District energy systems deliver heated or chilled fluids from an outside source through piping systems for the primary purpose of heating and cooling buildings or industrial processes. District systems typically generate this energy in a centralized location. Heated or chilled fluid is transferred through a system of insulated pipes for space and/or water heating, or to provide chilled water for space cooling or cooling of equipment. District energy systems are used for residential, commercial, industrial, and institutional applications. Plastic piping systems often play a role in efficient transfer of this thermal energy.


District energy systems in various forms have been used for over one hundred years. Many early systems transferred steam in cast iron pipes, often with dangerous results. As the use of heated and chilled water became popular, so did more advanced piping materials, including plastic pipes. While many legacy district steam systems still exist in large cities and major industrial complexes, modern district heating and cooling systems using water or water-based fluids are being applied in new applications. Many fluid-based district energy systems are ideal applications for high-temperature plastic piping systems manufactured by members of PPI’s Building & Construction Division.

The major benefits of district energy systems are fewer mechanical devices (e.g., boilers or chillers), reduced energy consumption, and decreased greenhouse gas emissions. Reliability is typically increased vs. decentralized systems that employ individual boilers, air conditioners, heat pumps, chillers, pumps, etc. Maintenance costs can be reduced, and each building connected to the district energy system does not need to contain its own heating and cooling equipment, increasing space efficiency.

Geothermal District Energy Systems

As opposed to using fossil fuels to generate heat (e.g., boilers), geothermal district energy systems use ground-source geothermal energy from the earth to heat and cool buildings through connected buried pipelines in a distribution network. Most of these systems use plastic pipes for the underground pipelines.

These systems can be designed in several ways, and are described using various terminology:

  • Geo Grids
  • Thermal Grids
  • Geo Energy Networks
  • Thermal Energy Networks (TEN)
  • Geothermal District Heating (GeoDH)
  • Ground Heat Exchange District (GHEXD)
  • Ambient Temperature (Geothermal) Loops (ATL)
  • Community Ground Source Heat Pumps (CGSHP)


The typical configuration of a district energy system is to connect piping trunk lines to the source of energy (e.g., centralized boiler plant, centralized chillers) and for these trunk lines to be buried under streets, where they connect to smaller branch lines which feed individual buildings with heated or chilled fluid. In a multi-building campus setting such as a university, hospital, or industrial complex, pipelines are sometimes run through underground tunnels for maintenance access to equipment such as valves and pumps. So-called four-pipe systems include heated fluid supply and return pipes plus chilled fluid supply and return pipes to allow buildings to switch from heating to cooling mode as needed. Each building typically utilizes an energy meter device that measures flow rate and change in fluid temperature to calculate its energy consumption for billing purposes.

In geothermal district energy systems (see above), buildings in the same district network can exchange thermal energy with the fluid in a single main pipeline, utilizing heat pumps within buildings to extract or return thermal energy to the main pipeline. In such a system, one building may be heating while another is cooling, with each building exchanging energy with the district pipeline (i.e., ambient temperature loop). In this case, the total energy consumption for the network is greatly reduced, minimizing the use of fossil fuels for energy production. Heated water can be used for applications such as traditional hydronic heating for space conditioning, radiant heating, outdoor snow and ice melting, and more. Chilled water is used for traditional hydronic heating for space conditioning, radiant cooling, and more.


Many fluid-based (i.e., not steam) district energy systems are ideal applications for high-temperature plastic piping systems manufactured by members of PPI’s Building & Construction Division. District energy systems may employ CPVC, HDPE, PE-RT, PEX, PP-R, or PP-RCT piping materials, with the selection of material depending upon the temperature and pressure requirements. Many of these piping materials (e.g., CPVC, PEX, PE-RT, PP) are rated for operation at 180°F (82°C), whereas others (e.g., HDPE) are better suited for cold or chilled water applications.

Note: These plastic piping materials should never be used for steam systems or hot water systems with temperatures that exceed 200°F (93°C), and the pressure rating of each piping material must be adhered to. See PPI’s webpage on HYDRONIC DISTRIBUTION PIPING for more details on those products.

HDPE, PEX, PE-RT, PEX-AL-PEX, and PP materials are also available as flexible pre-insulated plastic piping systems which combine carrier pipe, insulation, and a casing or cover as a pre-manufactured assembly.

Flexible Pre-Insulated Plastic Piping

To reduce installation time and simplify installation procedures, pre-insulated plastic piping systems are available in several configurations. They may include one or more carrier pipes within a common outer jacket, with the pipes surrounded by insulation to reduce heat transfer. By integrating all components into one pre-manufactured assembly, flexible pre-insulated plastic piping systems can provide significant time and cost savings, as compared with field-manufactured insulated piping systems constructed on-site of individual components while ensuring consistent performance of the piping system.

Because flexible plastic piping systems are often supplied in long continuous lengths, sometimes hundreds of feet in length, many joints and connections can be eliminated. Further time savings are realized when these relatively flexible pipes are easily bent around obstacles or follow undulations in the ground without the use of fittings and joints. Outdoor construction time can be reduced by more than 50%. See PPI’s webpage on FLEXIBLE PRE-INSULATED PIPING SYSTEMS for more details on those piping products.

District Energy in the Media

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