• Schedule professional maintenance before cooling season.

Our air conditioning coverage page explains what plans usually expect from homeowners.

Repair or replace?

When a major part like a compressor fails on an older system, replacing the whole outdoor unit, or the entire system, sometimes makes more sense. Factors include the system's age, its efficiency, the refrigerant it uses and whether other parts are near the end of their life. Ask your technician for both options in writing.

How home warranties treat central AC

Many home warranty plans cover central air conditioning components such as compressors, coils, motors, capacitors and thermostats. Common limits and exclusions include:

• Coverage caps per system or per claim.
• Exclusions for improper installation, lack of maintenance or pre-existing problems.
• Limits on refrigerant recovery or code upgrades.
• Ductwork may be covered separately. See ductwork coverage.

How Warranty Wizards can help

We help homeowners compare home warranty plans from third-party providers and explain how each one covers central air. We'll walk through limits and exclusions before you buy. Request a free quote.