Origins and evolution of radiant systems

The following image shows the typical heating system used by the Romans:

But based on the archaeological evidence we can understand that the Romans are not the real inventors of the heated floor. In fact, the first radiant systems were made by the Chinese, the Egyptians and the Greeks. The best known example is that of the Baths of Olympia (100 b.C.).

These were generally rooms used as spas or public baths and the technique used was rather rudimentary: an underground fireplace was created by conveying the fumes and hot air into a special cavity under the floor of the room to be heated ("hypokaio" = switch on under).

But not even the Greeks can be considered the inventors of the heated floor! Not everyone knows that this "one-room" heating system was already in use in Korean rural architecture and that the oldest remains date back to 300 b.C. The system was called “Ondol” (hot stone) and in its simplicity it shows a surprising ingenuity.

In the kitchen hearth, firewood, brushwood or grain remains were burned. The fumes and hot air were conveyed under the floor of the adjacent room, to then exit through a chimney on the opposite side. It is clear that in winter (severe cold) people lived in the room equipped with ondol, while in summer (hot and humid) people lived in an unheated room.

With the Romans the system was perfected in the so-called "Hypocaustum".

The oven acquires larger dimensions (Praefurnium) and in this way it is possible to heat more rooms (Tepidarium, Calidarium). To facilitate the outflow of fumes and hot air under the floor, some measures are adopted such as increasing the height of the cavity and the modified shape of the support pillars, made with a circular section or with rounded edges. To replace one or more chimneys, cavities made in the walls with special perforated bricks (tubules) are used, which guarantee the draft of the system. By increasing or decreasing the fire in the Praefurnium, it was possible to somehow regulate the thermal power diffused under the heated rooms. The Romans are therefore the forerunners of floor and wall radiant systems.

The Roman hypocaust technique remained practically unchanged for many centuries. When the Arabs conquered Syria in the 7th century a.C. they also took possession of the Roman thermal buildings, acquired the technique and built the so-called "hammam" (which spreads heat).

While the public bath falls into disuse in the West, Arabs and Turks continue to adopt the Roman heating system. It is precisely the expansion of the Ottoman Empire, throughout the Middle Ages, which allowed a large-scale diffusion of the construction method of the hammams.

To be able to talk about a real hydronic radiant system (that is, working with water instead of hot air) we have to wait for the beginning of the twentieth century.

It was an English thermotechnician named Arthur H. Barker (1870-1954) who conceived and patented a hot water radiant heating system. As a curiosity, the patent bears the following title: "System for heating rooms with hot water conveyed in pipes". In 1909 the Crittal Company bought the patent rights and built the first underfloor heating system in the Royal River building in Liverpool, with metal pipes embedded in the floors. This is the prototype of the radiant system of modern conception.

Everyone remembers the American architect Frank Lloyd Wright and his most famous project: "The house on the waterfall":

Perhaps very few people remember another of his projects: "The Jacobs House" in Madisons, Wisconsin:

In 1905 Wright went for the first time to Japan where, most likely, he had been able to admire some heated houses with an evolution of the ondol system. In 1909 he had certainly learned of Barker's studies (who in the meantime had also patented his system in the United States) and of the first case of application to the Royal River.

Based on this knowledge Wright designed the "Jacobs House" in 1937, where on the ground floor there were metal pipes for the hot water heating system. We are facing the first case of a combination of a radiant floor system and modern architecture.

In Europe radiant systems remained virtually unknown until the end of the Second World War, when the reconstruction works, together with the shortage of radiators and copper, created the conditions for a rapid spread of these heating systems. The technique was based on the use of 1/2 "or, more rarely, 3/4" steel pipes, embedded in the thickness of the floor without any insulating layer being subjected:

In Italy, the first examples of application are the Venice Railway Station (1952), designed by Eng. Paolo Perilli, and the Cathedral of Lodi (1958) designed by the engineer Franco Palmizi, which represents the first system of copper floors in our area:

Excluding the case just mentioned, which is a rare exception (copper was an expensive material in the post-war years), many buildings were built with the technique of anchoring steel pipes directly to the slab without using an insulating layer in all of Europe.

From this point of view, a Technical Bulletin published by the Dalmine company (leader in Italy in the production of steel pipes) in the 50s is extremely interesting, some passages of which are reported:

The cost of the panel system is much lower than that of the radiator systems, as shown by the fact that approximately 11 m of 1/2” pipe correspond, in terms of thermal emission, to 1 mq of radiators.

In panel systems, on the other hand, in addition to the difference in cost of the system itself, the following factors must also be considered:

• the installation of the panels takes less time than the installation of the radiators;
• a bricklayer is not required for the opening of holes, for the masonry of anchors, supports, etc ...;
• no painting is required, nor is it necessary to fix or reattach the radiators that hinder the finishing work;
• with the panels you avoid the need for modifications, and therefore additional expenses, in case of moving the dividers.

But the plants built in this way decreed an inexorable failure of radiant heating. In a few years there have been many episodes of disputes from end users. Numerous cases of malaise have been documented, even with the support of medical certificates: high blood pressure, poor circulation, chronic headache, excessive sweating, etc ... In some European countries, such as France and Germany, commissions have even been set up to investigate on reported episodes and try to answer the problem.

The final reports of these Commissions highlighted that the complaints were not without foundation and that the inconvenience caused by these plants was essentially due to two factors:

• Excessive floor surface temperature: today we know that well-being is guaranteed by a maximum surface temperature of 29 ° C, while in those years the first systems operated at 38-40 ° C.
• Excessive thermal inertia of the slab: no insulating material was used under the pipes and this led to a considerable accumulation of heat in the slabs, which continued to yield energy even when the system was off, causing the rooms to overheat and thus amplifying the discomfort of the inhabitants.

The Commissions of Inquiry determined the decline of radiant systems, even if in the published reports the opinion on this type of systems was not negative at all: if designed and built according to precise parameters, they could undoubtedly offer greater comfort than that of radiators or convectors. However, due to the supervening distrust of this type of systems, radiant systems were no longer used for many years.

The situation changed radically in the 70s with the energy crisis. In all European countries, new laws are enacted requiring the insulation of buildings to contain energy consumption.

In Italy, law 373 of 1976 is known: “Rules for the containment of energy consumption for thermal use in buildings”. At the same time, the first technical specifications to support the work of the designers begin to be issued. For radiant systems, the milestone is represented by the German standard DIN 4725 of 1992. Due to the energy crisis and thanks to the greater attention to the insulation of homes and the introduction of plastic pipes (the patent for cross-linked polyethylene dates back to 1965 ) we are witnessing a real rediscovery of radiant systems.

With the use of plastic pipes it is possible to reduce the installation distance (with the metal pipes a distance of about 30 cm was used) and thus guarantee a uniform distribution of heat, as well as greater reliability of the systems, speaking of resistance to corrosion and breakage (wear, scale and leaks in the joints were the biggest problems in metal pipes). The standards introduce the concept of maximum surface temperature (29°C) and prescribe an insulating layer under the pipes; the result is a reduction in the water temperature and a lower thermal inertia of the floor. All these factors favored a return of radiant systems in the 80s, especially in the Nordic countries, where there was greater attention to thermal insulation and where legislative activity was more effective.

In Italy, the success of radiant systems comes above all in the 90s, with the introduction of some innovations in the legislative field, but this is a completely different story ...

Please note. Image source: web