How Does an HVAC System Work?

Learn how HVAC systems work before you talk to a repair pro.

According to the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy, heating and cooling account for about 43 % of the energy use in a typical U.S. home, making it the largest energy expense for most homes. So, it's likely a good idea to know a little about how various HVAC systems work.

What Kind Of System?

There are several basic energy sources for home:

  • Natural gas is carried into your gas-fueled appliances such as the furnace, stove of dryer. Propane gas is usually in a storage tank on your property and underground lines carry it to the house.
  • Fuel oil is used to heat air, steam or water to provide heat in the house.
  • Electric furnaces heat the air as a toaster might heat a piece of bread. Or think of the difference of an electric or gas dryer.
  • Heat pumps offer an energy-efficient alternative to furnaces and air conditioners for climates with moderate heating and cooling needs. Like your refrigerator, heat pumps use electricity to move heat from a cool space into a warm, making the cool space cooler and the warm space warmer. During the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warm outdoors. Because they move heat rather than generate heat, heat pumps can provide up to four times the amount of energy they consume.
  • Hot water and steam heat systems are similar. A boiler heats water which is then fed through pipes as steam or water to radiators throughout the home.
  • Gravity furnaces make use of the principle that warm air rises and cold air falls. These tend to be in homes built during the 1930s and 1940s and are easy to identify because they look like giant octopuses with tentacles extending from a large belly, which is the heat exchanger.
  • Forced air: These systems are common. You set your thermostat at 72 degrees, for example. The furnace heats air in the chamber which is then blown into rooms through duct work and registers. Similarly, in summer you may set the temperature a 68 degrees and the blower pushes cool air from the air conditioning unit into living areas. A programmable thermostat can be a money-saver if you set it, for example, at a higher temperature during working hours but set it to cool down the house an hour before you get home.

How Does Air Conditioning Work? 

Air conditioners can range from small family window units to central air conditioning, sometimes with two or more units for large homes. Large office buildings may have 10 or more condenser units on their roofs.

An air conditioner works by removing warm air from your house and cycling it back as cooler air. The blower that's also often used for the furnace pushes the cool air throughout the house until the air reaches the desired temperature on your thermostat.

A central air conditioner is either a split-system unit or a packaged unit.

In a split-system central air conditioner, an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator. In many split-system air conditioners, this indoor cabinet also contains a furnace or the indoor part of a heat pump. The air conditioner's evaporator coil is installed in the cabinet or main supply duct of this furnace or heat pump. If your home already has a furnace but no air conditioner, a split-system is the most economical central air conditioner to install.

In a packaged central air conditioner, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof or on a concrete slab next to the house's foundation. This type of air conditioner also is used in small commercial buildings. Air supply and return ducts come from indoors through the home's exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors.

Air Goes Around and Around

Central air conditioners circulate cool air through a system of supply and return ducts. Supply ducts and registers carry cooled air from the air conditioner to the home. This cooled air becomes warmer as it circulates through the home; then it flows back to the central air conditioner through return ducts and registers.

Here's how it works:

  • Air conditioners employ the same operating principles and basic components as your refrigerator. Refrigerators use energy to transfer heat from the cool interior of the refrigerator to the relatively warm surroundings of your home; likewise, an air conditioner uses energy to transfer heat from the interior of your home to the relatively warm outside environment.
  • An air conditioner cools your home with a cold indoor coil called the evaporator. The condenser, a hot outdoor coil, releases the collected heat outside. The evaporator and condenser coils are serpentine tubing surrounded by aluminum fins. This tubing is usually made of copper.
  • A pump, called the compressor, moves a heat transfer fluid (or refrigerant) between the evaporator and the condenser. The pump forces the refrigerant through the circuit of tubing and fins in the coils.
  • The liquid refrigerant evaporates in the indoor evaporator coil, pulling heat out of indoor air and thereby cooling your home. The hot refrigerant gas is pumped outdoors into the condenser where it reverts back to a liquid, giving up its heat to the outside air flowing over the condenser's metal tubing and fins.

How Has HVAC Technology Innovated?

Throughout the second half of the 20th century, nearly all air conditioners used chlorofluorocarbons (CFCs) as their refrigerant, but because these chemicals are damaging to Earth's ozone layer, CFC production stopped in the United States in 1995. Nearly all air conditioning systems now employ halogenated chlorofluorocarbons (HCFCs) as a refrigerant, but these are also being gradually phased out, with most production and importing stopped by 2020 and all production and importing stopped by 2030. Production and importing of today's main refrigerant for home air conditioners, HCFC-22 (also called R-22), was phased out in 2010 and will stop entirely by 2020. However, HCFC-22 is expected to be available for many years as it is recovered from old systems that are taken out of service. As these refrigerants are phased out, ozone-safe hydrofluorocarbons (HFCs) are expected to dominate the market, as well as alternative refrigerants.

HVAC Efficiency

For furnaces, the acronym AFUE (annual fuel utilization efficiency) rates how efficiently the unit delivers warm air into the house. A furnace with a 78 percent AFUE rating, the minimum by federal standards, means that 78 percent of the heat the furnace creates gets into your home and the other 22 percent goes up the flue. High-efficiency furnaces can deliver as much as 96 percent. SEER is the acronym for cooling units. The seasonal energy efficiency ratio (SEER) is similar in that the higher number indicates better cooling efficiency. Federal law dictates that new central air-conditioning systems need a SEER of at least 13.

What Else Do Air Conditioning Units Do?

Air conditioners help clean your home's air as well. Most indoor units have filters that catch dust, pollen, mold spores and other allergens as well as smoke and everyday dirt found in the air. Some advanced filters can help if allergens interrupt sleep. Most air conditioners also function as dehumidifiers. They take excess water from the air and use it to help cool the unit before getting rid of the water through a hose to the outside.

Unit Maintainence

You can do basic maintenance, such as changing a filter monthly, but a pro may be needed if the device just won't work. Do know how to turn off the main system, such as the shutoff valve for gas furnaces and water heaters. Electric systems should have a switch or a circuit breaker or fuse. Fuel oil tanks also have shutoff valves. A good way to keep your equipment running is to have an HVAC professional check your furnace in the fall and air conditioning in early spring.

Source: U.S. Department of Energy

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