- The history of heating – from radiant to convective and … again to radiant?
- Heating at home – reality
- Description of radiant heating systems
- At the end
In this article: Radiant Heating – 10,000 Years of History; the first radiant heating systems; Russian stove – generator of infrared rays; radiation heat of the human body; types of modern household radiant heating systems; at the end – the conditions under which radiant heating will be more profitable than convective.
Approximately 200 years ago, the heating systems of our houses began to be reborn, stoves and fireplaces, popular for thousands of years, were called archaisms, they were replaced by a water heating system that gives convective heat. Over the course of a century, a cross was put on the radiant heat, it was written off to scrap, however, the studies of scientists carried out over the past half century show the completely opposite – the radiant heat surpasses convective heat in its characteristics, and in a number of characteristics. We propose to understand this issue and find out why radiant heating is better than convective.
The history of heating – from radiant to convective and … again to radiant?
For thousands of years, the first and only source of heating in a human dwelling was a fire, and the method of heating itself was convective-beam. During the burning of a fire in a primitive stove, and after that, when the fire was smoldering, infrared rays emanated from the stone portal, and as a result of convection, the air in the room was heated. The obvious disadvantage of this heating method is that when a fire was burning, the dwelling was filled with flue gases, creating an unbearable atmosphere. Therefore, at the top of the roof of the houses, a chimney hole was made through which hot smoke escaped along with the heated air, the main stake was made on radiant heating, since its intensity did not depend on the degree of air heating.
Two thousand years ago, new heating systems were created, based on channels under the surface of stone floors, along which flue gases from melted furnaces moved, heating the floors with their heat (hypocaust (Dr. Rome), gloria (Spain), ondol (Korea), dikan (China), etc.). The population of Europe, meanwhile, used a partially modified version of the fire – a cobblestone hearth heated in black. Only by the 15th century did the Europeans improve the stone hearth by bringing an exhaust pipe made of wood to it.
Heating system hypocaust
In the 17th century, the “Russian system” of heating was popular in the castle and palace complexes of Russia and Europe – the air intake shaft ran close to the furnace wall and along it, where the air was heated and, due to convection, rose through branched brick channels to the rooms that needed to be heated. Having given off heat, the air from the premises went out through the exhaust ducts outside the building. A heating system of this design completely excluded the possibility of flue gases entering the living quarters, which was an amazing know-how at that time. This heating system, called the “fire-air system”, enjoyed growing popularity until the middle of the 19th century, but by its end it was no longer in demand, which was facilitated by the constant low-frequency hum in the air ducts, excessive dry air, burning of dust with the deposition of dust soot on the walls and interior items.
At the end of the 18th century, the French engineer Jean-Simon Bonneman invented and built the first hot water heating system, in which the circulation of the coolant was carried out in a natural way. Half a century later, a heating system with natural circulation of the coolant, developed by Professor Petr Grigorievich Sobolevsky, appeared in Russia. Convection water, steam and fire-air types of heating have been gaining popularity from year to year, largely due to technical progress, the emergence and development of centralized sources of heating the coolant and systems for its delivery to consumer objects. The large-scale construction of typical high-rise buildings with minimal insulation of facades, low-quality overlap of window and door openings played in favor of convective water heating – radiant heating is effective only in a well-insulated building.
However, 150 years later, scientists have found that the perception of radiant heating is much closer to humans than convection heating of air. And not only for a person, but also for household items, as well as materials used in the interior decoration of premises.
Heating at home – reality
Have you ever been in an unheated or poorly heated room in the winter – a school classroom, an institute lecture hall, or an assembly hall at some institution? In response to the discontent of the audience, the teacher (lecturer) calms down – nothing, let’s breathe and in half an hour it will be warm. And indeed, after a while it gets warmer, but the reason for this is not at all connected with the term “breathed” – those present warmed the atmosphere of the room with thermal radiation generated by their own bodies. Infrared rays emanating from the bodies of those present in the audience heat up the objects located near them, which, in turn, generate their own radiation, transferring it to neighboring objects, and the heat of their surfaces to the air.
Any and every object with a temperature above absolute zero Kelvin (or –273.15 ° C) emits infrared rays. The higher the temperature of the object, the more intense the radiation – for example, the human body at its normal temperature (from 36.6 to 37 ° C) generates infrared rays of the medium wavelength range, with a wavelength of 5 to 25 microns. The consumption of human energy for infrared radiation is reduced if the temperature of the environment rises, but not the air, but the enclosing structures (walls, ceiling and floor) and furniture. The fact is that the air environment is transparent and permeable to infrared rays, respectively, cold walls and floors will draw infrared heat from human bodies even at a 25-degree room temperature – this is radiant heat exchange, explained by the laws of Planck and Stefan-Boltzmann.
Generations of townspeople are accustomed to living conditions in brick and panel houses, trying to compensate for the infrared energy of the body, which goes to heating the enclosing structures, with the help of various types of electric convectors. In the memory of the townspeople there was a vague conviction about the importance of wooden walls in the house, which are able to “breathe”, compensating for the humidity of the air – indeed, such an ability is present in unpainted timber and log walls, but the main role in wooden houses was played not by them, but by the Russian oven.
The massive construction of the Russian stove was given a significant place in the house, it kept heat perfectly and heated the whole house with infrared radiation. No water or air heating system can compare in its heating capabilities with a Russian stove! By the way, it is precisely because of the radial heating method that baked goods in a Russian oven turn out to be much more appetizing and tastier than in the most modern oven, the cooking principle in which is based on hot air (fire-air system).
The properties of radiant energy from the standpoint of heating were investigated by a laboratory at Yale University, funded by the John Bartlett Pearce Foundation – the results of an experiment conducted with the participation of volunteers were very revealing. At the first stage, the subjects were placed in a small room with artificially cooled walls, the air temperature in it was maintained with the help of fan heaters at 50 ° C – volunteers dressed in light clothing, after staying in this room, complained of extreme cold. During the second stage, the air temperature was deliberately lowered to 10 ° C, and the walls were heated with the help of pipes built into the interior, through which hot water circulated – the subjects, dressed all the same lightly, sweated profusely when they were in this room, they were hot.
However, each of us can check and personally experience the “vampirism” of cold and “donation” of heated walls at any time – you just need to come up and stand in front of the wall. In winter you will feel the cold coming from it, since the material forming the wall will absorb the infrared rays emanating from you, in the summer you will feel the warmth, that is, your body will already absorb the infrared radiation received by the wall from the Sun during the day.
Description of radiant heating systems
A massive stove was and remains an ideal source of radiant heating, however, in an apartment or office, and in many private houses, it is unrealistic to arrange such a stove. Consider modern radiant heating systems that make it possible to do without such a stove – “warm floor”, wall and ceiling radiant panels.
Underfloor heating systems differ in design and heating principle:
- convective systems include any systems with a water heat carrier, as well as cable, cable laying in heat-insulating plates and film (heating mats – a thin cable placed in a mesh base);
- radiation heat is generated by carbon film (heating element – graphite strips sealed in a polyester film) and core floors (their heating elements are also made of graphite).
The panels installed on the walls are modular blocks made of copper pipes, the heat carrier in them is hot water. Heat transfer of radiant heat from wall panels with circulating hot water at a temperature of 40 ° C is about 80%, the remaining 20% is due to convection – this is due to the permissibly high temperature of the coolant, which exceeds the maximum set by European standards of 30 ° C for a “warm floor”.
Copper modular blocks are installed on the wall surface using horizontal or vertical rod supports, before that a layer of insulation with aluminum foil is mounted on the wall surface. After installation, the wall panels are sealed with a 350 mm layer of plaster, covered with plasterboard or other hard coverings. In addition to external installation, modular blocks for radiant heating can be installed inside concrete walls – they are attached to a reinforcing frame with subsequent pouring with concrete.
The advantage of wall panels is lower thermal inertia, compared to “warm floors”, which is especially convenient for buildings with intermittent heating. It should be noted that for effective heating, wall panels need free space around the perimeter of the walls in which they are installed – with a large number of cabinet furniture, it is irrational to use them.
The first models of radiant ceiling panels were created long before the “warm floors” and wall panels, the interest of manufacturers in them was explained simply – the ceiling, and hence the ceiling panels, was located farthest from households, which made it possible to heat the panels to high temperatures without any damage to humans. The maximum temperature of modern ceiling panels depends on the height of the ceilings – the optimal difference between the air temperature in the room and the surface temperature of the beam panel is 10 ° C. Modern ceiling panels are not built into ceilings – they are installed on the surface of the ceiling, which simplifies their installation and maintenance.
At the end
The popularity of convection heating today is connected only with the fact that most houses have minimal heat-retaining characteristics – earlier this did not interest designers and builders, since their tasks were focused on reducing the cost of projects. Hence, houses glowing at night in infrared detectors, colossal heating costs and frequent cosmetic repairs. And precisely because of the high heat losses through the window openings, heating radiators were installed directly under them – to cut off the cold air from the street coming through the slots of the window frames and through their glazing.
Convective heating allows you to quickly and relatively inexpensively heat non-insulated rooms, but it does not allow you to avoid drying out the air, cold air at floor level (the warmest layer of air is collected at the ceiling), constant mold growth of the walls in the cold season (due to moisture deposition on their cold surfaces) and the need for frequent cosmetic repairs – the above facts are undeniable.
If the enclosing structures of the house are made of wood, brick or reinforced concrete, insulation (sandwich panels, thermal insulation materials followed by plastering, etc.) is carried out on the outside (street) side, and modern doors and windows are installed in the window and door openings with sufficient low rates of thermal conductivity, then solving the problem of heating with the help of a radiant heating system will justify itself. On the other hand, when insulating the enclosing structures from the inside of the room, which is performed especially often in multi-storey buildings of Soviet construction, it makes no sense to build a heating system on infrared heating, since the material from which the walls are made will not heat up and give off heat in the form of radiation, because wall surfaces are thermally insulated with thermal insulation materials.
Taking into account the new requirements for the thermal protection of buildings, set forth in SNiP 23-02-2003, radiant heating systems may well take over the leadership in convective heating. It will be much more pleasant and useful for households of any age to perceive infrared rays of a certain wavelength than to be in an air “aquarium” with constantly cold walls, filled with air heated as a result of convection and suspended dust.