District heating networks across Europe and Scandinavia were largely designed in an era when supply temperatures of 80°C to 120°C were standard engineering practice. Those temperatures made sense for the building stock and production assets of the time. In 2026, the picture looks very different. Aging building stock is being retrofitted for energy efficiency, heat pumps are replacing fossil fuel boilers, and operators are under mounting pressure to reduce thermal losses and cut emissions. The result is a growing imperative to reduce supply temperatures — and a genuinely difficult engineering question: how do you model and plan that transition in a network that was never designed for it?

Heat network hydraulic modeling has always required careful attention to both hydraulic and thermal behavior. But simulating a shift toward lower supply temperatures in an existing high-temperature infrastructure introduces a layer of complexity that static calculations simply cannot address. Physics-based thermal network simulation tools are now the standard approach for planning these transitions — enabling engineers to test scenarios, identify constraints, and make investment decisions before a single pipe is replaced or a single substation is reconfigured.

The engineering challenge of transitioning district heating networks

The core challenge of moving from high-temperature to low-temperature district heating is that the two operating regimes make fundamentally different demands on the same physical infrastructure. A network designed for 90°C supply and 60°C return relies on a specific hydraulic balance, pump configuration, and substation design. Reducing supply temperature to, say, 60°C or lower does not simply mean turning down a dial at the production plant — it changes the thermodynamic relationship between the network and every building it serves.

In a high-temperature system, substations are sized to transfer heat efficiently at a given temperature differential. When supply temperature drops, the same substation must either transfer heat across a smaller differential — which typically means higher flow rates to deliver the same thermal output — or the building’s heating demand goes unmet. This creates a cascade of hydraulic consequences: pressure differentials shift, pump operating points change, and sections of the network that previously operated within comfortable margins may become bottlenecks. Planning this transition without a physics-based model means navigating that cascade largely in the dark.

There is also a temporal dimension to the challenge. Most utilities cannot switch from high-temperature to low-temperature operation overnight. The transition is phased, often spanning years or decades, as production assets are replaced, buildings are retrofitted, and substations are upgraded. During that period, the network operates in a hybrid state — serving some consumers under conditions suited to low-temperature supply while others still depend on higher temperatures. This hybrid operating window is arguably the most complex phase to plan for, and it is where district heating network design software delivers its clearest value.

What low-temperature district heating demands from an existing system

Low-temperature district heating — broadly defined as systems operating with supply temperatures below 60°C, and in some fifth-generation concepts even lower — places specific demands on network components that were not part of the original design brief. Understanding these demands is the prerequisite for any meaningful simulation work.

Substation performance at reduced temperature differentials

Substations are the critical interface between the district heating network and the consumer’s internal system. In a high-temperature network, a substation sized for a 30°C to 40°C differential between supply and return operates with a comfortable margin. At lower supply temperatures, that differential narrows, and the heat exchanger must work harder to transfer the same thermal load. In practice, this often means that existing substations are undersized for low-temperature operation — a constraint that must be identified through simulation before it becomes a service quality problem in the real network.

Flow rate and pipe capacity implications

Reduced temperature differentials directly affect the mass flow rates required to meet consumer demand. The relationship is straightforward: if the temperature differential between supply and return decreases, flow must increase to deliver the same thermal power. In a network with fixed pipe diameters, this increased flow demand can push sections of the network beyond their hydraulic capacity, causing pressure drops that exceed what pumps can compensate for. District heating optimization software that couples thermal and hydraulic calculations simultaneously is essential for identifying where these constraints will emerge before they are encountered operationally.

Return temperature control

Low-temperature operation also demands tighter control of return temperatures. In high-temperature systems, a somewhat elevated return temperature is thermodynamically costly but operationally manageable. In low-temperature systems — particularly those integrating heat pumps or waste heat sources that are sensitive to inlet temperature — a high return temperature can directly reduce production efficiency or even prevent certain heat sources from operating at all. Managing return temperature across a large, geographically distributed network requires a simulation environment that can model consumer behavior, substation performance, and network hydraulics together.

Key variables a simulation must capture for hybrid network analysis

A simulation intended to support low-temperature transition planning in an existing high-temperature network must go beyond steady-state hydraulic analysis. The hybrid operating condition — where different parts of the network serve consumers with different temperature requirements — creates a dynamic that only a fully coupled thermal-hydraulic model can represent accurately.

The variables that matter most in this context include:

• Supply temperature profiles over time: The model must represent time-varying supply temperatures rather than a single fixed value, capturing how the network responds as temperatures are stepped down incrementally during the transition.
• Consumer demand patterns: Thermal demand varies by building type, occupancy, and outdoor temperature. A credible simulation accounts for this variation rather than applying a uniform load assumption across all connection points.
• Substation heat transfer characteristics: Each substation’s heat transfer coefficient and design parameters affect how efficiently it transfers heat at a given supply temperature. These parameters must be represented at the component level.
• Pipe thermal losses: In a low-temperature network, the temperature difference between the pipe and the surrounding soil is smaller, which reduces thermal losses — but the model must account for how this changes as supply temperature decreases, particularly in older networks with degraded insulation.
• Pump operating curves: As flow rates increase to compensate for reduced temperature differentials, pump operating points shift. The simulation must verify that existing pumps can meet the new operating requirements or flag where upgrades are needed.
• Production source temperature constraints: If the transition is being driven partly by the integration of heat pumps, solar thermal, or industrial waste heat, the simulation must represent the temperature constraints of those sources and how they interact with network demand.

Capturing all of these variables simultaneously requires a district energy modeling environment built on the physics of heat transfer and fluid flow — not simplified empirical approximations. This is precisely the modeling challenge that Fluidit Heat is designed to address, combining hydraulic and thermal simulation in a single platform that can represent the full complexity of a transitioning network.

A simulation-based approach to planning the temperature transition

Effective transition planning begins with a calibrated baseline model of the existing high-temperature network. This model should reproduce the current operating state — measured supply and return temperatures, flow rates, pressure profiles, and consumer demand data — with sufficient accuracy that deviations between simulated and observed values are within acceptable engineering tolerances. A baseline that has not been calibrated against real operational data will produce unreliable results when used to simulate a fundamentally different operating regime.

Once the baseline is established, the simulation-based planning process typically follows a structured sequence of scenario analyses:

1. Temperature sensitivity analysis: Systematically reduce supply temperature in the model and observe where hydraulic or thermal constraints emerge. This identifies the sections of the network and the consumer connections that will be most affected by the transition, allowing engineers to prioritize intervention.
2. Substation assessment: For each connection point, simulate performance at the target low supply temperature and compare it against the consumer’s peak demand. Connections where the existing substation cannot meet demand at the lower temperature are flagged for upgrade.
3. Phased transition modeling: Represent the hybrid operating state by assigning different temperature requirements to different network zones, reflecting the phased nature of the transition. This allows planners to assess whether the network can simultaneously serve high-temperature and low-temperature consumers without compromising either group.
4. Production integration scenarios: Model the introduction of new low-temperature heat sources — heat pumps, waste heat recovery, or solar thermal — and assess how they interact with the network’s hydraulic and thermal balance under different demand conditions.
5. Infrastructure investment prioritization: Use simulation outputs to rank infrastructure interventions — pipe replacements, pump upgrades, substation retrofits — by their impact on enabling the temperature transition, supporting a capital investment program grounded in engineering evidence rather than assumption.

This kind of structured scenario simulation is where district heating optimization software demonstrates its strategic value. The ability to test multiple transition pathways in a virtual environment — before committing capital to physical changes — significantly reduces the risk of costly misalignment between infrastructure investment and operational outcomes.

Common pitfalls in low-temperature network planning

Even with a simulation-based approach, there are several recurring mistakes that can undermine the quality of low-temperature transition planning. Awareness of these pitfalls is as important as having the right tools.

Treating the transition as a uniform network-wide change. In reality, different parts of a district heating network will reach the conditions for low-temperature operation at different times, depending on the pace of building retrofits and substation upgrades in each zone. Plans that assume a simultaneous network-wide transition typically underestimate the complexity of the hybrid operating period and the hydraulic challenges it creates.

Neglecting return temperature management. A focus on reducing supply temperature without equal attention to return temperature control can erode the efficiency gains the transition is intended to achieve. If substations in older buildings continue to return hot water to the network, the effective temperature differential remains low regardless of what the production plant is doing. Simulation must include return temperature as an active variable, not a fixed assumption.

Underestimating the impact on pump systems. Increased flow rates in a low-temperature network place higher demands on pumping infrastructure. Plans that focus on pipe and substation changes while overlooking pump capacity and control strategy often encounter operational problems that could have been identified and resolved during the modeling phase.

Using steady-state models for a dynamic problem. A transition that unfolds over years involves changing load profiles, seasonal temperature variation, and incremental changes to network topology. Steady-state analysis at a single design point cannot capture this complexity. Dynamic simulation — modeling the network’s behavior over time under varying conditions — is essential for credible transition planning.

Separating hydraulic and thermal analysis. Some planning workflows model hydraulics and thermal behavior in separate steps, using outputs from one as inputs to the other. This approach misses the coupled interactions between flow rates, temperature differentials, and heat transfer that are central to low-temperature network behavior. A thermal network simulation tool that solves hydraulic and thermal equations simultaneously produces results that more accurately reflect real network behavior.

For utilities navigating this transition, access to the right combination of modeling expertise and district heating software is critical. Fluidit’s Expert Consulting Services work alongside utility teams at exactly this stage — helping to build, calibrate, and run the scenario analyses that turn a complex transition plan into a defensible engineering program. Whether the need is a full model build from scratch or a targeted analysis of a specific network zone, having engineers who work with the simulation platform daily means the technical support is grounded in practical experience, not just theory.

If your utility is evaluating how to plan a temperature transition in your heat network, exploring Fluidit Heat as your district heating network design software is a natural next step — book a demo to see how the platform handles the specific modeling challenges your network presents.