Heat is delivered to connected buildings via a network of pipes. Heat loss in district heating occurs because some of the heat is released into the environment as it travels through the pipes.
The extent of these losses depends, amongst other things, on temperature, insulation, the pipe system, and network operation. This article explains how heat losses can be categorised and calculated per metre and for the entire district heating network. You will also learn which technical and operational measures help to limit them effectively.
Depending on the context, heat loss in district heating is described using three different parameters:
These key figures answer different questions and should not be treated as equivalent. W/m describes the heat loss along the pipeline, whilst kWh or MWh represent the resulting energy loss. Percentage values, on the other hand, relate to the balance of the entire district heating network.
The physically inevitable heat loss from a district heating pipeline cannot be completely avoided. However, the extent of this loss can be influenced by operating conditions, the design of the pipework system and the installation conditions.
Heat always flows from a warmer to a colder area. In the case of district heating pipes laid underground, heat is transferred from through the wall of the medium pipe, the insulation and the outer casing to the surrounding soil. The heat transfers medium, the greater the temperature difference between the heat transfer medium and the surroundings, the higher the heat loss from a district heating pipe tends to be. The insulation significantly limits this heat flow.
The amount of heat loss per metre in a district heating pipeline, depends in particular on the following factors. These factors interact and determine how much heat the pipe system loses under the respective operating and environmental conditions:
Among other things, the pipe dimension affects the heat-emitting surface area, whilst the material influences heat conduction through the service pipe. However, the overall structure comprising the service pipe, insulation and outer casing remains the decisive factor. Damp ground, as well as damaged or moisture-saturated insulation, can further increase heat loss in district heating systems.
Fittings, Valves and Joints also cause localised heat losses, which must be taken into account separately when assessing the overall system. Comparative figures are therefore only meaningful if the dimensions, temperatures, insulation and installation conditions are the same, and it is clear whether they apply to a single pipe or to both the supply and return pipes.
Heat losses are difficult to grasp, as they do not make a noise, they do not drip, and they do not lead directly to a breakdown. Without measurement or calculation, it therefore often remains unclear how much energy is released into the environment along a route. Yet even moderate heat losses per metre quickly adds up to significant amounts of energy and additional operating costs over the length of a district heating network.
A specific calculation reveals the power loss occurring along a route and how much energy is consequently lost over the course of the year. After all, only those who know the extent of the heat loss from the district heating pipeline, can take targeted countermeasures.
Heat loss per metre is not a fixed value, but the result of a thermal calculation. For the steady-state, simplified case of a single pipe, the basic principle can be expressed as follows:
Length-related heat loss rate qₗ in W/m = temperature difference ΔT in K ÷ length-related total thermal resistance Rₗ in m × K/W
The total thermal resistance per unit length takes into account, amongst other factors, pipe geometry, insulation material, insulation thickness, ground conditions and installation situation. In twin-pipe systems, the thermal interactions between the supply and return pipes must also be taken into account. The simplified basic equation alone is therefore not sufficient for their thermal design.
For design purposes, the W/m value is determined using a suitable calculation method or design software. Alternatively, it can be taken from technical product data at under documented conditions. Such a value can only be used if the underlying temperatures, dimensions and installation conditions match those of the pipe system in question.
If the heat loss per metre of district heating pipeline is given in W/m, it is multiplied by the pipeline length in question:
Heat loss in W/m × pipe length in m = heat loss in W
It must be established whether the initial value applies to a single pipe or to both the supply and return pipes.
Power loss initially describes how much heat the pipework system continuously emits under the conditions under consideration. To calculate the annual energy loss, the power in kilowatts is multiplied by the operating hours:
Power loss in kW × operating hours = energy loss in kWh
If conditions are assumed to remain constant throughout the year, 8,760 hours per year can be used for the calculation. However, as flow, return and ambient temperatures fluctuate in practice, this is a simplified estimate.
Standard reference: The basic equation shown and the subsequent extrapolation illustrate the calculation method but do not replace a complete thermal design. DIN EN 13941-1:2022-06 specifies requirements for the design of insulated single- and double-pipe composite systems for district heating networks laid directly in the ground.
Whilst W/m describes the heat loss per unit length of a defined pipework system, percentages indicate the losses across the entire network over a specified period. To calculate this, the difference between the heat supplied to the network and the usable heat delivered to customers or to the supplier’s own facilities is expressed as a proportion of the heat supplied to the network:
Relative network losses as a percentage = (network feed-in − usable heat output) ÷ network feed-in × 100
If, for example, 100,000 MWh of heat is fed into a district heating network within a year and 88,000 MWh is recorded as usable heat output, the loss amounts to 12,000 MWh. In this example, district heating losses stand at 12 per cent. To ensure accurate calculation, the feed-in and heat output must relate to the same period and the same accounting boundaries.
The percentage of heat loss in district heating depends primarily on the following factors:
W/m therefore quantifies the heat loss per unit length of a pipework system under defined conditions. The percentage figure, on the other hand, describes the balance of the entire district heating network. High percentage losses cannot therefore automatically be attributed to a single pipework system or a specific material.
Heat loss in district heating can be minimised through a combination of planning, operation and maintenance. It is crucial to consider the network temperatures, the pipework configuration and the actual condition of the route together.
Which measure achieves the greatest effect depends on the structure and condition of the network in question. Operators should therefore evaluate calculated heat losses per metre of district heating pipeline, measured network losses and operational data together. If losses decrease, less heat needs to be supplied at the generation point to achieve the same usable output. How this affects operating costs depends on energy prices and the specific operating conditions.
Low-loss network operation is particularly relevant for green district heating concepts that incorporate renewable heat sources or usable waste heat. This ensures that the heat reaches the connected buildings with the lowest possible transmission losses. The impact on emissions depends on the specific generation mix.
In pre-insulated plastic pipe systems, the medium pipe, insulation and outer casing act together as an integrated system. Due to the nature of the material, the plastic medium pipe has low thermal conductivity. However, it is primarily the factory-applied insulation that limits heat flow to the environment. The outer casing protects the system from moisture and mechanical stress.
The arrangement of the supply and return pipes also plays a role. In single-pipe systems, both pipes run in separate outer casings. Double-pipe systems carry the supply and return in a single shared outer casing with shared insulation. Their thermal interaction must be taken into account in the calculation. The extent of heat loss from the district heating pipeline therefore depends on its dimensions, temperature level and installation conditions.
aquatherm energy combines medium pipes made of PP-R or PP-RCT with PUR insulation and an HDPE outer pipe. The PUR foam meets the functional requirements of DIN EN 253. As a single-pipe system, aquatherm energy is available in sizes ranging from 32 to 355 mm and is suitable for heating and cooling networks with operating temperatures of up to 80 °C.
For supply and return flows in smaller diameters, there is aquatherm energy twin. The twin-pipe system comprises two PP-RCT medium pipes within a single outer casing. It is available in diameters ranging from 32 to 125 mm and is designed for continuous operation up to 75 °C as well as short-term temperature peaks of up to 100 °C.
The appropriate variant should be determined on the basis of the project-specific temperatures, pressure ratings and calculated heat losses. The selection of the pipe system is therefore one of the technical decisions that must be taken into account in the decarbonisation of district heating.
Heat loss in district heating systems can only be assessed effectively if the key figures are clearly distinguished from one another. W/m describes the heat loss per unit length of a pipework system, whilst kWh or MWh represent the total energy lost over a given period. Percentage values, on the other hand, show the overall balance for the entire district heating network and cannot be directly compared with W/m.
The extent of the losses depends on temperatures, pipework dimensions, insulation, installation conditions and network operation. Calculations should therefore be based on clearly defined boundary conditions. To reduce losses, the pipework system, operating mode, condition of the pipes and monitoring must be considered together. Only specific network and operational data can reveal which technical measures are suitable and what effect they will have on the network in question.
Every district heating network has different requirements in terms of pipework sizing, temperature levels, and pipe configuration. aquatherm supports planners and operators in technically evaluating suitable system solutions for new builds, refurbishments and network extensions.
Talk to our experts.
