HVAC · Lesson 5 of 13Intermediate8 min read

Air Conditioners

A central air conditioner is the refrigeration loop split in two: a hot half outside, a cold half inside, joined by two copper lines. This lesson walks through both halves, the water they make, how efficiency is rated, and the rules that govern the refrigerant.

A split system

Most central air conditioners in U.S. homes are split systems. The outdoor condensing unit holds the compressor, the condenser coil wrapped around its sides, and a big fan on top that pulls air through the coil and blows it upward. It also holds the electrical parts that start the compressor and fan, such as a contactor, which the thermostat’s 24-volt cooling signal switches on.

Inside, the evaporator coil usually sits on top of the furnace, or inside an air handler, in a box called the plenum. The furnace’s or air handler’s blower moves the air across it. A copper line set joins the two halves: a larger, insulated suction line and a smaller liquid line.

Other setups use the same loop. A packaged unit holds everything in one cabinet outdoors, connected to the ducts. A window or portable air conditioner is a tiny packaged unit. Ductless mini-splits are covered in Lesson 6.

A split-system central air conditioner. The indoor coil and the outdoor unit are the two halves of one refrigeration loop. Sources: U.S. Department of Energy, Energy Saver 101: Home Cooling; ENERGY STAR maintenance checklist.

Air conditioner parts and what they do
Part	What it does	Signs of trouble
Condensing unit	The outdoor cabinet with the compressor, condenser coil and fan.	Silent when it should run; humming; a fan that doesn’t spin.
Contactor and capacitor	The contactor is a heavy-duty switch the thermostat closes; capacitors help start the motors.	Humming with no start; a technician tests them.
Evaporator coil	The cold indoor coil that absorbs heat and condenses water from the air.	Ice, dirt, weak cooling.
Blower	In the furnace or air handler; pushes air across the coil and through the ducts.	Weak airflow, noise.
Line set	Two copper lines carrying refrigerant between the halves.	Torn insulation, ice, oily spots.
Drain pan	Catches the water that drips off the coil.	Rust, cracks, standing water.
Condensate drain	Carries the water outdoors, to a floor drain or to a condensate pump.	Water around the indoor unit; a musty smell.
Filter	Keeps dust off the coil and blower.	Dirty filters cut airflow and can lead to ice.

Where the water goes

Because the indoor coil runs below the dew point of the air crossing it, water condenses on its fins all summer long (Lesson 2). It drips into the drain pan and flows down the condensate line. You can often find the end of that line outside near the foundation, dripping steadily on a humid day, or near a floor drain in the basement. Where gravity can’t do the job, a small condensate pump lifts the water away.

If the drain line clogs, water backs up and overflows. ENERGY STAR notes that a plugged drain can cause water damage in the house and affect indoor humidity, so a technician checks it at each tune-up. EPA advises keeping drip pans clean and drain lines unobstructed and flowing. Your system’s manual describes any drain care the manufacturer recommends.

Efficiency ratings: SEER2 and EER2

A central air conditioner’s efficiency is rated two ways:

SEER2, the seasonal energy efficiency ratio: the cooling it delivers over a typical cooling season divided by the electricity it uses, in BTU per watt-hour.
EER2, the energy efficiency ratio: its efficiency at a single, hot peak condition, when it’s working hardest.

The “2” marks a change. On January 1, 2023, the Department of Energy began requiring a revised test procedure designed to better reflect real-world installation and use. Ratings under the new test (SEER2, EER2 and, for heat pumps, HSPF2) aren’t directly comparable to the old SEER, EER and HSPF numbers, so compare like with like. The same date brought new minimum efficiencies that depend on where you live:

DOE minimum efficiency for central air conditioners, from January 1, 2023
Region	Split systems under 45,000 BTU/h	Split systems 45,000 BTU/h or more
North	13.4 SEER2	13.4 SEER2
Southeast	14.3 SEER2	13.8 SEER2
Southwest (AZ, CA, NV, NM)	14.3 SEER2, plus an EER2 minimum	13.8 SEER2, plus an EER2 minimum

How the regional rules apply

In the North, the rule follows the date a unit was made: one built before 2023 to the old standard can still be installed. In the Southeast and Southwest, it follows the installation date for split systems: anything installed from January 1, 2023 on must meet the new minimums. In the Southwest, units must also meet an EER2 minimum (11.7 for smaller split systems with a SEER2 below 15.2) so they hold up on the hottest afternoons. ENERGY STAR certified models beat these minimums.

Refrigerants and the law

Certification. EPA’s Section 608 rules require anyone who attaches hoses and gauges to an air conditioner, adds or removes refrigerant, or otherwise opens the refrigerant circuit to be certified. There are four types: Type I for small appliances, Type II for high- and very-high-pressure equipment (which includes home central air conditioners and heat pumps), Type III for low-pressure equipment, and Universal for all of them. Technicians earn certification by passing an EPA-approved test given by an EPA-approved certifying organization, and the certification doesn’t expire. Apprentices may do the work only under the close, continual supervision of a certified technician.

No venting. It’s illegal to intentionally release refrigerant while servicing, repairing or disposing of equipment.

The R-22 phaseout. Since January 1, 2020, no new R-22 has been made in or imported into the U.S. EPA does not require homeowners to replace working R-22 systems, and recycled or reclaimed R-22 is still available for service. There are no “drop-in” replacements for R-22, and EPA doesn’t require converting old units to another refrigerant.

Lower-GWP refrigerants. Under EPA’s Technology Transitions rule, new residential and light commercial air conditioners and heat pumps must use refrigerants with a global warming potential of 700 or less. For self-contained units such as window and portable air conditioners, that applies to products made or imported from January 1, 2025. For split systems, it applies to systems installed from January 1, 2025, except systems whose components were all made or imported before that date: a May 2026 EPA rule removed the deadline for installing that older inventory. Existing systems can keep running and can be serviced and repaired for their whole useful life.

You canWhat you can safely do

Change or clean the filter on schedule (Change a Furnace Air Filter).
Keep leaves, grass clippings, plants and stored items away from the outdoor unit, following the clearances in its manual.
Know where your condensate line ends, and glance at it on humid days: steady dripping in summer is normal.
If the system won’t start, check the thermostat setting and the breakers (Reset a Tripped Circuit Breaker).
Schedule a cooling checkup in the spring, as ENERGY STAR suggests.

StopWhen to call a pro

Anything involving refrigerant: low charge, leaks, ice that keeps coming back after you’ve changed the filter. Ask for the technician’s Section 608 certification, as EPA suggests.
A humming outdoor unit whose fan won’t spin, or a breaker that trips again: turn the cooling off at the thermostat and call. The outdoor unit holds 240-volt circuits and parts that start motors; leave its panels closed.
Water overflowing from the indoor unit, especially near electrical parts.

Check yourself

Tap an answer to check it, or open “Show the answer.”

Which half of a split-system air conditioner holds the compressor?
Show the answer
The outdoor condensing unit. The compressor, condenser coil and fan live in the outdoor unit.
Why does an air conditioner produce water?
Show the answer
The indoor coil is colder than the air’s dew point, so vapor condenses on it. The coil runs below the dew point, so moisture condenses on it and drains away.
How do SEER2 ratings compare with older SEER ratings?
Show the answer
They come from a revised test procedure required since 2023 and aren’t directly comparable. DOE’s 2023 test procedure better reflects real installations, so compare SEER2 with SEER2.
Who may attach gauges to a home air conditioner or add refrigerant?
Show the answer
A technician certified under EPA Section 608, or an apprentice under close supervision. EPA requires Section 608 certification for that work; Type II covers home central air conditioners and heat pumps.
What changed for R-22 on January 1, 2020?
Show the answer
New R-22 could no longer be made in or imported into the U.S.. Production and import ended; existing systems can still run and be serviced with recycled or reclaimed R-22.

Go deeper

U.S. EPA: Homeowners and Consumers: Frequently Asked Questions (the HCFC-22 phaseout) EPA’s answers for homeowners about R-22 and refrigerants.
U.S. EPA: Technology Transitions HFC Restrictions by Sector EPA’s current refrigerant restrictions by type of equipment.
U.S. Department of Energy: Energy Conservation Standards for Central Air Conditioners and Heat Pumps, Frequently Asked Questions (October 2022) DOE’s FAQ on the 2023 standards and SEER2.
ENERGY STAR: Maintenance Checklist ENERGY STAR’s maintenance checklist.

Sources

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A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.