Floor Heating
Despite the fact that radiant, large-surface floor heating with warm air in floor ducts has been known for more than two thousand years, and hot-water radiant large-surface heating with heating pipes cast into the building structure (ceiling, wall and floor) for almost 100 years, heating systems with traditional heat emitters - radiators - and convective heat transfer to the interior still clearly predominate in practice.
So the basic question is: why convective and not radiant heating?
RADIANT HEATING
With radiant heating of interiors, the radiant heat flow of the heating surface (emitter) is used, which directly - i.e. without the mediation of the interior air - heats the surrounding building structures of the interior, with only a very small part of the heat flow transferred by the radiant surface via convection directly to the air in the heated interior (i.e. the opposite of convective heating of an interior by a convective heat emitter). This is also why the internal surface temperatures of the building structures forming the interior are, with radiant heating, higher than the air temperature in the interior heated this way (again, the opposite of convective heating of interiors).
The heat rays emitted (radiated) by the surface of heated solid bodies (radiant heating surfaces) are in fact electromagnetic waves with a wavelength of 0.78 to 400 (i.e. within the infrared part of the electromagnetic spectrum), propagating at a speed of 300,000 km.s⁻¹.
HEAT DISTRIBUTION IN RADIANT HEATING
When theoretically designing heating systems in the interiors of building structures, it is always necessary to determine the heating input, i.e. the heat flow needed to ensure the required thermal comfort for the person - the user.
In heating engineering, the heating input is most often calculated from the thermal balance conditions of the heated space (interior, room, etc.) in a steady state. Precisely in this calculation, it is essential to distinguish between convective heating (where the heat emitter transfers heat flow to the heated interior mostly by convection) and radiant heating, i.e. floor heating (where the heating surface transfers heat flow to the heated interior mostly by radiation).
From these considerations it follows that, with the radiant method of heating, the heating surface transfers heat flow by radiation - without the mediation of the interior air - to the surrounding cooler surfaces of the interior, whereby the effective temperature (i.e. the average temperature of the surrounding surfaces) is higher than the air temperature.
It is therefore clear that, with radiant heating, a substantial part of the heat is transferred by radiation and only a small amount of heat flow is transferred by convection. Therefore, when calculating the heating input, we must determine:
- the thermal balance of the radiant heating surface,
- the thermal balance of the air,
- the thermal balance of the radiant heating surface and the air,
- the thermal balance of the heated space.
TYPES OF RADIANT HEATING
Radiant heating can currently be divided as follows:
- large-surface heating (ceiling, wall and floor heating surfaces), known as floor heating
- overall heating with suspended radiant panels,
- individual heating with nearby panels (radiant panels),
- heating with infrared radiators.
From this classification it can be seen that, in principle, the radiant heating surface can be located within the building structure as an integral part of it (large-surface heating systems), or it can be formed as a separate heating surface freely placed in the space (radiant panels, heating panels, infrared radiators).
Based on this structural difference, there are also fundamental differences in terms of the surface temperature of the heating surfaces, and consequently also in the choice of the heat transfer medium and its temperature, and finally also in the related specific heat output of the radiant heating surface.
LARGE-SURFACE RADIANT HEATING
In large-surface radiant heating, the heating surface is usually one of the surfaces bounding the heated space - either the ceiling, wall or floor.
The surface temperature of the heating surface must be relatively low (40 to 45 °C for ceiling heating, 55 to 60 °C for wall heating and 25 to 30 °C for floor heating), which means the medium temperature will also be similarly low. The heating surface can be heated by:
- hot water (most common),
- warm air,
- electricity (resistance heating).
From the above it is clear that large-surface radiant heating systems operate with lower heat transfer medium temperatures. These low-temperature systems are suitable for using energy obtained from low-potential energy sources, such as geothermal water, solar energy, or waste heat and ambient heat. The heating surface in large-surface heating is one of the surfaces bounding the heated space. Depending on the surface used, large-surface systems are divided into:
- ceiling heating
- wall heating.
Large-surface radiant heating differs from convective heating by heat emitters in the way heat is transferred and in the thermal conditions in the room. With convective heating, the heat emitter heats the air in the room, which transfers heat to the walls. The indoor air temperature is higher than the wall temperature. A characteristic air flow pattern arises in the room, resulting in a relatively large temperature difference between the air near the ceiling and near the floor.
With large-surface heating, heat transfer takes place by radiation and convection. The share of radiant heat is 80% for ceiling heating, 65% for wall heating and 55% for floor heating.
The structural design of the heating surface can vary considerably. Basically, we distinguish two basic solutions:
- the heating surface is built in, i.e. it is an integral part of the building structure,
- the heating surface is a separate heated panel, either:
- attached to one of the building structures,
- or placed freely in the heated interior.
In the first case, the surface of the building structure is heated by hot water flowing through pipe coils (registers), which are cast into the building structure. When retrofitting radiant heating, the heating coils are subsequently suspended below the load-bearing ceiling in a completed building and covered with a layer of plaster in the suspended ceiling. Radiant heating panels, which are mounted on the building structures in a room (or placed freely in the space), are most often made of metal, ceramic or glass. These panels are usually heated electrically.
LARGE-SURFACE FLOOR HEATING
If the heating coil (register) is part of the floor of a given interior, we speak of large-surface floor heating. This is currently a heating system which, thanks to its many positive properties, is experiencing something of a renaissance and is being increasingly used in residential construction (especially in family houses), in public amenity buildings (kindergartens, nurseries, gyms, swimming pools, etc.), but also in large-volume halls (multi-storey car parks, industrial plants, agricultural buildings).
In the past, large-surface floor heating was used as a supplement to ceiling heating, when the ceiling surface was not sufficient to cover heat losses. Wider adoption of floor heating was also hindered by the requirement for a low floor surface temperature, which was linked to low heat output. New designs, architectural approaches, improved thermal properties of buildings and advanced construction materials for heating systems have now made it possible for a wide range of floor heating systems to emerge.
The choice of floor heating is primarily determined by the building itself. It must meet thermal-technical properties such that the average heat loss should be less than 20 W.m⁻³, or the average annual heat consumption should be lower than 70 to 80 kWh.m⁻². This shows that the minimum energy demand is primarily determined by the building itself, followed by the operating regime with the possibility of heat storage in the floor with a thermal inertia of approximately 4 to 8 hours, and with a high degree of self-regulation.
The optimal thermal condition of interiors with floor heating is usually achieved with resultant temperatures 2 to 4 K lower than with convective heating, together with an almost ideal vertical and horizontal temperature gradient.
HOT-WATER FLOOR HEATING
From the above it is clear that hot-water floor heating belongs among the radiant heating systems, although the share of the radiant component in the total heat transfer from the heating surface is only slightly higher than the convective heat flow (55% : 45%). At the same time, this very favourably combines the advantages of both methods of heat transfer to the interior.
Hot-water floor heating is also large-surface heating, i.e. the heating pipes are part of the floor structure. Due to hygienic requirements related to the limited surface temperature of the floor, and thus the relatively lower specific heat output of the heated floor, the heating pipes are almost always laid under the entire floor area. This has an exceptionally positive effect on the uniformity of heat transfer in the interior and helps create a thermally homogeneous, uniform environment, both vertically and horizontally.
The heating water temperature is usually below 50 °C, so hot-water floor heating can also be referred to as low-temperature heating, which has several advantages. These include ensuring substantial energy savings in operation, while also enabling the use of non-traditional low-potential energy sources such as solar radiation, geothermal water energy, or heat pumps utilising ambient heat.
These facts place hot-water floor heating among the progressive heating systems, guaranteeing all three E's, i.e. the energy, environmental and economic aspects of heating buildings. On the other hand, despite the existence of this type of heating more than 2,000 years ago, its greatest expansion and renaissance is happening precisely today.
It is therefore necessary to define, in a theoretically exact way, the principles for the practical application of hot-water floor heating, which relate to the choice, calculation, design, assessment, construction, installation, control, operation and applications.
CHOOSING BUILDINGS FOR FLOOR HEATING
Since not every building is suitable for the application of a floor heating system, it is essential that, when choosing the building, the following requirements are ensured:
- thermal-technical,
- hygienic,
- energy-related.
DETERMINING THE DECISIVE THERMAL-TECHNICAL QUANTITIES
The thermal-technical properties of building structures and buildings are ensured by the following quantities:
- thermal resistance of building structures,
- temperature damping of building structures,
- thermal absorptivity of floor structures,
- the amount of water vapour condensed and evaporated in building structures,
- air permeability of building structures and their joints,
- thermal stability of the room,
- energy consumption for heating.
When designing building structures and buildings defined by a set internal environment condition, the required criteria are the minimum internal surface temperature according to Article 3 of STN 73 0540 and the maximum energy consumption for heating according to Article 21 of STN 73 0540.
HEATING PIPES
The most important element of the heated floor is the heating pipes, so we will now characterise in more detail:
- pipes for heating,
- pipes for floor heating.
PIPES FOR HEATING
Currently, the following pipe materials are most commonly used for central heating:
- metal pipes,
- plastic pipes,
- composite pipes.
METAL PIPES
Steel (black), galvanised steel, stainless steel and copper are used. The material of fittings and couplings is quite varied (malleable cast iron, brass, bronze, gunmetal, etc.). The variety of materials is a source of contact corrosion, which has a major effect on the service life of the heating equipment.
a) Steel pipes
These are among the classic pipe materials. Smooth seamless steel pipes are predominantly used. Joining has become standardised on welding, and spatial changes in the pipe network are made by hot bending. Threaded joints are used only for connecting fittings and heat emitters. More than a century of experience has made it possible to eliminate most shortcomings. Only the high demands on welding work remain. This is precisely the factor now working against the introduction of thin-walled closed rectangular steel profiles, since mirror welding is not popular and is no longer commonly mastered today.
b) Galvanised steel pipes
Galvanised steel pipes are one of the oldest pipe materials, but require impeccable craftsmanship. The zinc layer does not always have the same thickness and the same quality. When using modern thread-cutting tools, significant local weakening of the pipe's load-bearing part often occurs during thread cutting. This, together with the removal of the galvanised surface, is the cause of crevice corrosion. Since screwed joints require expertise and are time-consuming, they are increasingly replaced by welding where possible. It can be said, however, that with the use of thick-walled pipe, these are already in decline.
c) Stainless steel pipes
Seemingly ideal stainless steel piping ran into problems during installation with joining. Welding in this case requires a highly specialised technological process, since otherwise undesirable transcrystalline deformations occur. Likewise, when hard-soldering with silver solder, there is often a risk of corrosion at the point of contact between the flux and the stainless surface. It was not until the early 1970s that a fundamental change occurred, when the Mannesmann company developed a method of joining stainless steel pipes using pressed sleeves made of authentic stainless steel.
This type of joining technique gradually spread to other types of pipework as well (e.g. plastic, where mostly bronze pressed sleeves are used). Pressing the sleeve with a special hand-held electric tool creates a watertight joint between the pipe and the sleeve in about 6 seconds. The only apparent weak point is the pressed-in sealing O-ring located at the end of the sleeve, which is usually made of butyl rubber. Opinions on the service life of this material differ, and sufficiently long-term experience has not yet been gathered.
d) Copper pipes
Copper is among the noblest pipe materials. However, it is essential to strictly maintain the dosage of inhibitor in the heating water. The reaction of water with the copper surface creates a protective layer of copper oxide on the one hand, while copper ions are released on the other. If, out of ignorance or necessity, a steel (or even galvanised) pipe or aluminium is placed downstream of a section of copper pipe, intense corrosive attack on that surface will occur. A suitable inhibitor can help here.
Joining copper pipework is a very important part of the work process. It is joined very advantageously by soft or hard soldering. Soft soldering, depending on the type of solder used, takes place at a temperature of 230 to 240 °C, and hard soldering at 710 to 730 °C. These temperatures must always be strictly observed. Only water-soluble fluxes are used, whose residues are flushed out of the pipe by rinsing.
If the joining conditions are observed and deoxidised copper SF-Cu with a copper content of at least 99.9% is used, high corrosion resistance and sufficient strength are guaranteed even for thin-walled pipes. Installation is relatively simple and fast. The pipework can be used up to operating pressures of 4 MPa and operating temperatures of up to 150 °C.
PLASTIC PIPES
Among pipes suitable for central heating, a distinction needs to be made between floor heating systems and classic systems. Floor heating systems require great flexibility of the pipe loops and are also less thermally stressed than classic heating systems.
Suitable types for floor heating are therefore PB, PE-X, PVC-C and PP-R. In this context, it should be noted that for PVC the maximum operating temperature is only 60 °C, while for PVC-C the continuous operating temperature is 90 °C and the short-term temperature is 110 °C. A similar range applies to PB, PE-X and PP-R. PVDF holds a special position, as it can be used up to 140 °C. Some values are continually being refined. For example, it was known that halogens (fluorine, chlorine, bromine, iodine) react with PE, causing molecular changes, embrittlement of the material and loss of strength even at temperatures of 20 °C. In the USA it has now been found that ageing and destruction of PB occurs after long-term use due to the effect of chlorine. However, this finding cannot be fully applied to our conditions, since the chlorine concentration in water is far higher in the USA (up to 2 ppm).
The main advantage of plastic pipework is its simple and fast installation. It is joined either by gluing (PVC), by welding (PB, PE, PP), or with sleeves and screw couplings (PB, PE-X, PP).
The main disadvantage of plastic pipework is its low pressure resistance at higher temperatures, low rigidity (dimensional stability) and, in particular, high linear thermal expansion. Compared to stainless steel, the expansion is 10 times greater for PB, 12 times for PP, 14 times for PE, and as much as 23 times greater for PE-LO. This must be taken into account when designing the fixing and routing of plastic pipework. Plastic pipework should not be installed at all without continuous supports and elimination of pipe expansion through compensation.
MULTI-LAYER PIPES WITH A METAL LAYER
Another line of development stemmed from the finding that metal provides one-hundred-percent protection against oxygen diffusion. Based on this, multi-layer plastic pipes with a metal layer were developed. So far, two basic types have become established:
a) One type with an internally built-in, longitudinally welded aluminium jacket about 0.5 mm thick. These pipes consist of an inner plastic layer (PB, PE-HD, PE-X, PP-R), an adhesive intermediate layer, the welded aluminium jacket, another adhesive intermediate layer, and an outer plastic jacket.
b) The second type was developed from a normal plastic pipe, tightly wrapped with an aluminium foil about 0.2 mm thick, provided with an adhesive layer on both sides, and, for external protection, covered with a thinner layer of plastic.
A composite of plastic and metal does not have to be formed only with aluminium. A plastic pipe with an applied adhesive layer can be inserted into a thin-walled stainless steel pipe, and subsequently pressed onto the steel jacket by heat treatment. Multi-layer pipes with a metal layer minimise linear thermal expansion to values approaching those of aluminium. They are more stable, which substantially reduces the number of fixing clips required.
This pipework can be joined using screw couplings, pressed sleeves, or direct pressing. The service life of the pipework is determined by two essential conditions. First, it is necessary to prevent any possible ingress of water or air moisture at the cut end face of the multi-layer pipe (preventing contact between water and aluminium), and second, it is necessary to avoid the possibility of electrostatic corrosion between brass or gunmetal and aluminium.
PIPES FOR FLOOR HEATING
From the previous section, it is clear that the range of pipe materials for central heating is very wide, but the following are particularly suitable for floor heating:
- copper,
- plastics,
- composite pipes.
COPPER PIPES FOR FLOOR HEATING
Copper as a working material has definitely influenced the history of culture. Since it also occurs in nature in its pure state, it could be worked as a metal since ancient times. In ancient Egypt, copper was used as currency. The first mentions of copper water pipes date back to 2500 BC, when hammered copper sheets were laid in the tomb of Sahure in Egypt.
Around 1900, copper began to be used as a high-quality roofing material, especially for prestigious representative buildings and churches. In 1933, the use of this material for non-strategic purposes was banned. In Western Europe, this material was released for heating and plumbing purposes in 1965. Unfortunately, the Eastern Bloc continued to use this material only for strategic purposes.
The versatile use of copper is possible mainly thanks to the high quality of the processed material, which is subject to high requirements:
- minimal installation costs,
- stable functional and operational safety,
- long service life.
Among the most significant advantages of copper pipe installations are:
• high corrosion resistance, and therefore, compared to steel pipes
- minimal wall thickness,
- minimal weight per metre,
• high material strength,
• simple and fast installation with low space requirements,
• good adaptability to construction conditions, and therefore exceptionally high suitability also for retrofitting,
• simple and safe joining technique (no need for screwed joints),
• bactericidal effect,
• no deposits in the pipework (copper pipes do not scale up), which ensures high flow rates and low pressure losses,
- the possibility of specifying an exact internal pipe diameter,
- good thermal conductivity,
• environmental friendliness of the material (100% recyclable, waste-free technology).
The material from which copper pipes are made is deoxidised copper (SF-Cu) with a minimum pure copper content of 99.90%.
Basically, we distinguish two types of joining copper pipes:
1. Demountable joints
• Union nut
-conical/conical,
-conical/flat,
-flat gasket.
• Union nut with a clamping ring
-metal seal,
-soft seal.
• Pipe couplings.
• Flanged joints.
2. Non-demountable joints
• Soft soldering.
• Hard soldering.
• Press-fitting.
• Welding.
This pipe is specially developed for distribution pipework for floor heating and for connecting heat emitters in single-pipe and two-pipe heating systems. The copper pipe is coated with a protective PVC sheath, which partly allows for expansion and protects the pipe against both physical and chemical effects of concrete or plaster mixtures.
Currently, no other installation material used in domestic installations in Western Europe is as widespread or has such versatile applications as copper. In Germany, approximately 70% of installations are currently made from copper pipework, and in Great Britain even 90 to 95%.
PLASTIC PIPES FOR FLOOR HEATING
The basic building blocks of plastics are carbon C and hydrogen H. Three basic processes are used in their production: polymerisation, polycondensation and polyaddition.
Polymerisation forms a polymer from monomers under the effect of energy, catalysts and other additives. Polymerisation is used to produce, for example, polyethylene and polypropylene.
Polycondensation creates a new substance from a large number of molecules of several low-molecular-weight substances. Polycondensation is used to produce, for example, phenol-formaldehyde resin. Polyaddition is irregular polymerisation. Polyaddition is used to produce, for example, polycarbonate.
Polymers can be divided into two groups:
• homopolymers, whose macromolecular chain consists of regularly repeating identical molecules,
• copolymers, in which units of a different type of molecule are attached to the basic chain. According to the spatial arrangement of the macromolecules, we distinguish linear and branched polymers.
Depending on the method of technological processing and behaviour of plastics when heated, they are divided into thermoplastics, thermosets and elastomers.
The properties of thermoplastics are influenced by the order or disorder of the macromolecules. Partially crystalline plastics are flexible and tough. Amorphous plastics, on the other hand, are hard and brittle. Thermoplastics can be reversibly processed and reshaped with heat.
Thermosets cannot be shaped or welded using heat. Resins are a clear example of this group. A special subgroup among thermosets are the thermoelastics, which retain their flexibility but cannot be thermally shaped or welded. The main representative of thermoelastics is cross-linked polyethylene.
Elastomers deform considerably under mechanical load. After the load is removed, they return to their original state. They cannot be thermally shaped and are not weldable. This group includes synthetic rubbers.
Types of plastics used in heating
The plastics used in our country for heating installations are mainly PEX, PP-B, PP-R, C-PVC, PB and PVDF.
Polypropylene block copolymer PP-B contains 20 to 30% ethylene content, which allows greater flexibility and toughness at low temperatures (down to -20 °C). Its pressure-free temperature resistance is up to 90 °C. Pipework made of this plastic is intended for floor heating. It is joined by welding or mechanical couplings.
Polypropylene random (statistical) copolymer PP-R was developed mainly for sanitary technology. It is characterised by good pressure resistance even at higher temperatures, and is used both for floor heating and for central heating. Its pressure-free resistance exceeds 100 °C. It is joined with mechanical couplings, butt welding or electrofusion (polyfusion) welding.
Polybutylene PB is highly flexible and has a high molecular weight. Along with high strength, it is also characterised by resistance to stress cracking. Its pressure-free temperature resistance exceeds 100 °C, and it becomes brittle at -18 °C. It is used for floor heating and central heating distribution. Because it has good mechanical properties, it is manufactured with thinner walls than other plastic pipework. It is joined with mechanical couplings, butt welding or electrofusion welding.
Chlorinated polyvinyl chloride C-PVC has an increased chlorine content, which is not a hygiene obstacle for transporting hot water through pipework made of this plastic. Its pressure-free temperature resistance exceeds 100 °C. This plastic is manufactured in various ways, so its properties differ somewhat between individual manufacturers. It is joined by gluing, and only pipes and fittings supplied by a single manufacturer, together with its recommended adhesive, should be glued together.
Polyvinylidene fluoride PVDF has high mechanical strength and considerable resistance even at high temperatures. It is also easy to process. Its usable temperature range is from -40 to +140 °C. It is resistant to UV and gamma radiation, giving it good ageing resistance. It is likewise resistant to abrasion, physiologically harmless and hard to ignite. Its price is relatively high. It is joined by electrofusion welding, infrared welding, butt welding or mechanical couplings.
Properties of plastics
Among the properties of plastics that we should definitely pay attention to are temperature resistance, flammability, water absorption, electrical conductivity, thermal conductivity and thermal expansion.
The temperature limit beyond which a polymer melts and softens without a change in structure is the limit of temperature resistance. If the plastic were to be subjected to further thermal loading, it would result in a change of mechanical-physical properties and gradually also in the degradation of the polymers, leading to their deterioration. When assessing the temperature resistance of pipework intended for heating installations, some caution should be exercised, since incorrect welding causes partial degradation through local overheating.
To prevent or reduce the ageing of plastics, stabilisers, pigments and antioxidants are added to the polymers. The ageing process refers to ongoing irreversible changes in the mechanical properties of plastics, caused by the effects of oxygen, temperature and UV radiation. Ageing proceeds slowly compared to thermal degradation.
Corrosion of plastics first manifests itself inside the plastic and later also on the surface. Corrosion of plastics manifests itself as a change in composition and a change in the properties of the plastic. This change is caused by the effect of chemical and physical influences. Some metallic materials, such as copper, cobalt and their alloys, can also have a negative effect. When a metal comes into contact with certain plastics, thermo-oxidation occurs, which is catalysed by metal ions and manifests itself as damage to the plastic's structure. For this reason, couplings and adapters with a suitable surface treatment, recommended by the plastic pipe manufacturer, should be used. Stress corrosion of plastics manifests itself as the formation of small cracks. For plastic products, we do not need to take into account changes in dimensional tolerances and changes in mechanical properties caused by moisture. Plastics are non-wettable and their water absorption and moisture uptake are very low. Plastics are generally considered insulators and are not electrically conductive. Another fact is that, with plastic pipework used to transport bulk materials or gases, an electrostatic discharge can occur. The surface resistance is around 1014 ohms and depends on the degree of surface contamination and the percentage of air humidity.
From the point of view of pipe network design, linear thermal expansion is very important. The thermal expansion of plastics is ten times greater than that of steel or copper. If the designer does not fully respect this property of plastics, it will cause many problems that can only be resolved by re-laying the pipework.
The thermal conductivity coefficient of plastics is very low; however, this fact does not entitle us to reject thermal insulation for plastic pipework. Plastic pipework cannot be classified as a thermal insulator, and many plastic distribution systems directly require general insulation protection.
Basic data used when working with plastic pipework
An important factor affecting the use of plastic pipework is its service life. It used to be said that the service life was 50 to 60 years, regardless of the operating conditions and the type of plastic used. It is now clear that this service-life limit needs to be revised downward and assessed very strictly in accordance with the plastic used and the operating conditions to which the plastic is exposed. During operation, plastic pipework is stressed simultaneously by operating pressure and operating temperature. Since the simultaneous action of pressure and temperature is a very significant fact, the minimum service life of the pipework is determined in practice. This service life is determined depending on the two above-mentioned parameters, according to experimentally established isotherms. The stress in the pipe wall, plotted on the y-axis, is directly proportional to the outer diameter of the pipe, the wall thickness and the internal overpressure, and inversely proportional to twice the wall thickness.
Plastic pipes are manufactured in the following series: PN2.5 PN4 PN6 PN10 PN12.5 PN16 PN20 PN25.
The number after the PN abbreviation specifies the maximum operating pressure (Pa) at a water temperature of up to 30 °C over 50 years. It follows that, for designed higher temperatures, a higher pressure series must be chosen for the same pressure and the same service life, since the maximum permissible operating pressure decreases as the water temperature increases.
Plastic pipes are marked in a millimetre series. The marking should include: the manufacturer's name, material designation, outer or inner dimension x wall thickness, operating pressure/temperature, date of manufacture, pipe name, or registration number.
Plastic pipework can be joined to itself, plastic pipework can be joined to pipework of another material, pipes to fittings, pipes to fixtures, or pipes to appliances. A whole system of pipe joining is used for this purpose. Joints must meet certain requirements, so they are checked for resistance to pressure, temperature or chemical effects, whereby functionality and tightness of the joint are required.
Oxygen diffusion through the walls of plastic pipes
Polyolefin heating pipes are often discussed, as they are a measure of the quality of the whole system and a target of doubt about it. They therefore deserve increased attention.
A current issue receiving increased attention is the diffusion of oxygen through the walls of plastic pipes.
It has been found that molecules of atmospheric oxygen penetrate the plastic lattice until they reach the heating water. Heating water, continuously saturated with oxygen, then becomes very aggressive towards metals. The amount of oxygen that penetrates depends on the type of plastic and the temperature. It has further been found that pressure changes at higher temperatures induce a tendency to form fatigue surface cracks, which increases oxygen diffusion. Since a molecule with a small atomic radius diffuses more easily into a lattice formed from atoms with a larger radius, the first approach was cross-linking of plastics. This almost eliminated the formation of fatigue cracks, but did not completely prevent oxygen diffusion. Therefore, the next line of development led to fully coating plastic pipes with a layer of special polymer, serving as a diffusion barrier. However, the percentage effectiveness and service life of this barrier depend on the polymer used.
Oxygen diffusion can be limited by separating the heating circuits from the pump circuit, increasing the wall thickness of the pipes, or adding an oxidising agent to the heating water.
Finally, it must be said as a principle that the cause of this problem is the operating condition of the floor heating system, which is permanently and irreversibly built into the floor structure. For this reason, efforts are made to convince the customer and user of the reliability and safety of the heating system, so that they need not worry about possible failure of the pipe network. One of the traditional causes is corrosion of the metal parts of the accessories and equipment of the heating system, since the pipes themselves are made of plastic.
The following solution has therefore been found. Reputable manufacturers use metal parts made exclusively of brass, which minimises the likelihood of corrosion. Furthermore, to prevent the transport of impurities, particularly from the heat source exposed to the possibility of low-temperature corrosion, heating system connections via a heat exchanger are increasingly being adopted, i.e. separating the primary boiler circuit from the secondary circuit of the floor heating surface. And finally, to achieve maximum certainty, efforts are made to exclude the possibility of oxygen entering directly through the walls of plastic pipes, i.e. by means of yet another covering layer - an oxygen barrier. There are even known extreme solutions where a third layer, based on metal, is added.
So the issue of oxygen diffusion will always mainly depend on the specific conditions of use, as well as on the investor's budget. In Germany, where floor heating is most widely used, approximately 60% of companies offer pipes with an oxygen barrier.
What is essential, however, is that, given the current lack of specific practical cases, research into not using an oxygen barrier is continuously ongoing.
Trends in the application of plastics
Germany - the total market is estimated at approximately 230 million metres of pipe, of which more than half is copper, while among plastics cross-linked PE is used the most (70%), followed by PPC (15 to 20%) and PB (5 to 10%),
Austria - steel and copper account for more than 55%, the rest are plastics,
Switzerland - plastics account for as much as 74% of the market, used mainly for floor heating, with a consumption of 4.6 m per inhabitant per year (mostly PEX; PPC and PB are 4 times lower than PEX),
Netherlands - plastics 10% in sanitary technology, 10% in central heating, of which PEX 60%, PP and PB significantly less (1 to 2%).
When using plastics for pipework in buildings, a service life of 50 to 55 years is assumed, i.e. installations are replaced once during the service life of the building.
Developments in the plastic materials market in Germany, Austria and Switzerland confirm the complete equivalence between pipework based on traditional metal materials and pipework based on plastics, whose share ranges from 40 to 75%.
Cross-linked polyethylene generally holds a dominant position in the production of pipe materials, at around 70%, with polypropylene and polybutylene ranking equally thereafter.
COMPOSITE PIPES FOR FLOOR HEATING
In recent years, multi-layer pipework, i.e. pipework composed of dissimilar materials, has begun to be used in our country. A multi-layer pipe consists of a basic plastic pipe, an aluminium jacket with a longitudinal welded seam, and a protective plastic layer. Good adhesion of both plastic layers to the aluminium foil is ensured by a special adhesive layer.
Composite pipes compensate for the handicap of plastic pipework in terms of linear thermal expansion. The linear thermal expansion values approach those quoted for aluminium; for example, manufacturers state a value of 0.025 mm.m⁻¹.K⁻¹. A further advantage is that the pipework can be fixed at greater intervals, since the sag after filling with water is smaller. This pipework is characterised by relative rigidity, while at the same time retaining considerable flexibility. Multi-layer pipework is joined mechanically or by electrofusion welding with a fitting. Before welding, all layers must be removed down to the basic plastic pipe.
Sources used:
Dušan Petráš: Hot-water floor heating
Dušan Petráš: Low-temperature heating and renewable energy sources
