The three functions of heating, ventilation, and air-conditioning are closely interrelated. All seek to provide thermal comfort, acceptable indoor air quality, and reasonable installation, operation, and maintenance costs. HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as room air distribution.
In modern buildings the design, installation, and control systems of these functions are integrated into one or more HVAC systems. For very small buildings, contractors normally "size" and select HVAC systems and equipment. For larger buildings where required by law, "building services" designers and engineers, such as mechanical, architectural, or building services engineers analyze, design, and specify the HVAC systems, and specialty mechanical contractors build and commission them. In all buildings, building permits for, and code-compliance inspections of the installations are the norm.
The HVAC industry is a worldwide enterprise, with career opportunities including operation and maintenance, system design and construction, equipment manufacturing and sales, and in education and research. The HVAC industry had been historically regulated by the manufacturers of HVAC equipment, but Regulating and Standards industries such as ASHRAE, SMACNA, ACCA, and AMCA, have been established to support the industry and encourage high standards and achievement. Most recently, the ICC has been established to create international standards that many countries, including the US, Canada, the UK, Australia and many others have been adopting.
HVAC energy efficiency and noise
Heating energy
Water heating is more efficient for heating buildings and was the standard many years ago. Today forced air systems can double for air conditioning and are more popular. The most efficient central heating method is geothermal heating.
Energy efficiency can be improved even more in central heating systems by introducing zoned heating. This allows a more granular application of heat, similar to non-central heating systems. Zones are controlled by multiple thermostats. In water heating systems the thermostats control zone valves, and in forced air systems they control zone dampers inside the vents which selectively block the flow of air.
Air conditioning energy
The performance of vapor compression refrigeration cycles is limited by thermodynamics. These AC and heat pump devices move heat rather than convert it from one form to another, so thermal efficiencies do not appropriately describe the performance of these devices. The Coefficient-of-Performance (COP) measures performance, but this dimensionless measure has not been adopted, but rather the Energy Efficiency Ratio (EER). To more accurately describe the performance of air conditioning equipment over a typical cooling season a modified version of the EER is used, and is the Seasonal Energy Efficiency Ratio (SEER). The SEER article describes it further, and presents some economic comparisons using this useful performance measure.