Angie's LIST Guide to
Air conditioning

A generation ago, whole-house air conditioning was a luxury, but now we think of it as a necessity. Learn how your home AC system works and how to keep it humming along efficiently.



How air conditioners work

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.

central air conditioning graphic

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.

Measuring AC 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.

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.

Air-source heat pumps

If you live in a climate that doesn't require round-the-clock cooling during warmer months, a heat pump may be a more cost-effective and energy efficient option.

Air-source heat pumps as cooling systems

Powered by electricity, heat pumps can provide both heating and cooling operations.

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.

Heat pumps cost less than centralized air-conditioning units and use considerably less energy. However, this method is only effective where outdoor temperatures do not get extremely hot.

Geothermal heat pumps

Geothermal air conditioning

Installing a geothermal heat pump is one of the most energy efficient upgrades a homeowner can make. Using the near-constant 50 to 60 degree temperatures found just a few feet below ground, a series of looped tubes supply or remove heat to and from a home using heat transfer.

Although highly efficient, geothermal systems require extensive excavation to install the heat transfer tubes and can cost significantly more up front than other heating or cooling appliances. However, they are typically far more durable than conventional HVAC equipment and the high upfront cost may be offset by federal, state or local tax credits.


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