Modern technical thought in heating was manifested by attempts at using an intermediate heating agent which started from experiments with steam. Water was heated in a covered vessel from which pipes were connected to heaters.
In the first systems water condensate was recycled into the boiler through the same pipes which supplied steam to heaters. An argument for using steam was the wide availability of boilers and – already during the industrial revolution – also steam engines. Of course the return of condensate should be ensured.
Initially, the pressure of steam used by such systems was 1-2 bars. Heaters were made of tubes: smooth or ribbed. Also, coils were used. In subsequent years low-pressure steam heating systems were introduced, but they had drawbacks as their construction was expensive and they could not be accurately regulated (adjustment by a change in pressure). Steam, as a heating agent, was predominant in the times of the so-called “pumpless age”, before water was used as a carrier of heat.
Around 1700, Russian engineers started designing water-based central heating systems. In the early 18th century water installations were put into service: in the Summer Palace in St. Petersburg (1710-1714), where a central water tank was installed along with a system of decorated porcelain pipes distributing hot water. In 1713 in France and the Netherlands equipment which could be deemed the prototype of the contemporary central heating system was installed. Similar structures were built in Sweden and England where in 1716 the first warm water central heating installations were put into service.
Around 1820 William Atkinson, an architect, developed an effective method of heating hothouses with hot water. Here, it should be noted that “warm water” implies temperatures below 100 °C – in “open” installations linked to atmosphere, whereas “hot water” means a liquid with a temperature exceeding 100 °C in a “closed” system under pressure. The systems had no pumps and water was circulated – as it was incorrectly defined – “by gravity”. In fact it was a common phenomenon of Convection in liquids and gases, based on the principle of difference in density.
This phenomenon is correctly termed thermosiphon (heat siphon) and is caused by “thermodynamic pressure” according to the first law of thermodynamics. An increase in internal energy is characterised by an increase in temperature and work (heat x weight) is a product of pressure and the thermal expansion coefficient. Thus, the circulation of water heated in a boiler and cooled in heaters giving off heat to the room, illustrating the first law of thermodynamics, is a thermal machine performing work to maintain the flow. Between 1818 and 1841 low-pressure water heating (t<100 °C) was used without any calculations. The theoretical relationships were first formulated by Hood who explained why water was circulating. Research by a French physicist, Péclet, (who proposed a popular formula of K coefficient) and Grashoff in the 19th century contributed to the development of thermal science and heating applications.
The use of a force other than the effect of the difference in the density of water resulting from differences in temperature, stimulating water circulation in low-pressure water systems – except for pumps – can be attributed to a Danish engineer named Reck. Similar heating systems with “accelerated water circulation” were developed by, among other inventors: Brückner, Körtin and Krell who used pulsometers. In heating with accelerated water circulation steam or air was supplied to feeders as they were lightweight substances causing water to flow faster.
The first water-tube boiler was built by John Blakey and patented in 1766. It was composed of a dozen or so inclined tubes with neighbouring ends connected. However, an American inventor, James Rumsey, was the first to invent a usable appliance of this type. In 1788 in England he patented more than a dozen boiler designs. Some of his patented solutions have been in use until now. The first boiler built as a combination of tubes connected to one tank was designed by John Stevens in 1805. The boiler had 12 horizontal tubes (1" in diameter and 50" in length) arranged in a circle and connected to a water tank at the bottom and to a steam tank at the top. The following years brought new solutions but a real breakthrough was the invention of Stephen Wilcox of 1856 in which inclined tubes connecting the water tank in front of the boiler with a steam tank in the rear were used for the first time. Nine years later this solution was used by Joseph Twibill in his sectional boiler.
At the early stage of development of heating systems steel boilers with brickwork were built. Americans were the first to use cast iron to produce boilers assuming that the devices operate at very low pressures. Thus, common use was made of a boiler type which:
• did not need brickwork;
• was made of identical segments, so it could be assembled in any size;
• could be produced on a mass scale;
• and could be most easily repaired by replacing the defective segment.
These rules were introduced in Europe by a German engineer, J. Strebel, who, having returned from the USA, in 1892 patented a cast iron boiler fired with coke and coal. A characteristic feature of this boiler was the semi-circular top part. At the same time scores of other boiler varieties were developed – more than a few hundred types. Some boilers built under Strebel's licence (Strebel ALFA, TERTIA, B2) were still in operation in the 1980s. The structure was so common that “Strebel” was used as a synonym for a hot water boiler. In Poland the history of steam boiler production is closely connected with the production of water pumps. Its origins date back to the mid-19th century. The Alexander Ironworks established in 1817 in Białogon near Kielce upon the initiative of Stanisław Staszic (in 1827 transformed into the Machinery Manufacturing Plant) apart from farming machinery produced various types of pumps and later also steam boilers. In the territory then under Prussian rule, on the borderline between Środula and Konstantynów, in 1880 Wilhelm Fitzner, a factory owner from Siemianowice, and the Swiss Konrad Gamper, built metal works.
In 1831 Angier Marsh Perkins, an American engineer working in England patented a boiler and an expansion valve which formed the basis for the new steam heating system. The inventor used wrought iron tubes with a diameter of 25 mm and a 6 mm thick wall (partially produced from excessive production of rifle tubes), in which steam pressure reached 50 bars and temperaturę approx. 200 °C. In subsequent years Perkins replaced steam with water which was much safer to use. He claimed that: there is no certainty that the best made boiler of the ordinary kind may not become red hot and explode by a sudden generation of steam. But we have a boiler which cannot explode or wear out, for it is completely separate from the fire…. This was an early version of the “indirect” heating system to be found in many installations today.
Unfortunately, the invention also had some drawbacks. In November 1841 The Times published a list of 23 installations seven of which had fires caused by direct contact between objects and hot heaters. In addition, in 16 other cases the apparatus had burst. However, Perkins regularly introduced improvements and modifications and the hot water heating system patented in 1841 (Perkins' Hot Water Apparatus) was installed in many buildings such as, for example, in the building of the Royal Society of Arts, the Patent Office in London and a number of divisions of the British Museum. Interestingly, installations dating back to the mid-19th century are still in operation in some churches in England and South Wales.
To use steam or hot water for heating suitable heat exchangers had to be designed and installed inside rooms. Nowadays these are called radiators. In 1824 Thomas Tredgold, a steam heating pioneer, proposed that heating tubes should be somehow “concealed” or that other, more aesthetic solutions should be used. Heaters used at that time were most often built of cast or wrought iron tubes with a diameter of 100 mm (similar to hothouses). One of the options was the use of cast iron coils with a tube diameter being respectively 2" for steam systems and 4" for water heating.
However, the father of the radiator is believed to be Franz San Galli who patented his invention in 1855 in St. Petersburg. Italian by descent, he was born in Kamień Pomorski (or Szczecin at that time known as Stettin) under Prussian rule. In the mid-19th century he designed a heater making use of compressed steam circulating through a system of pipes. San Galli called his invention a “hot box” and produced radiators under the name of “batteries” which in contemporary Russian is still a synonym for radiators. The first “hot box” was mounted in St. Petersburg in 1855. In England two inventors: Joseph Nason and Robert Brigss designed and produced in 1863 a radiator built from vertical tubes of wrought iron screwed into a cast iron base. In 1872, Nelson H. Bundy came up with the “Bundy Loop”, a radiator in the form of a cast iron loop. Initially the structure was adapted for use with steam heating but soon it was used in water heating systems.
The history of water-based heaters in Poland can be traced back to Stąporków where a forge operated from the end of the 15th century. From that time the village was famous for its cast iron radiator manufacturing plant – the only one in the Kingdom. A relic of the past excellence is the more than 2 m tall radiator monument standing in front of the Community Centre in Stąporków.
A real turning point for central heating systems was the use of circulating pumps around 1925. Previous solutions based on the so-called “gravity circulation”, were subject to considerable restrictions. Water, flowing through the system, had to overcome resistance in pipes, which is only possible when the pipes have an adequately large diameter. When this system was used in larger buildings, pipes with large diameters considerably increased the costs of installation.
Thanks to electric circulating pumps forcing the flow of water in the system it was possible to overcome strong hydraulic resistance occurring during flow through pipes with much smaller diameters than in “gravity heating”. In addition, pump systems were not subject to any restrictions regarding their size and range. The basic, most often used piping system was a double pipe system. Every radiator was individually connected to risers: flow and return pipes. Due to this fact the temperature at inlets to respective radiators was very similar. The temperature of the radiator was adjusted by means of a regulating valve attached to a radiator.
The development of central heating technology in the second half of the 20th century primarily referred to regu-lators, materials used to construct them and the use of more and more efficient sources of heat. The beginning of the 21st century for central heating was largely equivalent to the implementation of environmental postulates. New alternative sources of heat such as heat pumps or solar thermal collectors are incorporated in heating systems.