HVAC

From Free net encyclopedia

HVAC may also stand for High-voltage alternating current

HVAC (pronounced either "H-V-A-C" or, occasionally, "H-VAK") is an initialism/acronym that stands for "heating, ventilation and air-conditioning". This is sometimes referred to as climate control.

These three functions are closely interrelated, as they control the temperature and humidity of the air within a building in addition to providing for smoke control, maintaining pressure relationships between spaces, and providing fresh air for occupants. In modern building designs, the design, installation and control systems of these functions are integrated into a single "HVAC" system.

The term air handler can mean a whole unit including the blower, heating and cooling elements, filter racks or chamber and dampers, but not including the ductwork through the building.

Contents

[edit]

Heating

Heating systems may be classified as central or local.

[edit]

Central heating

Main article: Central heating

Central heating is often used in cold climates to heat private houses and public buildings. Such a system contains a central boiler, furnace or heat pump to heat water, steam, or air; 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. When so mounted it is often referred to as "radiant 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. The ductwork can be reused for air conditioning and the air can be filtered or put through air cleaners. Contrary to fiction, most ducts cannot fit a human being as this would create a potential security liability.

The heating elements (radiators or vents) should be located in the coldest part of the room, typically next to the windows. 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. Thus 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.

[edit]

Ventilation

Ventilation is the changing of air in any space in order to remove moisture, odors, smoke, heat, and airborne bacteria. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. Methods for ventilating a building may be divided into natural and forced types.

[edit]

Displacement ventilation

Airflow in ventilated spaces generally can be classified by two different types; mixing (or dilution) ventilation and displacement ventilation. Mixing ventilation systems generally supply air in a manner such that the entire room air is fully mixed. The cool supply air exits the outlet at high velocity, inducing room air to provide mixing and temperature equalization. Since the entire room is fully mixed, temperature variations are small while the contaminant concentration is uniform throughout the entire room.

Displacement-ventilation systems introduce air at low velocities which causes minimal induction and mixing. The displacement outlets are usually located at or near the floor. The system utilizes buoyancy forces (generated by heat sources such as people, lighting, computers, electrical equipment, etc.) in a room to move contaminants and heat from the occupied zone. By so doing, the air quality in the occupied zone is generally superior to that achieved with mixing ventilation.

Displacement ventilation presents an opportunity to improve both the thermal comfort and indoor air quality (IAQ) of the occupied space. Displacement ventilation takes advantage of the difference in air density between an upper contaminated zone and a lower clean zone. Cool air is supplied at low velocity into the lower zone. Convection from heat sources creates vertical air motion into the upper zone where high level return outlets extract the air. In most cases these convection heat sources are also the contamination sources, i.e. people or equipment, thereby carrying the contaminants up to the upper zone, away from the occupants.

Outlets are typically located at or near the floor level, and air is supplied directly into the occupied zone. This supply air is spread over the entire floor and then rises as it is heated by the heat sources in the occupied zone. Returns are typically located at or close to the ceiling and exhaust the warm contaminated room air.

Since the conditioned air is supplied directly into the occupied space, supply air temperatures must be higher than mixing systems (usually above 63 deg F) to avoid cool temperatures at the floor. By introducing the air at elevated supply air temperatures and low outlet velocity a high level of thermal comfort can be provided with displacement ventilation.


[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 fresh cool 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.

<math>Q_{S} = C_{d}\; A\; \sqrt {2\;g\;H_{d}\;\frac{T_I-T_O}{T_I}}</math>
U.S. customary units:
where:  
QS = Stack vent airflow rate, ft³/s
A = cross-sectional area of opening, ft² (assumes equal area for inlet and outlet)
Cd = Discharge coefficient for opening
g = gravitational acceleration, 32.17 ft/s²
Hd = Height from midpoint of lower opening to neutral pressure level (NPL), ft
NPL
  		= location/s in the building envelope with no pressure difference between inside and outside       (ASHRAE 2001, p.26.11)
TI = Average indoor temperature between the inlet and outlet, °R
TO = Outdoor temperature, °R
SI units:
where:  
QS = Stack vent airflow rate, m³/s
A = cross-sectional area of opening, m² (assumes equal area for inlet and outlet)
Cd = Discharge coefficient for opening
g = gravitational acceleration, 9.807 m/s²
Hd = Height from midpoint of lower opening to neutral pressure level (NPL), m
NPL
  		= location/s in the building envelope with no pressure difference between inside and outside       (ASHRAE 2001, p.26.11)
TI = Average indoor temperature between the inlet and outlet, K
TO = Outdoor temperature, K
[edit]

Forced ventilation

Forced ventilation may be used to control humidity or odours. Kitchens and bathrooms typically have mechanical ventilation to control both. 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.

Heat recovery ventilation systems employ heat exchangers to bring the fresh air temperature to room temperature.

Ceiling fans and table/floor fans are very effective in circulating the air in the room. Paradoxically, because heat rises ceiling fans may be used to keep a room warmer.

[edit]

Ventilation issues in houses

Proper ventilation in the attic:

  • Keeps the house cool in the summer. (Attics radiate heat downward in the summer when they are hotter than the living area.)
  • Keeps the attic cold in the winter, which can prevent ice dams.
  • Allows moisture to escape from the house. Some warm, moist air will always find its way into the attic, so ventilation is essential. This is important year-round, for preventing mold and rot, but it is especially important in the winter, when the moisture is more likely to condense.

Be aware that increased ventilation decreases the effectiveness of any insulation that is a poor barrier to air infiltration, such as fiberglass batts. The increased ventilation will create low pressure areas, so that the house will push conditioned air through the insulation faster than it normally would.

With insufficient ventilation:

  • Attic heat can penetrate into living areas during summer.
  • There will be excessive humidity, which can cause mold and, eventually, rot.
  • Water vapor can condense and collect on insulation, on rafters, and on the underside of roof sheathing. This will reduce the effectiveness of the insulation, and can greatly hasten the activity of mold and rot.
  • Condensation and mold will also occur in the living area, especially on perimeter walls (because they are coolest) and where ventilation is poorest, such as in corners and around furniture.

You will need more ventilation than usual if:

  • You live in a damp climate.
  • Your house is in the shade.
  • The crawlspace or basement has a dirt floor.
  • There isn't much wind.
  • You have a modern, super tight house.
  • You have a solid masonry house.
  • You have a house with impermeable siding such as vinyl or aluminum.
  • You do not have sufficient (or any) vapor barriers.

Most houses treat the attic and basement as unconditioned space. You can think of unconditioned space as outdoor space, minus the rain and snow. The unconditioned space surrounding the living area shouldn’t be wide open, but it shouldn't be sealed shut either. A good compromise is to have two foundation vents in the basement and two different types of vents in the attic. Vents should always exist in pairs (but not necessarily two of the same type) to allow for cross-ventilation. In an attic, one member of the pair should be low on the roof, and the other member should be higher up, so that outside air is pulled through one and out the other. Natural attic ventilation through these vents is usually sufficient. Powered vents in the attic may interfere with proper furnace and fireplace venting.

Some ways to ventilate an attic naturally:

  • Soffit vents.
  • Ridge vent (you can cover the ridge vent with shingles).
  • Gable vents.

Modern homes often incorporate all three types of attic vents, providing continuous cross-ventilation via multiple air pathways.

Make sure gable vents have screens to keep out insects and animals, and keep the screens clean to maintain proper ventilation.

Never close or block off the vents to a damp basement or crawlspace, except in extreme cold to prevent pipes from freezing. Closing the vents to a damp basement or crawlspace will cause mold, rot, and structural defects. Sometimes, a basement or crawlspace will look bone dry, but is transpiring moisture through the dirt floor at a rapid rate. To see if this is the case, lay down some clear plastic on the dirt floor for a few days and observe how much water collects on its underside.

If your basement or crawlspace is dry and has been dry for several years, you can:

  • Close the foundation vents in the winter to conserve energy, and open them again in the warmer months, to allow interior moisture from the house to escape.
  • Close the foundation vents permanently, install a polyethylene vapor barrier on the floor (just to be safe), insulate the basement or crawlspace walls, and part of the floor, if necessary, and include the basement or crawlspace as part of the conditioned space of the house. If you take this route, you don’t need to insulate the floor above the basement or crawlspace, but it doesn’t hurt if the floor is already insulated. Keep an eye on humidity. There will be less condensation on walls and pipes, but possibly greater humidity because of trapped air, requiring increased ventilation in the upper floors and attic to compensate.

Advantages of insulating a dry basement and crawlspace and making them part of the conditioned space of the house:

  • Decreased condensation, because walls are closer in temperature to the air inside the house, and because cold pipes are not exposed to outdoor air during the warmer months.
  • Reduced energy losses from ducts passing through the basement.
  • Reduced risk of pipes freezing in winter.
[edit]

Ventilation checklist:

  • Vent sources of moisture directly to the outside. This is especially important for the bathroom, which normally produces more moisture than any other room in the house, and for the dryer, which produces more moisture than any other appliance.
  • Do not vent moisture directly into the attic. The last thing you want to do is put warm, moist air into the attic. A whole-house fan is acceptable because of its usual location, installed in the attic floor near a gable vent, and because it is not directly connected to a source of moisture. The whole-house fan can help to remove cooking odors and can cool the entire house when it is not hot enough to turn on the air conditioning.
  • If you cannot vent the bathroom directly to the outside, install the vent up through the attic and down through a soffit vent. This will prevent water from dripping back down into the vent as it would if you installed the duct straight up through the roof. Wire the bathroom vent to a timer switch, so that people can turn it on without having to remember to turn it off.
  • Always vent the clothes dryer to the outside with a smooth-walled, metal (not plastic) duct that is as short as possible. To prevent a house fire, check the duct for clogs regularly. Do not vent the dryer directly into the laundry room. This puts much too much moisture into the house.
  • Kitchens should have a vent hood with an exhaust fan. The vent hood should have a back draft flap to keep out insects and cold air - but some cold air will inevitably seep in.
  • Install ceiling fans to improve ventilation and distribute heat. To disperse heat properly, run the ceiling fan in reverse, so that it pushes warm air up against the ceiling and down along the walls, where people tend to sit.
  • Do not block air intake vents for heating or air conditioning equipment. Blocking these vents will starve the equipment for air, causing it to run inefficiently.
  • If your unfinished basement has windows, keep them closed on hot, humid days to prevent moisture from condensing on the walls continuously, all day long. Open the windows once the humidity drops below the natural humidity of the basement, so that moisture doesn’t build up inside.
  • People, in their zeal to do a good job, sometimes pack insulation into the eaves, blocking the soffit vents, because they don’t know that the vents exist or don’t know what they are for. If you are installing insulation in the attic for the first time, do not cover the soffit vents with insulation.
  • If your attic is already insulated on the floor, make sure insulation is not blocking the soffit vents. This is more of a problem for loose-fill, since wind can scatter the fill around. To prevent loose-fill from scattering and covering the soffit vents, you can install baffles between the rafters. You staple the baffles to the underside of the roof sheathing, and the baffles maintain 2 inches of ventilation space next to the sheathing.
  • Wind coming through soffit vents can also push batt insulation up off the floor, causing cold airflow against the ceiling and cold spots high up on exterior walls. Baffles installed near the eave should also prevent this problem, by keeping the batts from flipping up and over.
  • If you are going to install batts or spray foam between the rafters, you should extend the baffles all the way up to the ridge vent. This will keep the sheathing dry and prevent it from rotting invisibly behind the insulation.
  • Likewise, when you insulate between the floor joists in the ceiling of an unconditioned basement or crawlspace, you should leave some space between the insulation and the sheathing (subfloor) to allow water vapor to escape.
[edit]

Air-conditioning

An air-conditioning system provides heating, cooling, ventilation and humidity control for a building. It is often installed in modern offices and public buildings, but is 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. The alternative to large ducts to carry the needed air to heat or cool an area is the use of remote coils or split systems. These systems are gaining popularity in commercial buildings although are most often seen in residential application. The remote coil is connected to a remote condenser unit using piping instead of ducts. The units usually have a fan to move air across the coil, although recent innovation have seen static units installed in some large office spaces.

A dehumidifier is an air-conditioning-like device that controls the humidity of a room or building. They are deployed in basements, which because of their lower temperature (and propensity for damp floor and walls) have a higher relative humidity. In food retailing establishments large open chiller cabinets are highly effective at dehumidifing 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 control system to maintain constant air quality.

[edit]

Thermostats

Thermostats control the operation of HVAC systems, turning on the heating or cooling systems to bring the building to the set temperature. Typically the heating and cooling systems have separate control systems (even though they may share a thermostat) so that the temperature is only controlled "one-way". That is, in winter, a building that is too hot will not be cooled by the thermostat. Thermostats may also be incorporated into facility energy management systems in which the power utility customer may control the overall energy expenditure. In addition, a growing number of power utilities have made available a device which, when professionally installed, will control or limit the power to an HVAC system during peak use times in order to avoid necessitating the use of rolling blackouts. The customer is given a credit of some sort in exchange.

[edit]

See also

[edit]

External links

de:Heizung id:HVAC it:HVAC nl:CV-ketel ja:暖房

Retrieved from "http://www.netipedia.com/index.php/HVAC"