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Which pipes to choose for the system?

Running water in the house and central heating are standard today. Sooner or later, each investor must decide what material to choose for the system. Only a few or so years ago, steel pipes were the basic installation material. Currently, they are used less and less often: they have been replaced by pipes made of materials such as copper and artificial materials. They are more resistant to corrosion and scale. It is also important that the installation process is much simpler and faster.

To create a cold and hot water system, similar as for central heating systems, three basic groups of materials are used:

In each of these groups, there are different types of pipes, for example, steel pipes: black, galvanized, copper pipes: hard, semi-hard or soft, and pipes of various types of artificial materials. Sometimes it's worth to take time and make sure we are making the right decision. Compare several products. Consider all the advantages and disadvantages, ease of installation, post-warranty service, and many important factors that are ignored when making a decision about the purchase. It may turn out that cheap solutions do not meet our expectations at all.

STEEL

Until recently, the basic material from which water systems were made was steel, which has very high mechanical strength. This results, among others the need for a dense arrangement of pipe fastening points. The undoubted advantage of steel is its resistance to high temperatures and the fact that it has a relatively low thermal expansion. However, it is heavy and not resistant to corrosion. It conducts the heat very well, the water in such pipes cools quickly. Resilient, transferring sounds well. It has a rough surface, which promotes deposits and quickly reduces the internal diameter of the pipes. Elements of the system made of steel are connected by thread or by welding. "Black" steel pipes not protected against corrosion are used for heating systems. They can be connected by welding and hot bent, which makes the installing process much easier. However, a very high temperature is needed for welding steel, which can only be obtained with the help of special oxygen-acetylene torches. Welding also enables to avoid the use of some of the fittings. These, in turn, increase the resistance of the wires. This is particularly important for traditional central heating systems with the convective flow. This steel is not suitable for drinking water - due to its lack of resistance to corrosion. Only galvanized pipes (''white'' steel) are suitable for drinking water systems. In turn, they cannot be used for heating installations, as the zinc film loses its protective properties already at 60°C. Galvanized steel pipes must not be welded or hot-bent. Therefore, the number of fittings used is high, which means a more complicated installation and greater flow resistance. Today in single-family housing, steel is already out of use. The material itself is several dozen percent cheaper than modern alternatives, but the weight and difficulties caused by the installation are almost one and a half times more expensive than other materials. Even worse, steel is not very durable. The durability of systems made of steel is primarily affected by the quality of the water. Steel pipes are susceptible to corrosion caused by water, oxygen, carbon dioxide, and dissolved mineral compounds. Calcium and magnesium ions deposit easily on the walls of steel pipes, which are not smooth, but rough. This is how limescale forms. Breakdowns caused by corrosion of systems occur after a few years. It is possible that the replacement of the system was necessary already after ten years.

COPPER

Undoubtedly, copper is a better material for the water system than steel. Copper is resistant to temperature, pressure, and ultraviolet radiation. It does not age. Pipes made of copper have little hydraulic resistance, so they can have a small diameter; this saves material and space, the installation is easy to place under the plaster, so they work well, for example, as branches departing from the cold and hot vertical water risers. Unlike some artificial materials, when it comes to copper, the oxygen does not penetrate (diffuse), which is very important for central heating systems. Installation is simple and quick, the workplace is small. The copper installation is resistant to limescale. Very important: copper prevents the growth of bacteria (including very dangerous Legionella) and algae, which are most often found in pipes made of artificial materials. The price of the material is 40% higher than the price of steel, but the cost of labor is 30% lower and the durability at least four times longer (50 to 100 years!). Copper pipes are not very resistant to scratches. Therefore, the systems made of copper must be well protected against the penetration of small solid particles, such as sand, mortar, and rust particles. There is a need to install a mesh filter with a mesh size not larger than 80 μm (0.08 mm) and the end of the pipe. Copper pipes are resistant to hot and cold water, but they can corrode if the water is soft and contains large amounts of aggressive carbon dioxide. In Poland, restrictions on the use of systems made of copper mainly apply to mountain areas, where the water is usually soft. Corrosion can also be caused by embedding in the copper installation, for example, faucets or devices made of galvanized steel or aluminum. Copper pipes are manufactured in three varieties: hard, semi-hard, and soft.

- Soft pipes can be profiled without major problems, which significantly reduces the number of connections and reduces the process of installing. Such pipes are mainly used for long sections of systems laid under the floor (floor heating), connecting radiators, or for water systems in the floor. They are the least resistant to mechanical damage and should not be connected with screwed fittings.

 - Semi-hard pipes are more resistant to mechanical damage than soft ones, but they can be easily bent using benders or internal springs, provided they are of the right size.

- Hard pipes, available in sections up to 6 m, are not bendable. Any change in the direction of the pipe requires the use of a suitable fitting, which is connected to the pipe by soldering or clamping. They are the most mechanically durable, which is why they are used in sections of central heating systems that are most likely to be damaged, cold and hot utility water run in furrows or directly on the wall. Hard copper pipes are ideal for piping within the boiler room.

ARTIFICIAL MATERIALS

Currently, water systems made of artificial materials are gaining popularity. They are resistant to corrosion and limescale. Such pipes do not transfer vibrations, they dampen vibrations and noise generated in the installation. They are chemically inert, so they do not react with water and the compounds it contains. They also do not affect the taste, color, or smell of water.  Pipes made of artificial materials are lightweight. They are easy to transport and install by yourself. Making connections is relatively easy and takes a very short time, but sometimes requires quite expensive welders or crimping tools.
The main disadvantage of pipes made of artificial material is the penetration of oxygen through their walls. The higher the working medium temperature in the pipe, the more oxygen penetrates. This does not affect the durability of the duct itself, but oxygen in the water in the system is detrimental to all metal parts and devices (for example, radiators) in the system.
In comparison to rigid steel pipes, pipes made of artificial materials require better mounting to the walls because, under the influence of high temperatures in hot water systems, they can be deformed. They also have very high longitudinal deformability, which is why they require compensation on longer sections. Unlike copper pipes, they have very thick walls, so they require very deep grooves to be placed under the plaster. A very important factor is that most artificial materials (except polybutylene) much worse than copper and steel protect against the development of bacterial flora in pipes, especially against the very dangerous Legionella bacteria.

Types of materials from which pipes are made:

- PVC polyvinyl chloride and CPVC chlorinated polyvinyl chloride,
- PE polyethylene (PE-LD soft and PE-HD hard),
- PE-X cross-linked polyethylene,
- PP polypropylene,
- PB polybutylene,
- Layer pipes (using various materials and spacers, for example, made of aluminum foil).

PVC (polyvinyl chloride) and CPVC (chlorinated polyvinyl chloride).

These are the oldest of all materials that the systems are made of.
PVC has very limited use. The low-temperature range (0 - 50°C) in which it retains its properties makes this material only suitable for cold water systems, PVC is brittle at negative temperatures, it is not resistant to stretching and bending. CPVC has better properties, which can be used not only for hot water, but also for central heating (temperature range 0 - 100°C), but the brittle temperature is still 0°C. CPVC can also be used for cold water, but it is more expensive than PVC.
Pipes made of artificial materials are delivered only in straight sections. The rules of their laying do not differ much from those that apply to steel systems. At higher temperatures, the mechanical strength of PVC decreases significantly. It begins to soften slightly below 80°C, but 60°C is considered a safe temperature.
The basic method of connecting PVC elements is gluing. At some points in the PVC system, it is necessary to connect with metal (outlet from the boiler, faucets, etc.). Threaded connections are used in such places. The connectors are factory prepared. The presence of threaded connections reduces the pressure resistance of the system.
Systems made of PVC and its varieties are very easy to make yourself, of course, according to the plan made by an authorized specialist. No special tools are needed for the work, a metal saw, a sharp knife and screwdriver are all you need.

PE polyethylene.

Another material used in water systems is PE polyethylene. It comes in many varieties.
The most popular are:

- "soft" type, marked with the abbreviation LDPE or PE-LD (Low-Density Polyethylene), designed for low-pressure systems,
"hard" type HDPE or PE-HD (High-Density Polyethylene) for high-pressure systems.

Both are characterized by high chemical resistance, low specific gravity, and high smoothness of the pipe walls. They can only be used for cold water systems; at temperatures above 20°C their strength decreases rapidly.
There is also medium-density polyethylene with increased thermal stability with the PE-RT designation. A much higher resistance - even up to + 95°C in continuous operation, and thus enabling the use of pipes for both types of water and for heating systems - is characterized by cross-linked high-density polyethylene PEX.
All pipes made of polyethylene are flexible and bendable so they can be bent, and you can save money on fittings, mainly elbows. An important advantage of polyethylene is the low brittle temperature, which is -25 °C. So there are practically no contraindications for using it outdoors in systems exposed to freezing. Welding and compression fittings can be used for connecting pipes.

PE-X cross-linked polyethylene

It is a polyethylene subjected to a special treatment (cross-linking, that is, the use of cross-links between polymer chains), available in several varieties, slightly different in terms of their properties. It is flexible and resistant to damage caused by stress. The pipes have shape memory and are resistant to metal ions and UV rays. Working temperature from -10 to 95°C allows for use in cold and hot water and heating systems.
It is a durable material (the service life of the system is over 50 years), there is no corrosion and development of the limescale, the pipes are lightweight and flexible, thanks to which they are easily laid. They are also resistant to accidental mechanical damages. The system made of PE-X suppresses sounds and does not transfer vibrations.
Cross-linked polyethylene is especially recommended as a material for drinking water pipes as it is non-toxic, free of heavy metal ions, and microbiologically resistant. Screw compression fittings or press connectors are used for connecting pipes. Cross-linked polyethylene pipes are most often produced as PE-X/AL/PE-X layer pipes with an anti-diffusion liner, limiting the penetration of oxygen into the inside of the pipe.

PP polypropylene

In the technique of water systems, PP polypropylene has been widely used, namely one of its variants, marked with the abbreviations PP-R or PP type 3, shorter PP-3.
There are normal and "heat stabilized" PP pipes. The first ones are used in cold water systems, and the second ones - for central heating and hot water systems. This is because although polypropylene withstands high temperatures (even above + 95°C), it also has a high coefficient of linear thermal elongation. When planning and making systems made of polypropylene pipes, this property of pipes should be taken into account. In "heat stabilized" pipes, the thermal elongation decreases up to six times.
There are also PP pipes with an anti-diffusion coating, which prevents oxygen from penetrating into the inside of the pipe (this adverse phenomenon accelerates the corrosion of steel parts of the system, for example, radiators).
PP pipes are quite stiff, so any direction changes require the use of fittings and connectors. They are connected to pipes by means of poly-fusion welding. In addition, screw connections, threaded ends and special connection blocks are used for connections.
Polypropylene pipes are very resistant to low temperatures (from -40° C) and are usually surface mounted. They can be uncovered, hidden in channels, or behind covering screens. If they are laid in furrows, the furrows must not be filled with mortar, as this would prevent the pipes from moving freely under the influence of temperature changes.
Ultraviolet radiation adversely affects polypropylene products, and therefore pipes exposed to UV radiation should be covered or protected by painting with a protective coating. Polypropylene accumulates static electricity on its surface and should not be used to transport flammable and explosive substances.

PB polybutylene

The youngest of the materials used for water systems is polybutylene. It is very flexible and - what is rare among artificial materials - it has no memory of shape (the pipe after unrolling from the coil does not spring). It is characterized by flexibility (thanks to which we can save money on connectors), impact strength (does not crack when hit), high creep resistance (slow deformation under long-term loading), abrasion, and stress cracks, as well as aging. Products made of PB polybutylene are easy to transport and install - pipes can be bent and routed like an electric cable.
The brittle temperature is -25°C. Above that temperature (that is, in our ambient conditions almost always), the pipe cannot be damaged by water freezing in the pipe. If an ice cork is formed, the pipe simply expands with it, and after thawing its contents it returns to its previous shape. It is also extremely resistant to high temperatures up to 90°C, so it is suitable for all types of water systems.
An important feature of PB is the ability to stop the development of bacteria. In this respect, it is not much different from copper, and the copper shows the strongest bacteriostatic effect among metallic materials. Sometimes manufacturers who cover the inner surfaces of steel pipes with a thin layer of PB because of this.
The installation process of the system is reduced to cutting off the relevant section of the pipe from the roll and sliding it into the appropriate connector - clamping or threaded one made of brass. Polybutylene pipes can also be connected by welding, which, however, requires appropriate equipment. Although the material itself is the most expensive among artificial materials used in water systems, the easy process of installation and utility advantages make water system from PB polybutylene competitive when it comes to price.

Multi-layer pipes

This solution allows you to combine the best properties of metals and artificial materials. The pipes consist of three layers: internal and external made of polyethylene PE-HD, cross-linked PE -X or polypropylene, and a central foil spacer - usually aluminum. Aluminum prevents oxygen from penetrating inside the pipe, significantly reduces its thermal expansion, and eliminates shape memory; the cable can be permanently formed according to our needs. The use of a metal insert also increases the heat resistance of the pipes - some withstand short exposure to a temperature of 110°C. Therefore, multi-layer pipes are mainly used in heating systems. The material ensures chemical resistance, smoothness of the external surface, thermal insulation, and suppresses noises of the system. On the other hand, metal prevents oxygen from penetrating inside the pipe, reduces its thermal expansion, eliminates shape memory, and increases temperature resistance. These features mean that multi-layer pipes are mainly used in heating systems.
On the other hand, multi-layer pipes in lagging protect the cold water system against ''sweating'', and hot water against heat losses. Their additional advantage is good noise suppression. The inner surface of plastic pipes is even several hundred times smoother than steel pipes. This greatly reduces the flow resistance, and no deposits form on the walls. Artificial materials are chemically inert, so they do not react with water or the compounds it contains. This is a big advantage. However, there are also disadvantages. Most artificial materials (except polybutylene) protect pipes against bacterial flora much worse than copper and steel.
Depending on the type of material, pipes are sold in rolls or straight sections. The pipes on rolls are flexible, so they can be used in underfloor heating systems, where they must be bent and should not be joined along the length.
Information about the material used for the production of the pipe is located on the side of each pipe. It provides materials for subsequent layers, for example:

PEX/Al/PEX (cross-linked polyethylene / aluminum / cross-linked polyethylene),
PP-R/Al/PP (polypropylene type 3 / aluminum / polypropylene),
PEX/Al/HDPE (cross-linked polyethylene / aluminum / high density polyethylene).

The basic method of connecting multi-layer pipes is the use of fittings - both screwed or pressed. In the case of PP with aluminum spacer, welding can also be applied by removing the AL layer on the pipe ends using a special stripper.

Coefficients of thermal expansion of artificial materials
artificial materials:                 multi-layer pipes    PVC and CPVC      PB      PE      PP     PEX
thermal expansion coefficients:       0,03 - 0,05         0,08              0,13   0,14   0,15   0,18

It is necessary to pay attention to the smallest permissible bending radius of specific pipes. Excessive bending causes the material to crack. Pipes made of artificial materials do not transfer vibrations. The durability of the systems depends on the temperature and pressure of the transmitted water. Too high a temperature accelerates the aging process of the material. The most resistant to it are polypropylene, polybutylene (up to 90°C), PE-X and multilayer pipes (up to 95°C). Therefore, cold water systems withstand the longest, but also, in this case, there are clear differences - PVC pipes are least resistant to low temperature (up to 0°C), and polypropylene pipes are most resistant (up to -40°C).

The main disadvantage of "plastic" pipes is the ease with which oxygen penetrates their walls (the higher the temperature of the working medium in the pipe, the more oxygen penetrates inside). Although this does not affect the durability of the pipes themselves, but oxygen in the water adversely affects all metal elements and devices in the systems (corrosion).

For this reason, pipes with an anti-diffusion coating (limiting the penetration of oxygen into the inside of the pipe), or with a metal insert that completely seals the pipe and limits dimensional changes under the influence of changing temperatures are better for central heating and hot water system.

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