The medium pipe of a conventional district heating system is made of steel since this material is particularly suitable for withstanding thermal and pressure loads. Plastic medium pipes can be more favourable than steel for small pipelines, even though they can only stand lower temperature and pressure loads.

The assembly procedure can be so advantageous that cost reductions for pipeline construction result even though the pipe material itself is often more expensive than steel pipes. To reduce the high investments costs of pipeline construction an attempt must be made to use these possible savings to the full. They are offered in the area of small pipelines and service connections for nominal diameters under DN 100.

Suitable pipeline systems with plastic medium pipes are being offered for sale in all the countries being considered here in Western, Central and Northern Europe, although not always to the same extent. For this reason, a comparison of the user practice and construction costs between the countries would be worthwhile. The work is an extension to an IEA-report, based on the status in 1985/86 which concerned the construction of pipelines in areas of low heat density.

A comparison of pipe systems on the market today with those considered at that time makes the rapid development experienced in this area quite clear. In fact, not one system is still being offered on the market without changes having being made. The many new developments mean that it may be assumed that there is still a considerable potential for a further development of these systems.

The status of the development today can be summarized in three sentences:

1. The experiences made by the manufacturers and from applications have produced a positive effect on the market, which has increased the expected turnover of established manufacturers and has also attracted new suppliers.
2. Manufacturers and users are following difficult development objectives. Plastic pipe systems will be open to professional applications and manufacturers will be able to increase their turnover only when all remaining application problems have been correctly solved.
3. The turnover of pipeline material based on cross-linked polyethylene is many times higher than the turnover of polybutylene systems.

These statements must be explained more in detail. The established manufacturer of plastic systems assesses the changes of these systems as positive. In addition, a competent company from Switzerland, Kabelwerk Brugg, has joined these suppliers and has introduced a new system. It is also interesting to note that manufacturers of conventional systems, i.e. of plastic jacket pipes, are increasingly extending their palette of products including plastic systems, e.g. Lögstör and Dansk Rörindustri. Wirsbo has even introduced a complete system although this company already has a very high position on the market as manufacturer of basic material.

Plastic medium pipes have been applied in a large number of reference "networks" which, for several products, have already shown periods of utilization of up to 12 years. It is noticeable that the main area of sales is the small, household-oriented construction of pipelines which are served by plumbers.

In the case of professional pipeline planners of the supply for communities, these systems have only been introduced in isolated projects where this type of design has been regarded as advantageous as a result of special experiences with plastic or for reasons of intentional industrial cooperations. In particular, the long-term experience is not sufficient for district heating suppliers. In general, it can be said here that the main problem lies in the permeability of gases through plastics. Even if on regards oxygen diffusion into the heating water as a problem that was originally feared but which can be solved with today's designs, the scientific discussion is being fed as a result of the fact that even water vapour escapes. The supplier can only expect frictionless sale of his products when this question has been clearly answered and the new systems show advantages when composed with the proved building constructions.

A final decision on the advantages of competing materials polyethylene PE-X (formerly VPE) and polybutylene PB cannot be made. The decisive advantage of PB lies in its weldability which makes jointing easier. However, this advantage has disappeared since the mechanical jointing elements for PE-X are meanwhile regarded as reliable. The connecting strainers, available today, have been classified both by the user as well as the manufacturer as so reliable that they can be buried in the ground. Further developments on these connectors are no longer concentrated on a reliable function but on reducing costs by means of saving material and simplifying assembly.

The work presented here had the objective of describing the systems which are offered today and to present the status of development which they have achieved. Problems which still hinder an unconditional application should be outlined. A large part of this work is devoted to cost considerations which compare which laying costs would be incurred by a particular system in a particular country.

In addition, this work includes a static consideration of the life of plastic medium pipes. This was needed because of the fact that system manufacturers were giving different load limits for the same pipe.

District heating pipelines with plastic medium pipes can offer advantages in public supply systems with diameters less than DN 100. The standard design with medium pipes of cross- linked polyethylene PE-X (previous VPE) and polybutylene (PB) seem to be suitable for loads of an operating pressure of 5 bar and variable supply temperature up to 90 °C.

Pipelines with plastic medium pipes have brought cost advantages of up to 40% compared with today's standard design of plastic compound jacket pipe with steel medium pipe. These advantages vary considerably from manufacturer to manufacturer; they differ in the northern and middle European countries considered here as a result of the level of the civil engineering costs. Savings are a result of the fast and flexible method of laying.

The low proportion of plastic pipelines in public district heating networks is primarily a result of the sensitivity to temperature and pressure and the lack of experience of the longterm behaviour. The use of these flexible systems will increase as long as the cost advantages remain and further positive operational experiences are made and temperature/pressure levels are decreased.