Network Topology and Structure

The network topology of a thermal network is structured into three pipe hierarchy levels — transmission, distribution, and service connections — and can be configured as a radial, ring, or meshed network. The two-pipe system with supply and return lines is the standard; three-pipe systems are used when different temperature levels are needed for space heating and domestic hot water. The choice of network structure is an early planning decision that determines supply security, heat losses, investment costs, and long-term expandability of the overall system.

Pipe Hierarchy

A thermal network is typically structured into three hierarchical pipe levels:

• Transmission and trunk line: Connects the energy centre with the supply areas. This is where the largest nominal diameters and volume flow rates occur. The transmission line bridges long distances with the lowest possible pressure losses.
• Branch line (distribution line): Branches off from the trunk line and serves individual streets or neighbourhoods. Nominal diameters decrease with increasing distance from the trunk line.
• Service connection: Connects the distribution line with the transfer station in the building. It has the smallest nominal diameters and is typically executed as a single line per building.

This subdivision is reflected in the hydraulic calculation: transmission lines largely determine the overall pressure loss of the network, while service connections are primarily relevant for the available differential pressure at the transfer station.

Classification by Number of Pipes

Two-Pipe System

The two-pipe system is the standard for modern thermal networks. It consists of a supply line and a return line forming a closed circuit. The heat transfer medium — typically water — is heated in the energy centre, transported to the customers via the supply line, releases heat at the transfer station, and flows back to the centre via the return line.

Advantages:

• Simple and cost-effective design
• Proven control technology
• Low installation costs

Virtually all currently planned and operated heating networks are based on the two-pipe system.

Three-Pipe System

The three-pipe system extends the two-pipe arrangement with an additional line. Two variants are common:

1. Two supply lines and one return: One supply line operates on a variable basis, the other at a constant temperature. The variable line serves weather-dependent consumers (space heating), while the constant line serves weather-independent consumers (domestic hot water, process heat).
2. One supply and two return lines: Separation of the return by temperature level, in order to provide a return temperature as low as possible for certain generators (e.g., heat pumps).

Advantages:

• Demand-responsive control of different consumer groups
• Lower heat losses in the variable-temperature line during mild weather
• Better integration of low-temperature heat sources

Disadvantages:

• Higher investment costs due to the third line
• More complex control and operation
• Greater space requirement in the trench

Four-Pipe System

The four-pipe system consists of two separate two-pipe circuits. Typical applications:

• Heating and cooling: One circuit for heat supply, a second for cooling supply — particularly in areas with simultaneous heating and cooling demand
• Different temperature levels: High-temperature circuit for process heat and low-temperature circuit for space heating

The four-pipe system is rarely used due to the high investment costs and large space requirement, and is only employed for specific requirements.

Network Structures

Radial Network

The radial network (also branched or tree network) is the simplest network structure. From a single feed-in point — the energy centre — pipelines branch out in a star pattern, subdividing into ever smaller branches. Nominal diameters decrease with increasing distance from the centre.

Advantages:

• Simple and clear structure
• Low investment costs
• Unambiguous hydraulic conditions

Disadvantages:

• No redundancy: in the event of a pipe failure, all downstream customers are cut off from supply
• Subsequent extension at the endpoints requires larger nominal diameters of the existing pipelines
• The critical point is always at the most remote end of the branch

The radial network is suitable for smaller networks with a manageable number of customers and low supply security requirements.

Ring Network

The ring network is a special case of the meshed network in which a closed ring line ensures the supply of connected customers. Feed-in occurs at one or more points on the ring line.

Advantages:

• Higher supply security: in the event of a pipe failure, most customers can continue to be supplied via the alternative supply route
• More uniform pressure conditions in the network
• More flexible integration of additional feed-in points

Disadvantages:

• Higher investment costs than for a radial network
• More complex hydraulic calculation

The ring closure is frequently realized in a later expansion phase when supply security needs to be increased.

Meshed Network

The meshed network is created when multiple energy centres are interconnected via meshed pipelines in combined operation. It represents the most complex but also the most supply-secure network structure.

Advantages:

• Highest supply security through multiple feed-in points and redundant pipeline routes
• Flexible deployment of various generation plants depending on the load case
• Good expandability

Disadvantages:

• High investment costs
• Complex hydraulic calculation and operation
• Complex control of the feed-in points

Meshed networks are typically the result of continuous expansion over many years and are found predominantly in larger urban district heating systems.

Sub-Distribution and Route Alignment

The type of route alignment determines how the distribution line is connected to the individual buildings. Four basic variants are distinguished:

• Standard route alignment: Each building is connected directly to the distribution line via its own service connection. This is the most common and most flexible variant.
• Building-to-building route alignment: Several buildings are grouped together and supplied via a shared spur line. The line is routed from building to building. This variant reduces the total route length but creates dependencies between the buildings.
• Loop-in route alignment: The distribution line is routed directly through the buildings. This variant is rarely used and is primarily considered with flexible pipe systems (PMR).
• Basement routing: The distribution line runs through the basements of the connected buildings. This reduces burial costs but requires appropriate penetrations and agreements with the building owners.

Evolution of Network Structure

The network structure of a thermal network typically evolves through four stages over years or decades:

1. Individual radial networks: During the development phase, independent radial networks are established around individual energy centres. Each centre supplies its own clearly defined supply area.
2. Interconnection of existing networks: Neighbouring radial networks are connected via tie lines. This enables load balancing between the centres and increases supply security.
3. Ring closure: Through targeted tie lines, open branches are closed into rings. This creates alternative supply routes and improves pressure conditions.
4. Meshed network: Through ongoing extensions and ring closures, a meshed network emerges with multiple feed-in points and redundant pipeline routes.

This evolution process is characteristic of many urban district heating systems and reflects the growing importance of supply security as customer numbers increase.

Conclusion

The choice of network structure is a central planning decision that determines the investment costs, supply security, and expandability of the thermal network in the long term. While radial networks are suitable for entering piped heat supply, ring and meshed networks offer the redundancy required for larger supply areas. The hydraulic effects of different network structures and route alignments can be investigated and optimized in detail with simulation software such as VICUS Districts.

Further reading: Pipe Dimensioning describes how the nominal diameters of the individual pipe hierarchy levels are calculated, Supply Area and Heat Demand Density explains the delineation and evaluation of supply areas as the basis for the network structure, and Planning Phases for Thermal Networks places the network structure decision within the overall planning process.

References and Standards

• AGFW FW 401 — Installation and Statics of Pre-insulated Bonded Pipes in District Heating Networks
• Nussbaumer, T.; Thalmann, S.; Zaugg, D.; Cueni, M. (2025): Planungshandbuch Thermische Netze. Version 2.0, EnergieSchweiz / Bundesamt fuer Energie BFE.
• AGFW FW 440 — Hydraulic Calculation of Hot Water District Heating Networks