HVAC

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

HVAC is an acronym for heating, ventilating, and air conditioning. It refers to a group of systems and machines used in buildings such as homes and buildings that regulate the internal to ensure comfort. 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. Generally speaking, the HVAC system accounts for the highest of energy expenses, but this system is also necessary for comfort. In 2009, 63% of Canadian residential energy use went entirely to space heating while 17% went to water heating. 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.

Thus it is important to consider energy efficient building design when making a home as well as including energy efficient furnaces, heat pumps, or air conditioners to ensure that the cost stays minimal. As well, combining proper equipment maintenance with adequate insulation, weatherization, and thermostat settings can all work to cut the cost of heating and cooling on energy bills.

Unit Operation

At the most basic level, an HVAC system works by sending warmed, cooled, or otherwise treated air through a series of ducts to be distributed through rooms in a building.

Air Conditioning

main page

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. Note that the above process is a split system, also available are "packaged" systems where the condenser and evaporator coil are held in the same place.

Heating

Generally, homes are heated through the use of furnaces. They can also be heated naturally, and use passive solar heating to diminish energy costs. As well, in 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 your home. More on the operation of furnaces can be found on the furnace page.

Heat pumps can also be used to cool air. When they are 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.

Figure 2. Diagram of HVAC system and vents in home.

Ventilation

All buildings need ventilation to remove stale interior air and moisture and provide fresh air to inhabitants. The ANSI standard recommends that houses have 7.5 cubic feet per minute of fresh air per bedroom, plus one extra cubic foot with additional air flow related to the overall size of the house. Natural ventilation is sometimes an option, as simply opening windows or doors can 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. Therefore sometimes it is best to use forced ventilation where excess humidity, odors, and contaminants are replaced with outside air through spot ventilation of bathrooms and kitchens. Simply put, large fans force uncomfortable air 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. In order to control airborne particulates such as pollen, fungal spores, animal dander, insect proteins, pesticides, lead, and infectious bacteria and viruses from circulating around a home, filters are used. 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.