HVAC
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2008-07-15T16:34:19Z
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{{dablink|HVAC may also stand for [[High-voltage]] [[alternating current]].}}
[[Image:Central ventilation tube2.jpg|thumb|250px|HVAC systems use ventilation [[Duct (HVAC)|air duct]]s installed throughout a building that supply conditioned air to a room through rectangular or round outlet vents, called diffusers; and ducts that remove air through return-air "grilles"]]
[[Image:Shaft dupont mississauga on.jpg|thumb|right|[[Fire-resistance rating|Fire-resistance rated]] mechanical shaft with HVAC [[sheet metal]] ducting and [[copper]] [[Pipe|piping]], as well as [[Joint (building)|"HOW" (Head-Of-Wall) joint]] between top of [[concrete]] block wall and underside of concrete slab, [[firestop]]ped with ceramic fibre-based firestop [[caulking]] on top of [[Mineral wool|rockwool]].]]
'''HVAC''' (pronounced either "H-V-A-C" or, occasionally, "''H-vak''") is an [[initialism]] or [[acronym]] that stands for "'''[[heating]], [[Ventilation (architecture)|ventilating]], and [[air conditioning]]'''". HVAC is sometimes referred to as '''climate control''' and is particularly important in the design of medium to large industrial and office buildings such as [[skyscraper]]s and in marine environments such as [[aquarium]]s, where humidity and temperature must all be closely regulated whilst maintaining [[Sick building syndrome|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''', or ventilating is dropped as '''HACR''' (such as the designation of HACR-rated circuit breakers).
Heating, ventilating, and air conditioning is based on the basic principles of [[thermodynamics]], [[fluid mechanics]], and [[heat transfer]], and two inventions and discoveries made by [[Michael Faraday]], [[Willis Carrier]], [[Reuben Trane]], [[James Joule]], [[William Rankine]], [[Nicolas Léonard Sadi Carnot|Sadi Carnot]], 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 three functions of 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 (architecture)|ventilation]], reduce air [[Infiltration (HVAC)|infiltration]], and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as '''[[room air distribution]]'''.<ref>''Designer's Guide to Ceiling-Based Air Diffusion'', Rock and Zhu, [[ASHRAE]], Inc., Atlanta, GA, USA, 2002</ref>
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 engineering|mechanical]], [[Architectural engineering|architectural]], or [[Building engineering|building services]] engineers analyze, design, and specify the HVAC systems, and specialty mechanical contractors build and commission them. In all buildings, building permits 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 organizations such as [[ASHRAE]], [[SMACNA]], [[Air Conditioning Contractors of America|ACCA]], [[Uniform Mechanical Code]], [[International Building Code|International Mechanical Code]], and [[AMCA]] have been established to support the industry and encourage high standards and achievement.
== 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 either ductwork, for forced air systems, or piping to distribute a heated fluid and [[radiator]]s 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 [[thermal radiation|radiation]]. The radiators may be mounted on walls or buried in the floor to give under-floor heat.
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 (secondary) heat exchanger inside a storage cylinder to provide hot running water.
Forced air systems send heated air through [[Duct (HVAC)|ductwork]]. During warm weather the same ductwork can be used for air conditioning. The forced air can also be filtered or put through air cleaners.
Heating can also be provided from electric, or resistance heating using a filament that becomes hot when electricity is caused 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, typically next to the windows to minimize condensation and offset the convective air current formed in the room due to the air next to the window becoming negatively buoyant due to the cold glass. Devices that direct vents away from windows to prevent "wasted" heat defeat this design intent. Cold air drafts can contribute significantly to subjectively feeling colder than the average room temperature. Therefore, it is important to control the air leaks from outside in addition to proper design of the heating system.
The invention of central heating is often credited to the [[Roman empire|ancient Romans]], who installed a system of air ducts called "[[hypocaust]]" in the walls and floors of public baths and private villas{{Fact|date=September 2007}}.
== Ventilating ==
[[Image:Air handling unit.JPG|thumb|left|225px|An [[air handling unit]] is used for the heating and cooling of air in a central location (click on image for legend).]]
[[Ventilation (architecture)|Ventilating]] is the process of "changing" or replacing air in any space to control temperature or 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.<ref>Ventilation and Infiltration chapter, Fundamentals volume of the ''ASHRAE Handbook'', ASHRAE, Inc., Atlanta, GA, 2005</ref> Ventilation is used to remove unpleasant smells and excessive moisture, introduce outside air, and 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. However, 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.
Ceiling [[Fan (mechanical)|fans]] and table/floor fans circulate air within a room for the purpose of reducing the perceived temperature because of evaporation of perspiration on the skin of the occupants. Because hot air rises, ceiling fans may be used to keep a room warmer in the winter by circulating the warm stratified air from the ceiling to the floor. Ceiling fans do not provide ventilation as defined as the introduction of outside air.
==== Natural ventilation ====<!-- This section is linked from [[Chimney]] -->
[[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 through 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 [[economizer]]s 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 are provided through the removal of heat. The definition of cold is the absence of heat and all air conditioning systems work on this basic principle. Heat can be removed through the process of [[thermal radiation|radiation]], [[convection]], and [[Heat conduction|conduction]] using mediums such as water, air, ice, and chemicals referred to as refrigerants. 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 standalone [[air conditioner]], provides cooling, ventilation, and [[humidity control]] for all or part of a house or building. The [[Freon]] or other refrigerant provides cooling through a process called the [[refrigeration cycle]]. The refrigeration cycle consists of four essential elements to create a cooling effect. A compressor provides compression for the system. This compression causes the cooling vapor to heat up. The compressed vapor is then cooled by heat exchange with the outside air, so that the vapor condenses to a fluid, in the [[condenser]]. The fluid is then pumped to the inside of the building, where it enters an evaporator. In this evaporator, small spray nozzles spray the cooling fluid into a chamber, where the pressure drops and the fluid evaporates. Since the evaporation absorbs heat from the surroundings, the surroundings cool off, and thus the [[evaporator]] absorbs or adds heat to the system. The vapor is then returned to the compressor. A 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 [[pathogen]]ic [[bacterium|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 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 evaporator 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.
==Energy efficiency==
For the last 20-30 years, manufacturers of HVAC equipment have been making an effort to make the systems they manufacture more efficient. This was originally driven by rising energy costs, and has more recently been driven by increased awareness over environmental issues. There are several methods for making HVAC systems more efficient.
'''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 valve]]s, and in forced air systems they control [[zone damper]]s inside the vents which selectively block the flow of air.
'''Ventilation Energy recovery'''
[[Energy recovery]] systems sometimes utilize [[heat recovery ventilation]] or [[energy recovery ventilation]] systems that employ [[heat exchangers]] or enthalpy wheels to recover [[sensible heat|sensible]] or [[latent heat]] from exhausted air. This is done by transfer of energy to the incoming outside fresh 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 efficiency|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.
==Major terms==
;Air Change per Hour (ACH)
:The number of times per hour that the volume of a specific room or building is supplied or removed from that space by mechanical and natural ventilation.
;[[Air handler]], or '''air handling unit''' (AHU)
:Central unit consisting of a 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.
;[[British thermal unit]] (BTU)
:Any of several units of energy (heat) in the HVAC industry, each slightly more than 1 [[kilojoule|kJ]]. One BTU is the energy required to raise one pound of water one degree [[Fahrenheit]], but the many different types of BTU are based on different interpretations of this “definition”. In the United States the power of HVAC systems (the rate of cooling and dehumidifying or heating) is sometimes expressed in BTU/hour instead of [[watt]]s.
;[[Chiller]]
:A device that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. This cooled liquid flows through pipes in a building and passes through coils in air handlers, fan-coil units, or other systems, cooling and usually dehumidifying the air in the building. Chillers are of two types; air-cooled or water-cooled. Air-cooled chillers are usually outside and consist of condenser coils cooled by fan-driven air. Water-cooled chillers are usually inside a building, and heat from these chillers is carried by recirculating water to outdoor cooling towers.
;Coil
:Equipment that performs heat transfer when mounted inside an Air Handling unit or ductwork. It is heated or cooled by electrical means or by circulating liquid or steam within it. Air flowing across it is heated or cooled.
;[[Condenser]]
:A component in the basic [[refrigeration cycle]] that ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condensers are heat exchangers, and can transfer heat to air or to an intermediate fluid (such as water or an aqueous solution of ethylene glycol) to carry heat to a distant sink, such as ground (earth sink), a body of water, or air (as with cooling towers).
;[[Constant air volume]] (CAV)
:A system designed to provide a constant air volume per unit time. This term is applied to HVAC systems that have variable supply-air temperature but constant air flow rates. Most residential forced-air systems are small CAV systems with on/off control.
;Controller
:A device that controls the operation of part or all of a system. It may simply turn a device on and off, or it may more subtly modulate burners, compressors, pumps, valves, fans, dampers, and the like. Most controllers are automatic but have user input such as temperature set points, e.g. a [[thermostat]]. Controls may be [[analog signal|analog]], or [[digital]], or [[pneumatic]], or a combination of these.
;[[Damper (architecture)|Damper]]
:A plate or gate placed in a duct to control air flow by introducing a constriction in the duct.
;delta T
:Delta T is a reference to a temperature difference. It is used to describe the difference in temperature of a heating or cooling fluid as it enters and as it leaves a heat transfer device. This term is used in the calculation of coil efficiency.
;[[Evaporator]]
:A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators can be used to absorb heat from air (by reducing temperature and by removing water) or from a liquid. The evaporator is the cold side of an air conditioner or heat pump.
;[[Fan coil unit]] (FCU)
:A small terminal unit that is often composed of only a blower and a heating and/or cooling coil (heat exchanger), as is often used in hotels, condominiums, or apartments. One type of fan coil unit is a unit ventilator.
;Fresh air intake (FAI)
:An opening through which outside air is drawn into the building. This may be to replace air in the building that has been exhausted by the ventilation system, or to provide fresh air for combustion of fuel.
;[[Furnace]]
:A component of an HVAC system that adds heat to air or an intermediate fluid by burning fuel (natural gas, oil, propane, butane, or other flammable substances) in a heat exchanger.
;Grille
:A facing across a duct opening, usually rectangular is shape, containing multiple parallel slots through which air may be delivered or withdrawn from a ventilated space.
;Heat load, '''heat loss''', or '''heat gain'''
:Terms for the amount of heating (heat loss) or cooling (heat gain) needed to maintain desired temperatures and humidities in controlled air. Regardless of how well-insulated and sealed a building is, buildings gain heat from warm air or sunlight or lose heat to cold air and by radiation. Engineers use a heat load calculation to determine the HVAC needs of the space being cooled or heated.
;Louvers
:Blades, sometimes adjustable, placed in ducts or duct entries to control the volume of air flow. The term may also refer to blades in a rectangular frame placed in doors or walls to permit the movement of air.
;Makeup air unit (MAU)
:An air handler that conditions 100% outside air. MAUs are typically used in industrial or commercial settings, or in "once-through" (blower sections that only blow air one-way into the building), "low flow" (air handling systems that blow air at a low flow rate), or "primary-secondary" (air handling systems that have an air handler or rooftop unit connected to an add-on makeup unit or hood) commercial HVAC systems.
;[[Packaged terminal air conditioner]] (PTAC)
:An air conditioner and heater combined into a single, electrically-powered unit, typically installed through a wall and often found in hotels.
;[[Packaged unit]] or rooftop unit (RTU)
:An air-handling unit, defined as either "recirculating" or "once-through" design, made specifically for outdoor installation. They most often include, internally, their own heating and cooling devices. RTUs are very common in some regions, particularly in single-story commercial buildings.
;Thermal zone
:A single or group of neighboring indoor spaces that the HVAC designer expects will have similar thermal loads. Building codes may require zoning to save energy in commercial buildings. Zones are defined in the building 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 can be combined into one zone. Small residences typically have only one conditioned thermal zone, plus unconditioned spaces such as unconditioned garages, attics, and crawlspaces, and unconditioned basements.
;[[Variable air volume]] (VAV) system
:An HVAC system that has a stable supply-air temperature, and varies the air flow rate to meet the temperature requirements. Compared to CAV systems, these systems waste less energy through unnecessarily-high fan speeds. Most new commercial buildings have VAV systems.
==Other HVAC equipment==
{{Col-start}}
{{Col-1-of-2}}
* [[Air conditioner]]
* [[Air filter]]
* [[Boiler]]
* [[Cooling tower]]
* [[Chilled beam]]
* [[Diffuser (thermodynamics)|Diffuser]]
* [[Duct (HVAC)|Duct]]
* [[Economizer]]
* [[Evaporative cooler]]
{{Col-2-of-2}}
* [[Fan (mechanical)]]
* [[Heat exchanger]], including 'coils'
* [[Humidifier]] / [[Dehumidifier]]
* [[Heat Recovery Ventilator]]
* [[HVAC control system]]
* [[Piping]]
* [[Pump]]
* [[unit ventilator]]
* [[Valve]]
* [[Variable Frequency Drive |Variable-frequency drive]], for fine control of pumps
{{Col-end}}
==HVAC industry and standards==
=== In America ===
==== USA====
{{Main|American Society of Heating, Refrigerating and Air-Conditioning Engineers}} (ASHRAE)
{{Main|Uniform Mechanical Code}} (Uniform Mechanical Code)
=== In Europe===
====In the United Kingdom====
The [[CIBSE|Chartered Institute of Building Services Engineers]] is a body that covers the essential [[Service (systems architecture)]] that allow buildings to operate. It includes the electrotechnical, [[heating]], [[ventilating]], [[air conditioning]], [[refrigeration]] and [[plumbing]] industries. 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 [[academic degree|degree]] in a branch of engineering, such as building environment [[engineering]], electrical engineering or mechanical engineering.
Within the [[construction]] sector, it is the job of the building services engineer to [[design]] and oversee the installation and maintenance of 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. Building Services is part of a sector that has over 51,000 businesses and employs over 500,000 people. This [[List of recognized economic sectors|sector]] has an annual turnover of £19.3 billion which represents 2%-3% of the [[GDP]].
The most recognized standards for HVAC design is based on [[ASHRAE]] data. ASHRAE is the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The ASHRAE Handbook's most general volume, of four, is Fundamentals; it includes heating and cooling calculations. Each volume of the ASHRAE Handbook is updated every four years. The design professional must consult ASHRAE data for the standards of design and care as the typical building codes provides little to no information on HVAC design practices; such codes, such as the [[Uniform Mechanical Code|UMC]] and IMC, do include much details on installation requirements, however. Other useful reference materials include items from [[SMACNA]], [[ACCA]], and technical trade journals.
== See also ==
{|
|-valign=top
|
* [[Absorption refrigeration]]
* [[American Society of Heating, Refrigerating and Air-Conditioning Engineers]] (ASHRAE)
* [[Architectural engineering]] -- Mainly N.A.
* [[ASHRAE Handbook]]
* [[BACnet]] -- An open-source network protocol used for Building Automation Control
* [[Baseboard heating]]
* [[Building engineering]] -- Mainly UK
* [[Building automation]]
* [[Building Indoor Environment]]
* [[Central heating]]
| width=40 |
|
* [[CIBSE]] Chartered Institute of Building Services Engineers
* [[Geo-exchange]]
* [[LonWorks]] -- A competing network protocol used for Building Automation Control
* [[Mechanical engineering]]
* [[Refrigeration]]
* [[Solar power]] and solar refrigeration
* [[Pressurisation ductwork]]
* [[Smoke exhaust ductwork]]
* [[Thermal Comfort]]
* [[Underfloor heating]]
|}
==References==
{{reflist}}
{{refbegin}}
*''Modern Refrigeration and Air Conditioning'' (August 2003) by Althouse, Turnquist, and Bracciano, Goodheart-Wilcox Publisher; 18th edition
*''International Mechanical Code'' (March 6, 2006) by the International Code Council, Thomson Delmar Learning; 1 edition
{{refend}}
==External links==
* [http://www.ashrae.org ASHRAE] - The American Society of Heating, Refrigerating and Air-Conditioning Engineers
{{wikibooks|How to reduce home energy usage#Residential ventilation issues}}
{{commonscat|Climate Control}}
*[http://www.hvacwebconnection.com Directory and Information resource for HVAC industry professionals]
*[http://www.price-hvac.com/media/trainingModule.aspx Complete HVAC Visual Training]
*[http://www.borpin.co.uk/sbfaq/faq51095.htm The UK Selfbuild FAQ]
*[http://www.price-hvac.com/catalog/J_all/SectionJIndex.aspx?pageRequest=DG The effects of displacement ventilation]
*[http://www.wbdg.org/design/naturalventilation.php Natural Ventilation] - by Andy Walker of the National Renewable Energy Laboratory
*[http://www.ecbcs.org IEA Energy Conservation in Buildings and Community Systems Programme.]
*[http://www.cooloff.org/sub_cool.html BTU Calculator] A worksheet by the Association of Home Appliance Manufacturers to help you estimate how much cooling capacity you need.
*[http://www.computrols.com/formulas HVAC Formulas] - Common HVAC industry formula resource
* [http://high-performance-hvac.com High Performance HVAC] - Comprehensive articles about HVAC, HVAC applications, and saving energy with HVAC systems
*[http://www.hvac-for-beginners.com HVAC for Beginners]
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