Ancient Romans, known for their love of beauty and luxury, would spend their spare time in public baths called thermae – an invention they had taken over from Greeks in around the 3rd century BCE and in the 1st century BCE adopted some technical improvements. One of the greatest architects and engineers in ancient times, Marcus Vitruvius Polio (70-20 BCE), in his book on architecture elaborated on their baths design, construction and heating. The baths were presented as rooms grouped in a specific order, according to the principle of economic use of heat.


The ancient hypocaust


The baths were presented as rooms grouped in a specific order, according to the principle of economic use of heat. The closest to the heat source were caldaria, that is, rooms with a hot plunge bath, and laconica – dry sweating rooms or speaking in contemporary terms: saunas. A bit farther were rooms offering baths in tepid water, so-called tepidaria, and the farthest located were frigidaria, that is, cold bath rooms. Auxiliary rooms (vestibulumapoditeriumunctorium) were situated on the perimeter of the bath complex along with numerous galleries, shops and sports grounds. Hot baths were at that time referred to as balneae pensiles, that is, hanging baths, and their heating system was called suspensura. It is believed that Caius Sergius Orata of Naples, a venturesome breeder of sea bream – fish at that time desired for their delicious taste, and a friend of Cicero, bred the fish in artificial pools with suspensura heating in the area of Baiae, a then fashionable resort. 



On the other hand, improvement of the heating system in thermae is bestowed on Vitruvius. Namely, it was a temperature regulator shaped as a bronze disc raised and lowered by means of a pulley. 


An archaic form of this system was used in ancient Greece, e.g., in steam and sweating rooms in the bath complexes in Olympia (4th/3rd century BCE) and in Gortys, Arcadia (2nd century BCE). The floor had an opening with a sliding cover and a hypocaust basement located underneath. The opening was used to adjust the inflow of heat into the room. The most grandiose Old Roman baths were built from the 1st to 4th century CE. Those were the thermae of Nero, Caracalla, Diocletian in Rome or the Stabian baths, the Forum baths and the Central baths in Pompeii. The expanded Roman thermae, compared to the Greek ones mainly designed for sporting purposes, became peculiar leisure complexes. Their most important element was the baths accompanied by regeneration rooms (spa), libraries, sports grounds and eating places.



Ancient central heating with warm air, called the hypocaust, is a classic form of radiant heating system. Radiation in heating means heat radiating from heated flat surfaces. The hypocaust was built as a vaulted furnace chamber (praefurnium) which by means of a small channel was connected to a hypocaust basement supported on pillars (suspensura) and with vertical ducts (tubuli), which originally carried off smoke outside. In general the operating rule of the hypocaust assumed that the room is heated from the underneath by heat radiating from a previously heated floor surface, and later also wall surface. The solution was used to heat rooms or pools with water mostly in ancient baths. Apart from thermae, certain residential and office buildings, both in ancient Rome and its northern provinces, were also heated by hypocausts.


So far the use of tubulatio in all the walls of a heated room was believed to be the so-called ducted heating. However, archaeologists found there had been a different variety of ducted heating without a hypocaust basement and often not involving tubes, in particular in walls. They also discovered numerous examples of a mixed variety, most frequently using brick, vaulted ducts radiating from a small central hypocaust basement supported on low pillars.



Hypocaust versus ducted heating


The use of the hypocaust, originating in ancient Greece and Rome, spread far to the north of the Roman Empire. It is believed that the classic form of the hypocaust involving suspensura was extinct in Europe between 6th and 7th century CE, making way for ducted heating. It was connected with the emergence of a new religion – Christianity – for the purposes of which water was heated in the early baptisteries. Ducted heating in the times of Merovingian and Carolingian dynasties was propagated in Western Europe, in particular in the empire of Charles the Great, and currently heating technology believes it to be a development of the hypocaust.


The ducted system had brick heating ducts passing through a solid material (stone, brick, or clay). The ducts forked from a cubical or a domed furnace. Seen from above, the arrangement of the ducts was mostly shaped as letters: X, Y, L, T or stellated. The ducts were distributed directly underneath the floor. Sometimes they were fitted with covered openings. Those elements suggest they are an intermediate link between the ancient hypocaust and the medieval storage central heating. The most interesting examples of innovations in the ducted system were found in Saalburg, Weiersbach, and Lalonquette, and mixed systems were discovered in Lalonquette, Monmaurin and Valentine.


In Europe the classical hypocaust was somehow revived in the 14th century. It was consciously modelled on Roman thermae from the 1st century CE in steam baths in castles and palaces of the noblemen in France and Flanders, e.g. in the seat of Margaret, Countess of Flanders, in Dijon (14th century).



Heating in the Middle Ages


The popularisation of the hypocaust in the territory of Poland is attributed to monks (mainly Cistercians), who implemented this system in their monasteries. Furnaces built in Teutonic castles (several dozen furnaces in Malbork) and hospital furnaces have been preserved.


Late Gothic hypocaust furnaces, also present in the territory of Poland from the 13th century, remained in operation until as late as the 17th century. They were made from bricks and consisted of a few parts: the furnace, heat storage chamber with ribs made from bricks and field stones accumulating heat, and barrel vaulting with vents through which heated air was released. The furnace was built next to a vestibule from which it was fired and where wood was stored. The furnace was elongated and low and at the back it had a flue carrying smoke off into a chimney stack. Over the furnace crosswise clay-joined brick ribs were built between which a small gap was left. Large irregular stones and smaller pebbles were loosely put on the ribs thus filling up to 3/4 of the heat chamber. The gap above the stones was used to accumulate hot air. The whole structure was covered by barrel vaulting with heat duct openings closed with a metal or stone lid mounted in the floor of the heated room. The flame produced by burning wood in a long furnace could evenly heat up the stones above and partly the main walls of the furnace which also stored heat. When the furnace fired the lids mounted in the floors of heated rooms were closed and the resulting flue gas and smoke were carried off through the chimney. The temperature in rooms located immediately above the furnaces was raised by releasing heat accumulated in the storage chamber by lifting the lids until stones were cooled.



Hypocausts were quite expensive to maintain because they consumed large amounts of fuel (sometimes wood was replaced with peat), so they were gradually replaced by tile stoves, which heated a separated area only. Single stoves made it possible to minimise the use of fuel to the amount actually needed to heat up a single room. At the same time there were also other, primitive forms of heating, mainly in rural cottages.


The stove was a fixed element of a Slavic chamber (…). This was necessitated by the northern climate. There are doubts about whether the household stove originated from a combination of an open hearth and a bread oven or evolved directly from an open hearth covered from all sides. Originally, a household stove was low and it filled the room with smoke. Smoky chambers were once common. Later, when chimneys were built to carry off smoke, such rooms disappeared but were preserved in isolated areas. [A. Brückner, Old Polish Encyclopaedia, Warsaw 1939, p. 122]


In smaller chambers occupied by well-to-do peasants initially open hearths placed in a central point were sufficient. With time they were enclosed with stone lining. Smoke escaped through openings in the roof. Those were domed stoves which in residential premises were less common than fireplaces. Their origins can be traced to wrapping fires with field stones. With time fires were covered with a clay dome which was opened at the topmost point or could even have an upright chimney.


Development of warm air heating


The Industrial Revolution and the availability of new materials had an impact on the development of warm air heating technology. An Englishman, William Strutt, in 1805 designed a boiler which was a riveted cast iron chamber enclosed by bricks a few inches apart to enable air circulation. The external part of the boiler was made of two chambers: the bottom chamber which cool air entered and the upper one from which hot air escaped. Ducts supplying air to rooms were fitted with dampers used to increase or reduce temperature.


At the same time air heating boilers were also built in France. They were called caloriferes and were composed of numerous cast iron tubes in the wall around the furnace. Air, flowing between hot iron tubes, was warmed up and passed to ducts leading to rooms.


Both Strutt’s boilers and the French caloriferes were important inventions; however, in terms of technology they were based on ancient solutions. Professor Paul Traugott Meissner presented a more scientific approach in his work Warm air heating system, published in 1821, in which he described various designs of boilers and air distribution methods in buildings of different types. A heating system designed by him was put into service for the first time in a sugar factory near Vienna (1820), and next in the Upper Belvedere in Vienna (1826). 




In 1840 in the White House in Washington work was started to install a hot-air heating system. The system was composed of an autonomous boiler with a cast iron furnace in a plastered brick enclosure. Ducts through which air was supplied to rooms ran both from the boiler itself and from the room in which it was installed. The boiler room was situated in the basement, in the central point of the Executive Residence – now housing a diplomatic reception area. Initially only rooms on the ground floor and the hall were heated, but five years later President James K. Polk ordered reconstruction of the system which afterwards was also connected to the so-called State Floor and the living quarters of Mr and Mrs President on the second floor. Ducts supplying warm air were terminated with ornamental grilles, which, depending on the representative function of the room, were made of silver, brass or iron. In 1853 Franklin Pierce was elected US President and – introducing further improvements to the White House – had the heating system changed into a hot-water heating system. 


A prototype of a hot-air heating boiler, which after 1850 gained immense popularity in the United States, was built in 1835 in Worcester. Insofar as previously there had been no specific boiler making industry (they were built ad hoc for the needs of specific solutions), the incorporation of Richardson & Boynton Co. in 1837 (in operation until 1942) can be regarded as the birth of the warm air heating equipment market in the USA. A real boom in the industry started after the American Civil War and lasted until the beginning of the following century. Hot air systems were believed to be modern, comfortable and efficient and also safe. At the same time they were much easier to install and operate than steam and hot water systems and significantly reduced the risk of explosion. 


In 1895 Ernest Bryant and Ezra Smith, two businessmen from Iowa, ordered the manufacturing of cast iron components for new boilers of their own design to Dave Lennox’s company. When, after the order was completed it turned out that the businessmen had no money to pay for the service, Lennox took over their patents, and later reworked the original designs according to his own idea. A structure made of riveted steel plates, under the brand name of Torrid Zone, was very successful.



Hot air heating boilers produced on a mass scale in the 19th and 20th century were not standardised in any way. Their parameters were determined by specifying the size of the furnace or the area of the living quarters the boiler was able to heat. Often identical boilers made by different producers had totally different parameters. The free-for-all practices in the industry resulted in a pricing war that in the early 20th century left many producers bankrupt. Many businesses were amalgamated into larger enterprises.


Troubles in the industry became worse when hot water heating systems, hitherto a niche solution, gained in popularity. In 1905, US businessmen established the Federal Furnace League with the main task of ensuring warm air heating its well-deserved position. Activity in support of standardizing and improving the quality of the products was carried out in the following years until 1914 when the National Warm Air Heating and Ventilating Association (NWAHVA) was established. At the same time, at the University of Illinois a number of studies and tests were performed. Their results, published by NWAHVA, defined the development trends for the industry for subsequent years. 


In the first decade of the 20th century fans were used in warm air heating systems. Previously only natural circulation of warm air had been used, which was not always effective enough. The first fans, for installation in return passages, were produced by Emerson Electric as early as 1908. However, generally they were not widely used until the late 1920s. 


Contemporary blow-in heating systems are based on various sources of heat, either solid or gaseous fuel boilers or heat pumps. Air cleaned by filters and then heated is distributed by a system of inlet ducts and enters respective rooms through adjustable inlet grilles. Simultaneously, used air from the room is sucked into the return passage. Before it is recycled into the furnace, it is mixed with fresh air from the outside and filtered. Manufacturers offer a wide range of equipment which, apart from heating, humidify air and in the hot summer season – cool it.