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Heating, ventilation, and air conditioning

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HVAC may also stand for High-voltage alternating current
"All-air" and "air-water" HVAC systems use air ducts, outlets, and inlets installed throughout their buildings. The "outlet vent" is actually an "air outlet", or more specifically in this case an "air diffuser".

HVAC (pronounced either "H-V-A-C" or, occasionally, "H-VAK") is an initialism/acronym that stands for "heating, ventilating, and air conditioning". This is sometimes referred to as climate control and is particularly important in the design of medium to large industrial and office buildings such as sky scrapers and in marine environments where humidity and temperature must all be closely regulated whilst maintaining safe and healthy conditions within. In certain regions (e.g., UK) the term "Building Services" is also used, but may also include plumbing and electrical systems. Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR.

Heating, Ventilation, and Air conditioning is based on the basic principles of thermodynamics, heat transfer and to inventions and discoveries made by Michael Faraday, Willis Carrier, James Joule, William Rankine, Sadi Karnot, and many others. The invention of the components of HVAC systems goes hand-in-hand with the industrial revolution, and new methods of modernization, higher efficiency, and system control are constantly introduced by companies and inventors all over the world.

The HVAC industry is 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 creat international standards that many countries, including the US, Canada, the UK, Australia and many others have been adopting.

The three HVAC functions, heating, ventilating, 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 infiltration, and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as room air distribution.[1]

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.

Major Terms

  • Air handler, or air handling unit (AHU) can mean a whole unit including the blower, heating and cooling elements, filter racks or chamber, dampers, humidifier, and other central equipment in direct contact with the airflow. This does not include the ductwork through the building.
  • Btu or British thermal unit, is the standard unit of measure of heat in the HVACR industry. it is the amount of heat required to raise one pound of water one degree farenheit Heating and Air Conditioning systems are measured by the rate of cooling or heating or in Btus/ hour.
  • Controllers are devices that execute functions such as turning on and off, or modulating, burners, compressors, pumps, valves, fans, and dampers. Most controls are automatic but have user-inputs such as temperature setpoints, e.g., a thermostat. Controls may be analog or digital, or a contain a combination of both analog, digital or pneumatic elements.
  • Fan Coil Unit (FCU) is a small terminal unit that is often composed of only a blower and either a heating or a cooling coil. Often used in hotels, condos, or apartments.
  • condenser is a component in the basic refrigeration cycle that ejects or removes heat from the system. condesners can be air cooled, water cooled, or cooled with another chemical called ethelyne glycol more commonly referred to as anti-freeze
  • Constant Air Volume (CAV) is an all-air or air-water HVAC system that has variable supply air temperature, but a constant flow rate of air. Most residential forced-air systems are small CAV systems with on/off control.
  • evaporator is a component in the basic refrigeration cycle that absorbs or adds heat to the system evaporators can be used to absorb heat from air or from a liquid. The evaporator is the component that actually creates the cooling effect.
  • Furnace is a component of a HVAC system that adds heat to air through the use of the combustion of natural gas, oil, propane, butane, or other flammable liquids or gasses in a heat exchanger. The process of combustion raises the temperature of the heat exchanger, and then a fan blows air across the heat exchanger which in turn is also heated.
  • Heat load, Heat loss, and Heat gain are engineering terms used to determine the amount of heating (heat loss) or cooling (heat gain) needed to maintain temperatures in commercial and residential HVACR. regardless of how well insulated and sealed a builing is, buildings gain heat in warmer climates and during warmer seasons, and lose heat in colder climates and during cooler seasons. Engineers use a heat load/ loss calculation to determine the HVACR needs of the space being cooled or heated.
  • Makeup Air Unit (MAU) is an air handler that conditions 100% outside air. MAUs are typically used in industrial settings, or in once-through, low-flow, or primary-secondary commercial HVAC systems.
  • Rooftop Unit (RTU) is an air handling unit, of recirculating or once-through design, that is for outdoor installation. They most often include, internally, their own heating and cooling devices. RTUs are very popular -- particularly for single-story commercial buildings.
  • Variable Air Volume (VAV) is an all-air or air-water HVAC system that has a relatively fixed supply air temperature, but the volumetric flow rate of air varies to meet the thermal load. Most new commercial buildings have VAV systems due to their reduced fan energy consumption, as compared to CAV.
  • Thermal Zone is a single or group of neighboring indoor spaces that the HVAC designer or contractor expects will have similar thermal loads. Energy codes require zoning to save energy in commercial buildings. Zones are defined to reduce the number of HVAC subsystems, and thus initial cost. For example, for perimeter offices, rather than one zone for each office, all offices facing West could be combined into one zone. Small residences typically have only one conditioned thermal zone, plus unconditioned spaces such as attached garages, attics, and crawlspaces. Basements may be either conditioned or unconditioned.

Heating

Heating systems may be classified as central or local. Central heating is often used in cold climates to heat private houses and public buildings. Such a system contains a boiler, furnace, or heat pump to heat water, steam, or air, all in a central location such as a furnace room in a home or a mechanical room in a large building. The system also contains piping or ductwork to distribute the heated fluid, and radiators to transfer this heat to the air. The term radiator in this context is misleading since most heat transfer from the heat exchanger is by convection, not radiation. The radiators may be mounted on walls or buried in the floor to give under-floor heating.

In boiler fed or radiant heating systems, all but the simplest systems have a pump to circulate the water and ensure an equal supply of heat to all the radiators. The heated water can also be fed through another heat exchanger inside a storage cylinder to provide hot running water.

Forced air systems send heated air through ductwork. During warm weather the same ductwork can be reused for air conditioning. The forced air can also be filtered or put through air cleaners. Most ducts cannot fit a human being (as they do in many films) since this would require a greater duct-structural integrity and create a potential security liability.

Heating can also be provided from electric, or resistance heating using a filament that glows hot when you cause electricity to pass through it. This type of heat can be found in electric baseboard heaters, portable electric heaters, and as backup or supplemental heating for heat pump (or reverse heating) system.

The heating elements (radiators or vents) should be located in the coldest part of the room and typically next to the windows to minimize condensation. Popular retail devices that direct vents away from windows to prevent "wasted" heat defeat this design parameter. Drafts contribute more to the subjective feeling of coldness than actual room temperature. Therefore, rather than improving the heating of a room/building, it is often more important to control the air leaks.

The invention of central heating is often credited to the ancient Romans, who installed a system of air ducts in walls and floors of public baths and private villas. The ducts were fed with hot air from a central fire. Generally, these heated by radiation; a better physiologic approach to heating than conventional forced air convective heating.

Ventilation

Ventilation is the changing of air in any space to remove moisture, odors, smoke, heat, dust and airborne bacteria. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types.[2] Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, and keep to keep interior building air circulating, to prevent stagnation of the interior air.

Mechanical or Forced ventilation

"Mechanical" or "Forced" ventilation is used to control indoor air quality. Excess humidity, odors, and contaminants can often be controlled via dilution or replacement with outside air. But in humid climates, much energy is required to remove excess moisture from ventilation air.

Kitchens and bathrooms typically have mechanical exhaust to control odors and sometimes humidity. Factors in the design of such systems include the flow rate (which is a function of the fan speed and exhaust vent size) and noise level. If the ducting for the fans traverse unheated space (e.g., an attic), the ducting should be insulated as well to prevent condensation on the ducting. Direct drive fans are available for many applications, and can reduce maintenance needs.

Heat recovery ventilation systems employ heat exchangers to recover some heat from exhausted air to preheat the incoming outside air.

Ceiling fans and table/floor fans are very effective in circulating air within a room. Counterintuitively, because hot air rises, ceiling fans may be used to keep a room warmer. Ceiling fans do not provide 'ventilation', however.

Natural ventilation

Natural ventilation is the ventilation of a building with outside air without the use of a fan or other mechanical system. It can be achieved with operable windows when the spaces to ventilate are small and the architecture permits. In more complex systems warm air in the building can be allowed to rise and flow out upper openings to the outside (stack effect) thus forcing cool outside air to be drawn into the building naturally though openings in the lower areas. These systems use very little energy but care must be taken to ensure the occupants' comfort. In warm or humid months, in many climates, maintaining thermal comfort via solely natural ventilation may not be possible so conventional air conditioning systems are used as backups. Air-side economizers perform the same function as natural ventilation, but use mechanical systems' fans, ducts, dampers, and control systems to introduce and distribute cool outdoor air when appropriate.

Air-conditioning

Air Conditioning and refrigeration, is provided through the removal of heat. The definition of cold is the absense of heat, and all air conditioning systems work off of this basic principle. Heat can be removed through the process of radiation, convection, and conduction using mediums such as water, air, ice, and special refrigerants sometimes referred to as freon. In order to remove heat from something, you simply need to provide a medium that is colder- this is how all air conditioning and refrigeration systems work.

An air conditioning system, or a stand-alone air conditioner, provides cooling, ventilation, and humidity control for all or part of a house or building. The freon or refrigerant provides cooling through a process called the refrigerant cycle. The refrigerant cycle consists of four essential elements to create a cooling effect. a compressor provides compression for the system, a condenser ejects or removes heat from the system, the evaporator absorbs or adds heat to the system, and the metering device acts as a restriction in the system at the evaporator to ensure that the heat being absorbed by the system is absorbed at the proper rate.

Central, 'all-air' air conditioning systems are often installed in modern residences, offices, and public buildings, but are difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required. A duct system must be carefully maintained to prevent the growth of pathogenic bacteria in the ducts. An alternative to large ducts to carry the needed air to heat or cool an area is the use of remote fan coils or split systems. These systems, although most often seen in residential applications, are gaining popularity in small commercial buildings. The remote coil is connected to a remote condenser unit using piping instead of ducts.

Dehumidification in an air conditioning system is provided by the evaporator. Since the evaporator operates at a temperature below dew point, moisture is collected at the evaporator. This moisture is collected at the bottom of the evporator in a condensate pan and removed by piping it to a central drain or onto the ground outside. A dehumidifier is an air-conditioner-like device that controls the humidity of a room or building. They are often employed in basements which have a higher relative humidity because of their lower temperature (and propensity for damp floors and walls). In food retailing establishments, large open chiller cabinets are highly effective at dehumidifying the internal air. Conversely, a humidifier increases the humidity of a building.

Air-conditioned buildings often have sealed windows, because open windows would disrupt the attempts of the HVAC system to maintain constant indoor air conditions.

HVAC Energy Efficiency

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 their performance. The Coefficient-of-Performance (COP) does, but this dimensionless measure does not enjoy wide use like the dimensional 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 .

HVAC Industry in the United Kingdom

NOTE: The information in this section applies only to the United Kingdom

Building Services is a construction body that covers the essential services that allow buildings to operate. It includes the electrotechnical, heating, ventilating, air conditioning, refrigeration and plumbing industries.

Building Services is part of a sector that has over 51,000 businesses and employs over 500,000 people. This sector has an annual turnover of £19.3 billion which represents 2%-3% of the GDP.

Within the construction sector, it is the job of the building services engineer to design, install and maintain the essential services such as gas, electricity, water, heating and lighting, as well as many others. These all help to make buildings comfortable and healthy places to live and work in.

To train as a building services engineer, the academic requirement is GCSEs (A-C) / Standard Grades (1-3) in Maths and Science, which are important in measurements, planning and theory. Employers will often want a degree in a branch of engineering, such as building environment engineering, electrical engineering or mechanical engineering.

HVAC System - Illustration

Symbols

  • H = Heating, Air or Hydronics including radiant, baseboard, fan/coils, towel warmers etc.
  • V = Ventilation, Natural or Mechanical including exhaust and make up air, heat recovery ventilators, energy recovery ventilators.
  • A/C = Air Conditioning, (conditioning the air) including humidification, dehumidification, filtration, heating and cooling.

HVAC Equipment

See also

References

  1. ^ Designer's Guide to Ceiling-Based Air Diffusion, Rock and Zhu, ASHRAE, Inc., Atlanta, GA, USA, 2002
  2. ^ Ventilation and Infiltration chapter, Fundamentals volume of the ASHRAE Handbook, ASHRAE, Inc., Atlanta, GA, 2005

3. Modern Refrigeration and Air Conditioning by Althouse, Turnquist, and Bracciano, Goodheart-Wilcox Publisher; 18th edition (August 2003)
 4. International Mechanical Code by the International Code Council, Thomson Delmar Learning; 1 edition (March 6, 2006)

Wikibooks has a book on the topic of: How to reduce home energy usage#Residential ventilation issues
Wikimedia Commons has media related to Climate Control.


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