How air conditioners work

Air conditioning has evolved from a luxury to a necessity for many in the past generation. Whole-house central air conditioners provide relief from the blazing heat and humidity of summer. Homeowners can choose from a variety of options to cool their homes, each with its own unique pros and cons.

An air conditioner, no matter what type, makes up half of one of the most complex and important systems in your home. Homeowners should understand the basics of their system, the types of maintenance they can perform on their own, how to spot imminent problems, and when to call in an expert for help. 

We understand how home heating works: Use energy to heat air inside a furnace, then blow it into the house. But how do you make air cooler?

The answer has to do with the physics of evaporation. When a liquid turns into a gas through evaporation, it absorbs heat. This is one reason you sweat when your body gets too hot: As sweat (a liquid) evaporates on your skin, it takes heat with it, cooling your body.

An air conditioner relies on the same principles but the evaporation takes place within a closed loop as a chemical compound is converted from liquid to gas and back to liquid in a continuing cycle. Known as a "refrigerant," the chemical compound easily converts to gas at relatively low temperatures compared to water, for example, which must be extremely hot to convert to gas.

The mostly widely known refrigerant, Freon, is actually a trademarked refrigerant combination owned by DuPont. Used widely up until recently, many refrigerants including Freon relied on chlorofluorocarbons, or CFCS. In residential heating and cooling, CFC-containing refrigerants are also known as R-22 type refrigerants. However, since CFC products were found to contribute to the depletion of the ozone layer, they have been phased out of the HVAC industry.

Today, most new residential HVAC systems rely on a special class of chlorine-free, non-ozone depleting and more environmentally friendly refrigerants. Known in the industry as R-140 refrigerants, Puron, EcoFluor, Genetron are some common brand names you may hear of.

What happens indoors:
If your furnace is in the basement or a utility closet, the central air components will be with it. The cooling process takes place within the main air handler unit, which houses the evaporator coil. The evaporator coil turns liquid refrigerant into its gaseous form, thereby creating evaporation which removes heat.

The central air handler blows air taken from the home's return supply duct over the evaporator coil, cooling it. The cooler air is then blown to the rest of the house, reducing the temperature.

What happens outside:
Once finished undergoing the evaporation process, the refrigerant (still in a gas state) is moved through the loop system to the outdoor air conditioning unit. Here, a compressor squeezes the volume of the gas to convert it back to a liquid, which allows it to cycle through the system again.

Just as evaporation removes heat, compression creates heat. That's why this part is outside. The heat is expelled from the outdoor system via a set of condensing coils. The big fan you see on top of the outdoor unit blows the excess heat out into the air.

Types of air conditioning systems

Air conditioners come in many types, ranging from window units to complex whole-house zoned systems. Here are a few of the basics:


The system most people have in mind when they think about residential air conditioning, this uses a central unit and a system of ducts throughout the house to push cool air to each room.


Ductless air conditioning systems provide another option for homes. Unlike forced-air systems, a ductless air conditioner uses an outdoor compressor unit to provide refrigerant, electricity and drainage into indoor systems mounted on the top of walls of the rooms to be cooled. Since cold air doesn’t get lost by moving through a traditional duct system, this method reduces energy usage by as much as 30 percent. On average, a ductless system costs between $3,000 and $5,000.

Variable-speed air handler

This isn’t a separate type of air conditioner, but rather an alternate means of moving the cool air around. In a standard forced-air system, the air handler operates at full blast, delivering air into the home at 100 percent capacity, turning off when it reaches the desired temperature, then activating again when needed. A variable-speed unit starts at full speed in the morning to quickly establish a comfortable temperature, then slows down but continues to operate continuously through the day to maintain the temperature. This saves energy by operating at lower speeds and preventing the need for repeated starts and stops, which require more electricity and add wear and tear to the system. 

Air-source heat pumps

If your home relies on cheap electricity rates, a high-efficiency heat pump may be a better choice, both in cost and energy efficiency, although it’s best suited for climates that don’t require round-the-clock cooling during warmer months. Used for both heating and cooling, an air-source heat pump condenses warm air and moves it either indoors or outdoors. Air-source heat pumps also tend to last longer, with average lifespans between 20 and 25 years.

A heat pump uses the principle of heat transfer, which relies on the difference between indoor and outdoor temperatures to provide heating or cooling to a home. In cooling mode, it acts much like an air conditioner, cooling by drawing heat out of a home and pumping it outdoors. In heating mode, think of a heat pump as an air conditioner that can go in reverse. It absorbs heat from the air, condenses it and pumps it into the home.

According to the Department of Energy, the heating efficiency of an air-source heat pump is determined by its heating season performance factor, or HSPF. It's a measure of the total space heating required during the heating season, expressed in BTUs, divided by the total electrical energy consumed during the same season, expressed in watt-hours.

Overall, they tend to require less energy than other kinds of cooling, and they're relatively comfortable. They tend to cost about $6,000 to install in the average home. Because many air-source heat pumps have difficulty working in extremely warm temperatures, they're best suited for temperate climates, and will usually require an alternate backup air conditioner.

Geothermal heat pumps

geothermal heat pump is one of the most energy-efficient methods of heating or cooling a home. Relying on a series of liquid-filled pipes, a home can be heated in cooler months by transferring and condensing energy from the 50- to 60-degree temperatures found just a few feet below ground. In the summer months, the process reverses, thereby cooling a home by removing heat. Underground temperature remains constant no matter the climate, so geothermal systems provide consistent and reliable heating and cooling.

Geothermal offers maximum efficiency while costing less to operate and lasting longer. Upfront installation costs are high, ranging from $10,000 to $30,000, depending on the site conditions, but some of these costs may be recouped through local, state or federal tax incentives.

Installation involves digging long trenches or deep holes to install the pipes, so it's labor-intensive on the front end. However, you'll save money over the long term because of the lower energy costs. Also, a geothermal system tends to last about twice as long as conventional air-source units. The lack of moving parts means you're less likely to spend big on repairs down the line. The buried loop system can be expected to last for 50 years. They're significantly more efficient then even the highest-efficiency air source units. 

Until recent years, geothermal systems required significant real estate to lay the pipes, but recent innovations in boring and drilling straight down or at an angle make geothermal systems plausible even for homes with very small yards. 

Don't underestimate those federal tax incentives, either. For systems installed through Dec. 31, 2016, homeowners can receive a 30 percent tax credit, so that $20,000 geothermal system translates to $6,000 on your next tax return. Various state incentives can offer even more immediate recoups of the investment. 

Geothermal pipes were traditionally laid horizontally (C), but recent innovations allow them to be installed diagonally (A) or vertically (B), requiring less real estate.

Measuring air conditioner efficiency

Just as the energy efficiency of heating systems can be measured as a ratio of heat created compared to energy used, a similar metric can be applied to cooling systems.

In this case the acronym is "SEER," which stands for "seasonal energy efficiency ratio." The SEER ratio of a cooling system is calculated by dividing the appliance's cooling output in BTUs for an entire season by its total electrical energy use for the same period. An air conditioner with a higher SEER number is more efficient than a device with a lower SEER number.

Recent years have seen great leaps forward in energy efficient models. An air conditioning system installed in 2000 may have a rating of only six to 10 SEER. Currently, all units sold in the United States are at least 13 SEER, with some models as efficient as 27 SEER. Improved SEER ratings bring lowered energy costs and cooling times, but they come with a corresponding cost increase. The hotter your local climate, the more likely a higher SEER rating will benefit you and justify its cost. 

When shopping for a new air conditioner, look for the "Energy Star" label. Although they're usually more expensive than their non-label counterparts, Energy Star cooling appliances meet higher energy efficiency standards, saving you money in utility bills in the long run.

Various regions and the federal government provide their own guidelines for HVAC efficiency. Find more details on HVAC standards here


You can’t prevent all heating and air conditioning problems, but paying proper attention to regular maintenance and having the air conditioner inspected by a professional at least once per year can head off many problems before they start.

Change your air filters regularly. Air conditioning professionals recommend this constantly, and they’re not exaggerating — it’s as important as a regular oil change in your car. A dirty filter forces the system to work harder, run longer, consume more energy and ultimately shorten its life through wear and tear. Furthermore, a fresh, clean filter more effectively traps contaminants and debris in the air. Energy Star recommends replacing your filter once every three months. During high usage months, check the filters at least monthly. You should also make sure your outdoor unit is free of obstructions such as grass, growth or vines, as anything impeding airflow will adversely affect operation.

Call in your HVAC contractor at least once a year, preferably in the spring, to test your AC system, clean and adjust it. This work tends to cost around $99.  A proper inspection doesn’t just look for problems. The contractor should also examine and wax the outdoor condensing unit, measure temperature and airflow values and adjust them to the manufacturer’s specifications. You should alsoconsider a maintenance contract with an HVAC company, which typically runs between $159 and $199 per year and covers visits for both the heating and cooling sides. This ensures the same company will be checking your system every time, keeping records over the years, and often gets you preferred service and discounts when something breaks.

Should I repair or replace my air conditioner?

No matter what, even the best-maintained air conditioner will eventually fail. HVAC professionals recommend following this basic rule when deciding whether it's worth investing in a repair: If the unit is more than 6 years old, and the repair will cost more than half the price of a new system, it's best to buy new. Energy Star recommends replacing an air conditioner that’s more than 10 years old, even if it’s still working. The system won't have many more years left in it, and it's likely to be using more energy than necessary. 


When you’re buying a new air conditioner, you need to take into account both “first cost” – the cost you pay up front on the price tag – and “lifetime costs,” such as the factors that affect its output and energy usage for years to come. A good HVAC contractor can help you sort out the details and select the best possible unit.

AC units are measured in tons, which refers to the amount of heat they can remove from a home in one hour. The larger your house, the more cooling power you need; however, more than just square footage goes into this calculation. A good AC service professional will take into account room heights and other variables that determine the most appropriate size for your house.

Air conditioner prices vary greatly based on these variables. For a basic, two-ton model, expect to pay $3,000; a mid-range unit will run approximately $5,000 and top-of-the-line air conditioners can creep up over $10,000.

When you hire someone to install a new system, considering these four areas:

Sizing: A professional can calculate the home’s cooling load and choose the most appropriate size for your air conditioner.

Ductwork: Leaky dusts will waste energy and result in a less-cool home. Installers should use mastic sealant or metal-backed duct tape to make the ductwork airtight.

Airflow: Each system has an appropriate airflow rate determined by the manufacturer. If the air moves through the system too fast or too slow, it will be less effective.

Refrigerant: Installers will consult manufacturer requirements to determine the ideal amount of refrigerant necessary. If it’s not properly charged, you lose cooling capacity and end up with higher energy costs.

You can also retrofit central air conditioning into a house that currently lacks it. 

Hiring and how to avoid scams

How to avoid air conditioning scams

Because air conditioners are such crucial parts of a home and repairs can involve large sums of money, unscrupulous contractors sometimes target homeowners with unnecessary work or bait-and-switch schemes. Follow these tips to minimize the chance of unpleasant surprises:

Check their paperwork

Ensure that a company is licensed, bonded and insured before you let them near your system. 

Ask for references

An HVAC contractor will have a list of recent customers you can contact to verify that the work was done well and in a timely manner.

Know your contractor

The most effective way to protect yourself is to build a long-term relationship with a contractor you trust. Good HVAC providers will arm their clients with as much information as possible to make an informed decision. Effective contractors will also provided detailed documentation of their work. Also, regular maintenance from the same contractor means you’re less likely to suffer a sudden, catastrophic breakdown that leaves you vulnerable to a scam.

When in doubt, get a second opinion

No reputable HVAC company will object to your asking for a second opinion, especially when thousands of dollars are involved. Similarly, be extremely skeptical of a contractor who tells you that you must commit to major work right away, without a chance to think it over. Be wary of scare tactics – if a contractor is making urgent appeals to the safety of your family or property, don’t agree to work right then. Discontinue use of the system until you verify with a second opinion. 

Other things you need to know

Since air conditioning usually requires large, unsightly exterior units, homeowners can choose from several options to make them a little more aesthetically pleasing. Landscaping features near the units such as ornamental grasses or shrubs can block the view of the unit, although you should leave at least three feet of empty space around the unit to maintain smooth airflow and allow service technicians to access the unit. A fence built around the unit can achieve the same effect.

You also want to be sure the unit has at least five feet of clearance above it. HVAC contractors recommend not using fitted cloth or vinyl covers, which can impede air circulation. Additionally, if you place materials around the AC to cover the bedding, rock is a better choice than mulch, as mulch can get caught in the fan and damage the unit.

Reaching peak energy efficiency

You can improve your comfort, extend the lifespan of your system and lower energy bills by ensuring optimal air conditioning efficiency. That yearly maintenance appointment with your HVAC company is a crucial part of this, as they can make sure no ducts are leaking, no grills are poorly placed, and no settings on the system are incorrect. Maintaining the exact level of optimum refrigerant also has a big effect.

Regularly cleaning your AC coils and making sure that filter gets replaced on a regular basis will also boost efficiency. Insulate air conditioning ducts and seal leaks with caulking and weather stripping. Any ceiling fans in your home should be switched to turn counterclockwise, pushing cool air down from the ceiling. Window film, tint or even heavy curtains can block bright sunlight and require your system to work less hard. 

Like what you see?

Become an Angie's List member now and get all our amazing content on all your devices.

Join Today
Furnaces & Home Heating SystemsAir ConditioningHVAC Maintenance
You got three bids for furnace or A/C replacement, but you’re not sure which is best? Ask yourself and contractor these four questions to pick the right one.
Indoor Air QualityFurnaces & Home Heating SystemsAir Conditioning
If you're looking to give your home's air quality a boost, consider a HEPA filter.
Air ConditioningHVAC MaintenanceFurnaces & Home Heating Systems
Are there any savings to be had with a smart thermostat?
Air Conditioning
Your A/C Unit should last for 15 or more years. But a neglected unit can konk out after seven years. HVAC experts list ways you may be killing your unit.
Air ConditioningEnergy EfficiencyHVAC Brands
The price tag on a high-efficiency A/C may not reveal the lifetime savings on energy costs.


Local Offers <
Trending <