HVAC

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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".
"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.

The HVAC industry is huge, worldwide, with career opportunities ranging from operation and maintenance, to system design and construction, to equipment manufacturing and sales, and to education and research. Annual sales are in the US$100's of billions. There are also hundreds of related local, national, and international professional, technical, trade, and labor organizations such as ASHRAE, SMACNA, ACCA, and AMCA, to name just a few, that support the industry and encourage high standards and achievement.

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.

Contents

[edit] 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.
  • 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.
  • 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.
  • 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.
  • 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. Zones are defined to reduce the number of HVAC subsystems, and thus initial cost. 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.
  • 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 combination.

[edit] 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.

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 is often fed through another heat exchanger inside a storage cylinder to provide hot running water.

Forced air systems send air through ductwork. During warm weather the same ductwork can be reused for air conditioning. The forced air can 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.

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.

[edit] 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]

[edit] 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.

[edit] 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.

[edit] Air-conditioning

An air conditioning system, or a stand-alone air conditioner, provides heating, cooling, ventilation, and humidity control for all or part of a building. '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.

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.

[edit] HVAC Energy Efficiency

[edit] 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.

[edit] 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 .

[edit] 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.

[edit] 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.

[edit] HVAC Equipment

[edit] See also

[edit] 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

[edit] External links

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