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Radiator as an exhibit at the Museum of Applied Arts (Vienna)
Ribbed radiator made of gray cast iron , example from the 1920s

Radiators are parts of heating systems in buildings. The hollow bodies, mostly made of steel, are also known as radiators , convectors or heating strips and, as heat exchangers, release some of the thermal energy transported by the heating medium (usually water, in electric radiators oil) to the environment in order to increase the room temperature. The heat dissipation occurs mainly through heat conduction to the surrounding air, which then distributes the heat in the room via natural convection , and to a very different extent via thermal radiation .

The radiator was invented by the Prussian-born Russian businessman Franz San-Galli in 1855.


Ribbed radiator

Examples of finned
heating pipes in a catalog of Centralheizungswerke AG from 1913

Ribbed radiators are cast iron radiators and were the first designs - the development is attributed to Hermann Rietschel . They had a smaller surface area than modern panel radiators with convection plates, a high weight and were very corrosion-resistant. The individual ribs were screwed through internal nipples so that the radiator could be put together in the desired size. Radiators for high pressure steam heaters are also held together by continuous tendons. In the past, ribs with cast-on feet were used as the first and last element in order to be able to set up the radiator on the floor, as wall mounting was often difficult due to its heavy weight. At the turn of the century and in Art Nouveau , ribs with an ornamental relief were also common.

Porcelain radiators

During the time of National Socialism, radiators made of porcelain were offered in order not to remove iron from the armaments industry. The idea was picked up in the GDR. From around the mid-1950s, porcelain radiators were manufactured in blocks of 4 and 5 ribs and joined together using flanges or spindles. There were also blocks of 3 and 6 more rarely. In the flanged design there was also a rare low version, which was manufactured as blocks of 5, 6 and 7. The blocks of 7 were mainly built into electric radiators. Porcelain radiators were manufactured in the VEB ceramic factory in Haldensleben and in the VEB electrical porcelain factory in Großdubrau.

They were mainly used in public buildings such as hospitals, boarding schools, etc., and occasionally also in residential construction. Various companies also made mobile electric radiator heaters from the porcelain blocks. The largest producer of these radiators was Gottschalk & Co Dresden. From the beginning of the 1960s, the production of all porcelain radiators was discontinued.

Sectional radiators

For sectional radiators , several sheet steel elements are welded together. A wide variety of lengths can be easily produced.

Panel radiators

Plate radiator made of sheet steel

The ribbed construction made of cold-formed and roll-welded sheet steel is characteristic of the most common design since the 1960s, the panel radiator or panel radiator. Large surfaces are achieved with air baffles or lamellas. The radiators produced in this way are painted or powder-coated .

Panel radiators are available in a wide variety of designs (type of connections, length, width) and colors and can be used immediately after installation. Their construction has been continuously optimized over the decades. By laying the convection sheets on the hot water channels and primarily by enlarging the convection surfaces, the heating output could be increased in the course of development. Today's panel radiators have a much higher material efficiency than z. B. older ribbed radiators and work because of the significant increase in the convection surface even at low flow temperatures. This is achieved through multi-layered water pockets and folded sheets in the convection stream. By reducing the volumes of heat transfer medium, faster reaction times can be achieved compared to external heat gains or losses. This means that low-temperature radiators can provide warmth in the house with water at 55 ° C, while older radiators sometimes need water up to 90 ° C for the same heating output in the same installation space.

Plate radiators are divided into different two-digit type numbers (e.g. type 11, type 21, type 22, type 33). The first number indicates the number of plates, the second number the number of convection plates.

Low temperature convectors

Modern condensing boilers work more efficiently the lower the return temperature of the heating circuit is. Heat pump heating systems can only be operated efficiently with a low flow temperature. In order to achieve sufficient heat emission from the radiators despite the low system temperature, they must be dimensioned larger.

If blowers are used, as with fan heaters , to increase the heat dissipation through forced convection , the size of the radiators can be greatly reduced. These can then also be used in confined spaces.

Available are:

  • Fan bars that can be retrofitted onto existing convectors.
  • Standard convectors with factory installed fan. These are called fan convectors, fan convectors or fan convectors.
  • Heating register that is integrated into an existing ventilation system and z. B. ventilation convectors are called.

The additional power consumption is usually very low. Noises, vibrations and the swirling of dust caused by the fans can be disadvantageous. It is usually not possible to record consumption with heat cost allocators , as the heat output varies. Instead, heat meters must be used.

Special forms

Towel radiator made of tubular steel

In addition, tubular steel radiators are increasingly being used; This design is preferred for practical ( towel dryer ) and aesthetic reasons, especially in the sanitary sector .

Further special forms are underfloor and wall heating as well as radiant ceiling panels. Here the surfaces of the walls, floors or ceiling panels provided with heating loops or electrical heating wires serve as large-area radiators. Another special form are hygiene radiators , which are particularly easy to clean and z. B. be used in clinics or in food production.

Heat distribution in the room

The heat radiation emanating from the radiator increases comfort . The heat radiation hits ceilings, floors, walls and furniture and heats them up. The heated surfaces and objects themselves then also emit thermal radiation.
Both the radiators and the heated surfaces emit their thermal energy through thermal conduction to the adjacent air layer. The heated air rises while the air cooled on the cold outer walls flows downwards: a convection current is formed.
Thermal radiation and circulating convection currents work together to ensure a relatively even distribution of heat within the room. As a rule, the air temperature near the ceiling is only a few degrees above the air temperature near the floor.

The heat transfer between the radiator and the room is always based on the principle of convection and thermal radiation. Convection heating and radiant heating therefore only differ in degrees:

  • With convection heating , the distribution of heat through the movement of the air predominates. The air temperature in a room heated by convectors is usually one to two degrees warmer than in rooms with surface heating .
  • With radiant heating , the average temperatures of walls, floors and ceilings are almost as high as the air temperature or, ideally, can even be higher. Unless air is circulated through large, cold window areas or the incoming air flow from a ventilation system, the heating heat is largely distributed via thermal radiation.

The amount of heat emitted by any body increases roughly to the fourth power of temperature. I.e. When the surface temperature of a radiator is doubled, the heat energy emitted increases by sixteen times. A small area of ​​very high temperature (such as a hot oven , an open fire, an infrared heater or a steam-operated radiator) can thus produce as high a radiation output as floor or wall heating operated at low temperatures .

Heated outdoor spaces as well as large, non-continuously heated rooms such as churches and factory halls are heated by heating pipes, plates or mushrooms that are brought to a high temperature either electrically or by gas. Thus, more energy is transmitted through thermal radiation in the infrared range than is used to heat the ambient air, which in such situations is largely drawn up unused.

In modern, highly insulated new buildings, the type of energy supply plays a less important role, since a relatively balanced temperature between the room air on the one hand and the floor, ceiling and walls on the other is established within the thermally insulated building envelope. A controlled domestic ventilation system continuously supplies air, the temperature of which is ideally slightly below room temperature, resulting in optimal (temperature) comfort.

In the case of poorly insulated old buildings, on the other hand, as high a proportion of radiation as possible for the necessary heating output is desirable. To compensate for the low surface temperatures of those walls, ceilings and floors that form the outer shell of the building, the heating system must otherwise generate a correspondingly large amount of warm air, which can be detrimental to health and comfort. In earlier times, tiled stoves and open fireplaces were sources of radiation that heated both the people in the room and the wall surfaces. At the same time, the chimney draft combined with leaky windows and doors in winter ensured a constant supply of cold fresh air.

In the post-war period, these fresh air sources were reduced by dismantling the chimneys and installing tight windows and doors. In particular, since after the energy crisis more and more powerful convectors have been used and operated with low flow temperatures, the rooms are heated less by radiant heat than by heating the room air. If the outer walls have no thermal insulation, there is a greater temperature difference between the heated room air and the cold outer wall surface in winter. This has several disadvantages:

  • The warm room air can absorb large amounts of moisture that condenses on the cold outer wall . Larger amounts of condensate can lead to the formation of mold , especially behind furniture, curtains and other hidden areas of the exterior wall, which cool down more than the rest of the wall. The moistening of the outer wall reduces its insulation value , whereby the surface temperature drops further.
  • In order to achieve the same level of comfort despite the cold exterior wall surfaces, the room air must be heated to over 20 ° C. It is believed that the warm, dry room air leads to health problems, as the mucous membranes dry out and more dust is transported than in cooler, more humid air.
  • With the increased temperature difference compared to the outer wall, the ventilation heat losses increase. If the room air temperature is 1 ° higher, an average of 5% more heating energy must be used.

Placement of the radiators

Traditionally, the radiators were installed under the windows. In this case, two circulating convection rollers can form: Warmed air rises from the radiators, while cooler air sinks down from the windows and outer wall above the radiators. Both air masses meet above the radiators and move towards the center of the room. At the latest when the air flow hits the inner wall on the opposite side of the room, it divides into an upper and a lower rotating air cylinder. Since the temperature of the rising and falling air mass on the outer wall equalizes to a mean value soon after the meeting, the drive of the circulating air masses is lost towards the center of the building (due to the difference in density between cold and warm air). This circulation movement is therefore so weak that it is hardly noticeable. If the radiators are instead placed on the inner wall, a single, significantly stronger air roll forms, which covers the entire room and is perceptible as a draft at very cold outside temperatures or inadequate insulation of the outer wall.

In today's very well insulated building envelopes, the installation of the radiators plays less of a role than the distribution and alignment of the air inlet openings in a ventilation system that may be present . Because in low-energy houses , more energy is often lost through ventilation than through the exterior of the building. The surface temperatures of the walls, floors and ceilings of a well-insulated building can be approximately equalized via the thermal radiation, so that there is no significant air circulation within the rooms.

Heat distribution in the radiator

Based on the English term for radiator ( "radiator" ), cast iron and tubular radiators are often referred to as radiators in German , although the radiation ( "radiation" ) in the heating circuit is low at today's temperatures. Radiators with heat or air baffles are called convectors . The air baffles transfer the heat from the radiator to the room air flowing through it from bottom to top.

Radiators should not be adjusted, covered or covered by curtains so that the air can circulate freely and as much radiant heat as possible reaches the room. Convectors that consist of several plates emit more radiant heat if the warm, inflowing water is first directed into the front plate facing the room and only then into the rear plates. (However, this will only be the case for some special designs in 2018.) Radiators that are installed in front of glazed facades must be provided with a cladding towards the glass, which prevents too much heat energy from being radiated to the outside. It also happens that the heat protection glass tears when tensions arise in the glass due to the local heating at cold outside temperatures.

On thermostatic valve is the temperature of the heater at its highest. The thermal energy is released into the room through thermal radiation and convection , while the carrier medium moves through the radiator. Towards the outlet valve , the temperature of the heating medium drops accordingly. The cooled medium is returned to the heat generator via the return line. If a large amount of air collects in the radiator, circulation is impeded and the upper part of the radiator remains cold. The ventilation takes place via a ventilation valve , by loosening the screw connection of the thermostatic valve or by flushing the heating circuit.

In widely ramified heating systems, the radiators should ideally feel a little colder in the lower area than in the upper area. If a radiator is equally warm above and below, more water will flow through it than necessary. This can mean that other radiators that are further away from the heating circuit pump receive too little flow. Then the pump output must be increased, which increases energy consumption and flow noises can arise in the circuit. It is therefore better to carry out hydraulic balancing using presettable thermostatic valves or adjustable lockshields . If a radiator only feels warmer near the thermostatic valve or in the entire upper area than on the rest of the surface, the flow rate is usually so low that the medium remains in the radiator for too long and cools down in the process.

If a radiator feels warmer in the front area over the entire height than in the rear area, the flow and return connections of the radiator may have been swapped. The heated medium enters the lower area of ​​the radiator and flows straight up to the outlet above without heating the rear area. If the thermostatic valve is opened further or the flow is increased in some other way, the warm water should be better distributed in the radiator. Rattling noises that may occur or difficulties in setting an average room temperature can in some cases be eliminated by using a thermostatic valve specially designed for this application.

At least since the oil price crisis in the 1970s, heating systems have been provided with controls that adjust the flow temperature of the heating circuit (and thus also of the radiator) to the outside temperature. The constantly high flow temperatures (often between 70 ° C and 90 ° C) of old systems caused unnecessary heat losses in the boiler and distribution lines during the transition periods .

Reflective foils, which are attached behind the radiators on the inside of the outer wall, can reduce the heating energy consumption of a building built in 1980 according to the thermal insulation standard of that time by 4%. A building with an average k-value of the wall of 0.5 W / (m² · K) results in a saving of 1.6%.

Determination of the radiator size

The most important factors for calculating the radiator size are the heating load or the standard heat demand of the building as well as the heating output of the heating system. The size of the radiator also depends on the window width and the sill height.

Various structural conditions influence the calculation of the heating load . In addition to thermal insulation, influencing factors are the area of ​​the external components, the number of joints, the size of the rooms and the difference between the external and internal temperatures. Accordingly, the heating load in an old building is greater than that in a well-insulated new building.

The heat output of the radiator describes the total heat output that a radiator has to generate in order to heat a room to the desired temperature. According to DIN 4701, the internal temperature should be 20 ° C in the rooms, 15 ° C in the corridor and 24 ° C in the bathroom. The first step in calculating the heat output is the room size. It is determined by multiplying the length of the room by the width of the room. The room size is in turn multiplied by a basic value. The basic value can be calculated as 80 watts per square meter. At 80 watts one assumes a well-insulated house / apartment. For a room with an area of ​​6 m × 8 m, this results in 3840 watts. You need a radiator with 3.6 kW. In a poorly or not insulated building, the guideline value is 150 watts per square meter of living space.

Radiator exponent

The radiator exponent describes the influence of changed temperature differences of a certain radiator type on its heat output compared to the standard values .

See also

Web links

Commons : Radiator  - collection of pictures, videos and audio files
Wiktionary: Radiators  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. the Sangalli / San Galli family
  2. ^ Johnny Acton, Tania Adams, Matt Packer: Origin of Everyday Things . Sterling Publishing Company, Inc. , 2006, ISBN 1402743025 , p. 205 (Retrieved February 4, 2015).
  3. Radiator types
  4. [1]
  5. A moisture content of 4% causes the thermal insulation value to drop by half. See Heinz Albrecht Beyer: Thermal comfort and low-dust indoor climate - building biologically sensible heating systems , title: low-temperature radiant heating, excerpt from: Healthy living and living
  6. Heinz Albrecht Beyer: Thermal comfort and low-dust indoor climate - building biologically sensible heating systems , title: low-temperature radiant heating, excerpt from: Healthy living and living
  7. Peter Rauch: Comfort in closed rooms - thermal radiation and thermal convection , IB Rauch
  8. a b Dietrich Beitzke: Why does a radiator have to be colder below than above? , in: Heizungsbetrieb.de, as of January 17, 2017, accessed in June 2018
  9. This effect also occurs when both the inlet and outlet are in the upper area of ​​the radiator. The medium then flows across it without heating the lower area.
  10. N. König: The influence of heat-reflecting foils in radiator niches on the heating energy consumption of a house , IPB Communication 58, 8 (1980) New research results, in short, Fraunhofer Institute for Building Physics
  11. Archived copy ( Memento from September 12, 2009 in the Internet Archive )