Climate control and comfort in modern buildings is a major design issue. Heating, ventilation and air conditioning (HVAC) systems help to control the climate, and keep occupants comfortable by regulating the temperature and air flow. HVAC systems are also important to occupants' health, because a well regulated and maintained system will keep a home free from mold and other harmful organisms. In some environments, such as museums, HVAC systems are vitally important for the preservation of historic artifacts.

In choosing any type of HVAC system, finding one that is size-appropriate to the building is important in terms of achieving the best efficiency and comfort level. Beyond understanding the maintenance and installation of the systems themselves, sizing the systems appropriately is perhaps one of the most difficult tasks of an HVAC contractor. If your HVAC contractor recommends a heating system that is too big or too little for your home or business, the result will be an inefficient system that will cost more money in the long run. An oversized air conditioner will not efficiently dehumidify the air; and an oversized furnace is likely to cause a large temperature swing and create an uncomfortable climate inside the target area.

Determining the size of HVAC systems only by square footage is largely inaccurate, since it does not take into account the energy-efficiency or design of the building. Another common mistake made by many contractors is simply replacing an HVAC unit with a newer unit of the same capacity. This is not always the best strategy. Older homes, for example, often had larger-than-necessary furnaces. An older home that has undergone significant renovation with new windows, weatherstripping, insulation and caulking, will be more energy-efficient, and may be better served by replacing the older, larger heating unit with a smaller one.

Gas and electric utility companies sometimes offer to perform a sizing calculation either for free or for a minimal charge. A good contractor will follow procedures set forth by trade organizations such as the Air Conditioning Contractors of America (ACCA) in regards to appropriate sizing, and may use software programs to assist in the calculation.

In larger, industrial systems, HVAC software is used to manage the HVAC system with respect to other areas such as scheduling, dispatch, billing, maintenance, inventory. Specialized HVAC software is also used by engineers to design ductwork and piping plans. These software programs usually integrate with CAD (computer aided design) software.