Hot-Air Heating

a system for heating premises by means of hot air. A hot-air heating system includes air pre-heaters, in which air may be heated by hot water, by steam (in air heaters), by heat given off by the burning of various types of fuel (in flame air preheaters), or by electricity (in electric air preheaters); air ducts, which deliver the air to the premises to be heated; air-supply and air-intake grilles, through which the air is fed to the area to be heated and collected for supply to the air preheater; and shutoff and regulating dampers in the air ducts. Air ducts, grilles, and dampers are not necessary if the air preheater is located directly in the area to be heated.

There are various types of hot-air heating systems, including recirculating (return) air heating, in which all of the air supplied to the air preheater is taken from the area being heated; and combined hot-air heating and ventilation, in which the air supply comes partially from the area being heated and partially from outside air. (The ratio between the recirculating and outside air volumes is adjustable over a wide range.) Combined hot-air heating and ventilation systems that operate only on outside air (without recirculation and return ducts) are sometimes called direct-flow systems. Such systems are used, for example, in residential buildings, where one air preheater serves several apartments. (In this case, a recirculating system would result in the undesirable entry of air from one apartment to another.) Nonrecirculating hot-air heating systems are also installed in industrial premises containing production processes that are accompanied by the emission of noxious gases or dust. The transfer of air in hot-air heating systems (in both recirculating and heating-ventilation systems) may be natural (owing to air temperature and density differences before and after the air preheater) or mechanical. An electric fan is installed for mechanical air transfer.

The chief advantage of a hot-air heating system over other types of central heating is the smaller amount of metal used owing to the fact that hot-air heating systems do not require the installation of heating equipment and piping such as that used, for example, in hot-water and steam heating systems. At the same time, the combined hot-air heating and ventilation system solves the problem of air exchange in an area and sometimes, in the case of preconditioning of the air supplied to an area (humidification, cooling, drying, and so on), the problem of air conditioning as well. In industrial and commercial workshops, the halls of public buildings, and buildings with a large number of rooms (in which the building structure makes it possible to use existing spaces as passages), a hot-air heating installation can be considerably simpler than other types of central heating. In the USSR hot-air heating, which is widely used in industrial workshops, is usually accomplished by means of units installed in the areas to be heated. Heating capacities for industrial hot-air heating units range from 5.8-11.6 watts to 5.8 megawatts (5-10 to 500,000 kilocalories per hour).

Combined hot-air heating and ventilation systems (without recirculation) have also begun to be used in schools and residential dwellings with four, five, or more stories. Hot-air heating is used for single-apartment residential dwellings in the USA and several other countries.