Heating, Ventilating, and Air Conditioning (HVAC), related processes designed to control conditions within buildings for comfort or for industrial purposes. Heating an area raises temperature to a more satisfactory level than that of the atmosphere. Ventilation controls both the supply and exhaust of air within an area in order to provide sufficient oxygen to the occupants and to eliminate odors. Air conditioning creates and maintains desirable temperature, humidity, air circulation, and air purity for the occupants of a space or for the industrial materials that are handled or stored there.
Heating Heating may be direct, as from a fireplace or stove in one room, or indirect, as in a central system. An open fire served as the earliest heating system. The first fireplaces were hearths, recessed into the walls of buildings, with short flues open to the air. Fireplaces with chimneys high enough to provide adequate air current for the fire were introduced during the 12th century, but 85 to 90 percent of the heat from the burning fuel went up the chimney. The stove, an enclosure of metal or ceramic materials in which fuel is burned, is more efficient, delivering about 75 percent of the energy of the fuel.
Central Heating Central heating systems, in which one centrally located heating unit warms several rooms or an entire house, were developed in the 1800s. Present-day systems provide heat from a central furnace for a single building or a group of buildings. Furnaces for heating systems are usually fired with oil, gas, or coal. As the fuel burns, it heats metal surfaces that transfer the heat to water, steam, or air. Most furnaces, large and small, are automatically responsive to remote thermostats that control their operation.
Radiators and convectors transfer heat to the area to be warmed. These units are placed in enclosures designed to permit air circulation; thus, heat is provided largely by convection, rather than by radiation. The term radiant heating describes systems in which walls, floors, or ceilings act as radiating units. Heating conduits are placed in the walls or floors during construction of a building.
In the simplest warm-air heating system, cold air is heated by contact with the hot surfaces of the furnace. As the air is heated, it rises through ducts to individual grills or registers in each room of the upper floors. In a forced-circulation system, a fan or blower in the furnace casing ensures the circulation of a large amount of air.
Modern hot water heating systems usually employ a boiler, in which water is heated. The water then circulates through pipes to radiators in various rooms. Circulation can be accomplished by pressure and gravity, but forced circulation using a pump is more efficient. Steam heating systems closely resemble hot water systems.
The practice of using electric energy for heating is increasing not only in residences but in public buildings as well. Electric heating generally costs more, but the convenience, cleanliness, and reduced space needs of electric heat often justify its use. The heat can be provided from electric coils or strips in varying patterns.
A heat pump provides both heating and cooling. Instead of creating heat, the heat pump transfers heat from one place to another. In heating season, a cold liquid absorbs heat from outside. It then flows to a compressor, which turns the liquid into a vapor that flows to an indoor coil. There heat radiates or is blown into the space to be heated. For air conditioning, valves reverse the flow.
Solar energy is often more than enough to heat a well-designed building, provided enough solar absorbing surface can be installed and enough heat storage is available for periods of darkness and inclement weather. Houses lacking central heating systems are equipped with various types of portable heating devices, many of which can be moved from room to room as needed. The most common types are kerosene stoves and electric heaters.
Ventilation
Buildings in which people live and work must be ventilated to replenish oxygen, dilute the concentration of carbon dioxide and water vapor, and minimize unpleasant odors. The small amount of air movement ordinarily provided by air leakage through small openings in a building's walls may suffice for homes, but not for public buildings. Factory ventilation systems must remove hazardous airborne contaminants. Simple ventilation devices include fans or blowers that exhaust stale air from a building, force fresh air into a building, or both. Ventilating systems may be combined with heaters, filters, humidity controls, or cooling devices.
Air Conditioning
An air conditioning system consists of centralized equipment that provides an atmosphere with controlled temperature, humidity, and purity year-round, regardless of weather conditions. In popular usage, however, the term air conditioning is often applied improperly to air cooling. Many so-called air conditioning units are merely blower-equipped refrigeration units that provide only a flow of cool, filtered air. Air conditioning systems provide fully controlled heating, cooling, and ventilation to theaters, stores, restaurants, and other public buildings. Such complex systems generally must be installed when the building is constructed; in recent years, these systems have increasingly been automated by computer technology to conserve energy.
Heat,
in physics, transfer of energy from one part of a substance to another, or from one object to another, because of a difference in temperature. Heat flows from a substance at a higher temperature to a substance at a lower temperature, provided the volume of the objects remains constant. Heat does not flow from a lower to a higher temperature unless another form of energy transfer, work, is also present. See Power.
Until the beginning of the 19th century, it was thought that heat was an invisible substance called caloric. An object at a high temperature was thought to contain more caloric than one at a low temperature. However, British physicist Benjamin Thompson in 1798 and British chemist Sir Humphry Davy in 1799 presented evidence that heat, like work, is a form of energy transfer. In a series of experiments between 1840 and 1849, British physicist James Prescott Joule provided conclusive evidence that heat is a form of energy in transit and that it can cause the same changes as work.
Temperature and Temperature Scales
The sensation of warmth or coldness is caused by temperature. Adding heat to a substance not only raises its temperature, but also produces changes in several other qualities. The substance expands or contracts; its electric resistance changes; and in the gaseous form, its pressure changes. Five different temperature scales are in use today: Celsius, Fahrenheit, Kelvin, Rankine, and international thermodynamic.
Heat Units
Heat is measured in terms of the calorie, defined as the amount of heat necessary to raise the temperature of 1 gram of water at a pressure of 1 atmosphere from 15¡ to 16¡ C. This unit is sometimes called the small calorie, or gram calorie, to distinguish it from the large calorie, or kilocalorie, equal to 1000 small calories, which is used in nutritional studies. In mechanical engineering practice in the United States and the United Kingdom, heat is measured in British thermal units (Btu). One Btu is the quantity of heat required to raise the temperature of 1 pound of water 1¡ F and is equal to 252 calories.
Latent Heat
Almost all substances expand when heated and contract when cooled. The process of changing from solid to gas is referred to as sublimation; from solid to liquid, as melting; and from liquid to vapor, as vaporization. The amount of heat required to produce such a change of phase is called latent heat. If water is boiled in an open container at a pressure of 1 atmosphere, its temperature does not rise above 100¡ C (212¡ F), no matter how much heat is added. The heat that is absorbed without changing the temperature is latent heat; it is not lost but is expended in changing the water to steam.