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Figure 1. The outdoor condensing portion of an air conditioning unit. The air conditioning unit is one portion of an HVAC system.[1]

HVAC is an acronym for heating, ventilating, and air conditioning. The term refers to a group of systems and machines used in buildings such as homes and buildings that regulate the indoor temperature and air quality to ensure comfort.[2] In modern homes, these three functions are often combined into one system. These systems heat and cool air to a comfortable temperature, as well as providing comfortably humid air while improving air quality by bringing in fresh air. This incorporation of fresh air is particularly important in small spaces or high-odour areas.[3] In Canada, the HVAC system accounts for the largest share of residential energy use, but this system is also necessary for comfort.[3] In 2009, 63% of Canadian residential energy use went entirely to space heating while 17% went to water heating.[4] Since space heating takes up such a large portion of all energy used, it is important to understand how to make devices that heat spaces as energy efficient as possible. Proper thermal insulation is also essential.

Energy efficient building design has a significant impact on both carbon footprint and energy efficiency. Improving energy productivity means making using energy efficient furnaces, heat pumps, or air conditioners to ensure that the cost stays minimal. In addition, combining proper equipment maintenance with adequate thermal insulation, weatherization, and thermostat settings can all work to reduce the cost of heating and cooling on energy bills.

Unit Operation

An HVAC system sends warmed, cooled, or otherwise treated air through a series of ducts to be distributed through rooms in a building.[5]

Air Conditioning

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The main goal of any air conditioner is to move cooled air from one area to another. Generally speaking, this involves taking indoor air and changing its temperature to what is comfortable and distributing this treated air around a building. Many central air conditioning units are known as "split system", as there is an outdoor condenser and an indoor evaporator coil that work to cool the house.

It is through a substance known as refrigerant that heat is moved around in an HVAC system. First, the outdoor unit heats the refrigerant to a high pressure, high temperature gas that then condenses again into a high pressure and temperature liquid. This extremely hot liquid then flows to the indoor evaporator coil. At this point, the liquid refrigerant is allowed to expand again, turning it into a low temperature and low pressure gas. It is at this point that the cooled gas absorbs the heat from the air circulating in the building's duct work, which leaves cooler air inside to be distributed. The refrigerant gas now returns to the outdoor unit to begin the cycle again.[5] Note that the above process is a split system. Packaged systems are also available, which have the condenser and evaporator coils in the same place.


Generally, homes are heated through the use of furnaces. They can also be heated naturally, and use passive solar heating to diminish energy costs. Moreover, places where a traditional furnace may not be enough, heat pumps can be installed to warm a home. The process a heat pump uses to warm a home is the reverse of what an air conditioner uses in the summer. It takes heat out of the outside air (or out of the ground if a geothermal heat pump is used) and it moves that heat inside, where it is transferred from the evaporator coil to the air circulating through the home.[5] More on the operation of furnaces can be found on the furnace page.

Heat pumps can also be used to cool air. When used in this manner, they operate very similarly to an air conditioner since air conditioners are simply heat pumps that only work to cool. When cooling, the heat pumps draws the warm air from inside the home and transfers it through refrigerant to the outside, expelling the extra heat and cooling the house.[6]

Figure 2. Diagram of HVAC system and vents in home.[7]


All buildings need ventilation to remove stale interior air and moisture and provide fresh air to inhabitants. The ANSI standard recommends houses have at least 200 liters per minute of fresh air per bedroom.[8] Natural ventilation is sometimes an option by simply opening windows or doors to increase the flow of fresh air into a home. However, this can sometimes allow outdoor air with contaminants to bypass filters and permit the introduction of excess moisture and isn't always the best option.[9] Therefore, forced ventilation is best wherever excess humidity, odors, and contaminants are replaced with outside air through spot ventilation of bathrooms and kitchens.[8] Simply put, large fans force the air currently in the room out through duct work and replace it with treated air. More on the function of these systems can be found on the forced ventilation page.

Air Filtration

Air filtration is the last component of an HVAC system that is important. Filters are used in order to control airborne particulates such as pollen, fungal spores, animal dander, insect proteins, pesticides, lead, infectious bacteria and viruses from circulating around a home. These filters are placed on ventilation systems and ducts to catch these particulates and should be regularly monitored and replaced to ensure they function correctly.[9]

For Further Reading


  1. "Air conditioner armaflex insulation"Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Air_conditioner_armaflex_insulation.jpg#/media/File:Air_conditioner_armaflex_insulation.jpg
  2. HVAC Training 101. (March 21, 2015). What is HVAC? [Online]. Available: http://www.hvactraining101.com/what-does-HVAC-mean.html
  3. 3.0 3.1 Natural Resources Canada. (March 21, 2015). HVAC [Online]. Available: http://www.nrcan.gc.ca/node/6599
  4. Natural Resources Canada Office of Energy Efficiency. (April 28, 2015). Energy Efficiency Trends in Canada [Online]. Available: http://oee.rncan.gc.ca/publications/statistics/trends11/chapter3.cfm
  5. 5.0 5.1 5.2 Consumer Energy Center. (March 21, 2015). Heating and Cooling [Online]. Available: http://www.consumerenergycenter.org/residential/heating_cooling/heating_cooling.html
  6. Natural Resources Canada. (April 28, 2015). What is a Heat Pump? [Online]. Available: https://www.nrcan.gc.ca/energy/publications/efficiency/heating-heat-pump/6827
  7. US Environmental Protection Agency. (May 6, 2015). Duct Sealing [Online]. Available: https://www.energystar.gov/ia/products/heat_cool/ducts/DuctSealingBrochure04.pdf
  8. 8.0 8.1 University of Kentucky. (March 21, 2015). Heating and Ventilation [Online]. Available: http://www.uky.edu/bae/sites/www.uky.edu.bae/files/Chapter%207%20Heating%20Ventilation%20Air%20Conditioning.pdf
  9. 9.0 9.1 US Environmental Protection Agency. (March 21, 2015). HVAC Systems [Online]. Available: http://www.epa.gov/iaq/schooldesign/hvac.html