LOW-TEMPERATURE DH: HOW TO REDUCE THE TEMPERATURE IN EXISTING DISTRICT HEATING NETWORKS

Road map to low-temperature district heating

When working with heat supply planning in Denmark, the temperature level is never an objective by itself. The objective of all energy supply is always to meet the overall energy policy objectives in the most cost-effective way, including costs of the environmental impact.

This can be narrowed down to the requirement of the Danish Heat Supply Act that all investments in the district heating sector shall be cost-effective for society, including costs to the environment, and for providing consumers with cost-effective and sufficient quality energy.

District heating companies face the challenge of increasing their market share by connecting more consumers to the existing network, increasing heat storage capacity, and installing large heat pumps to utilise low-temperature heat. To succeed, they must carefully balance the supply temperature to optimise both network capacity costs and production costs.

Each network requires its own pathway

Therefore, the optimal temperature levels in a district heating system in Denmark will depend on the actual local situation and the stage in a transition from high-temperature energy to low-temperature renewable energy. In other words, low temperature is the lowest possible temperature and changes from day to day and therefore requires constant focus.

Thus, the optimal level of low-temperature district heating should be determined based on the following criteria:

• The normal supply temperature to the district heating network should be adjusted constantly throughout the year to efficiently utilize low-temperature heat sources from ambient energy (e.g., wastewater-, air-, and surplus heat).
• As district heating in Denmark is always used to produce both space heating and domestic hot water, the supply temperature from district heating to the building should in general, not be below 60°C to prevent risk of legionella (unless local measures, e.g., temperature boosting, is installed) and offer sufficient quality of domestic hot water.
• The maximal supply temperature to the network on cold days should not exceed 90-95°C to allow pressure less or atmospheric heat storage facilities.
• The return temperature from buildings should never exceed 40°C, as thermostatic valves and coils, radiators and floor heating systems in buildings can provide temperatures below this level at modest costs.

Legal, institutional, and technical incentives

There is a wide range of incentives and measures to lower the temperature in the district heating system and in buildings, which also ensures compliance in the entire supply chain.

The road map is inspired by the experiences from primarily Denmark, but also Germany and other countries that DEA collaborates with, for the benefits and applicability of LTDH.

Some incentives will have a positive effect already in the short term, whereas others will be a long-term investment. Similarly, some investments will be very cost-effective whereas others may be too expensive considering the local conditions, and the local conditions may change in time.

The legal, institutional, and technical incentives can be grouped within the following headlines:

• Legal framework – Regulating district heating for low-temperature and Building regulations for low-temperature.
• Institutional set-up and soft measures – Ownership of the consumer heat substation.
• Tariffs incentives and smart meters.
• Technical measures in the building.
• Technical measures in the district heating substations.
• Technical measures in the district heating system.
• Gradual transition from natural gas to district heating.

The report comes with a number of examples within each subject heading.

8 steps to low-temperature

The report describes measures that help to reduce temperatures in district heating systems, with a focus on legal, institutional as well as technical measures in buildings, where the key takeaways are:

Introduce a building code for the design of installations in buildings to ensure that all new and old renovated buildings are prepared for low-temperature district heating

Introduce incentive tariffs for lower return temperature from buildings

Invest in electronic remote meters (smart meters) for improved energy management in buildings, implementation of incentive tariffs, and improved communication between the district heating company and building owners

Move the point of delivery from the primary to the secondary side of the district heating substation, which relieves the customers of operation and maintenance responsibilities and gives the company access to all the relevant data so that errors can be identified and acted upon immediately.

Upgrade the facade of old buildings (outside insulation and new windows, if acceptable) to lower the heat demand, thereby lowering the necessary temperatures

Install local temperature boosting at critical high-temperature consumers to enable the reduction of the overall temperature in the district heating system. In the case where there are only a few consumers that need a significantly higher supply temperature compared to the typical consumer (e.g., industrial consumers or some hospitals), a local heat booster at the consumer, e.g., a boiler, can be installed to upgrade the district heating supply temperature.

Utilize the return temperature of high-temperature consumers by connecting low-temperature consumers with a 3-pipe connection on the return pipe. If the district heating system consists of a mix of high- and low-temperature consumers, and the return temperature is too high, it is an opportunity to utilize some of the low-temperature consumers to reduce the return temperature in the network. This can be done by connecting the low-temperature consumers to the return pipe.

Allow a higher flow temperature from low-temperature heat sources (by local temperature boosting). This is only to be an option during the coldest winter hours to avoid unnecessary investment costs in the upsizing of the district heating network.

Regulation-driven shift to low-temperature DH

The district heating and building sector in Denmark kick-started the transition to lower temperatures in the years after 1980 due to intensive heat supply planning and tax on fossil fuels as a form of incentive to reduce and use low-temperature heat from electricity generation.

Today, most Danish district heating companies operate at 70°C supply and 40°C return on average, which is defined as low-temperature district heating. This definition is based on the optimal utilization of low-temperature energy sources and on the prerequisite of a 60°C supply temperature at the consumer level to ensure safe domestic hot water production, meaning without the risk of legionella in the domestic hot water tank.

Danish experience, therefore, shows that conversion to low-temperature district heating is possible but time-consuming. Putting in place legislation and regulative measures takes time, but also investment in new technical approaches can take time.

German experience meets the Danish

The transition to low-temperature district heating in countries such as Germany is slightly behind compared to, for example, Denmark. This is due, among other things, to a different political focus historically, as well as other local conditions, such as their building stock and a greater need for process heat. However, several of the measures from Denmark can be implemented either directly or with minor adaptations to German district heating systems.

The following three main initiatives have been identified:

Long term:

• Implement technical solutions for the further improvement of the district heating networks

Short term:

• Consumers should be included as key stakeholders in the process of transformation of heating networks toward net-zero emissions
• Greater focus on the digitalization of the substations in the buildings to provide 1) the possibility for the consumers to monitor their energy demand as well as return temperatures, and 2) tariff incentives for lowering the return temperature.
  The shares of temperature levels in district heating networks in Germany can be seen in Figure 2. Less than 25% related to the total length are operated below 90°C.

Figure 2: Temperature in German district heating systems related to the network kilometers (AGFW Data status official statistics, 2020)

In Germany, subsidies provided by the German bank KfW provide incentives to increase the energy efficiency of existing buildings. The sector works with a number of current measures, tools, and incentives for energy regulation and optimization. Focusing on the following headlines:

• Regulatory framework
• Technical guidelines
• Remote meters
• Tariff incentives

As an example, using technical guidelines, the district heating supplier in Berlin, which operates a large part of the district heating system in Berlin, specifies a maximum return temperature of 56°C in its Technical General Terms and Conditions. In general, they differentiate two supply areas: East and West.

Area West has a supply temperature of a minimum of 110°C and a return temperature of 55°C (=Temperature spread of 55 K), and Area East with a supply temperature of in general minimum of 135°C and a return temperature of 45°C (=temperature spread of 90 K).
Some of the measures from the Danish experience are in the report analysed regarding their possible realization in Germany.

Many networks are already being operated with optimized supply temperatures, considering the season, time of day, and requirements of special consumers. Systems where this is not being done already lack the required incentive to do so. Existing systems are often designed around very few supply plants, with long travel times and temperature losses between production and far-out consumers.

Those systems can benefit from a more spread-out production landscape or local measures to increase supply temperature for certain consumers, like bypasses. Low-temperature base load plants appear to be rare still, but more and more heat sources with low supply temperature are being considered (industrial waste heat, heat pumps, geothermal sources, and others).

Metering together with tariff structures are key

For establishing tariff incentives, as in Denmark, initially, smart meters are necessary. This measure can, therefore, be introduced at the earliest when smart meters are available in district heating systems. Currently, smart meters are often installed primarily not to facilitate the introduction of tariff incentives but to identify consumers who breach their existing contracts with excessively high return temperatures.

To lower the overall return temperature in German district heating networks, three key measures have been identified:
First, there are technical solutions for the further improvement of the district heating networks themselves.

Second, consumers should be included as key stakeholders in the process of transformation of heating networks towards net-zero emissions, which has not been done so far on a large scale.

Third, the digitalization of the substations in the buildings has to be focused on providing the possibility for the consumers to monitor their energy demand as well as return temperatures, and thus that supply companies can effectively offer tariff incentives for lowering the return temperature. While the technical measures are more likely long-term measures, the other two should be implemented in the short term.

Experience from other countries

In many other countries establishing low-temperature central heating in buildings and new district heating system is a challenge but can be introduced by coordinating with city-wide energy planning with the detailed renovation of the buildings following the experiences from Denmark.

New district heating countries have the possibility to start from where Denmark is today and establish new low-temperature systems utilizing renewable heat sources efficiently.

Figure 3: Low-temperature district heating is essential if the vision is to create livable cities with smart solutions for the citizens.

Summary

Low-Temperature District Heating (LTDH): Pathways to Efficient and Sustainable Systems

The article highlights the benefits of low-temperature district heating (LTDH), which improves energy efficiency, reduces heat losses, and enables better integration of renewable and surplus heat sources. Based on findings from a Ramboll report for the Danish Energy Agency, it outlines how Denmark and its partner countries (e.g., Germany, the UK, and the Netherlands) can transition existing networks toward LTDH.

A roadmap is presented, emphasizing that temperature reductions must balance cost-effectiveness, environmental impact, and energy quality, considering local conditions. Key principles include maintaining supply temperatures above 60°C (to avoid Legionella risk) and maximizing the use of ambient low-temperature sources.

Eight main steps are recommended, ranging from updating building codes and installing smart meters to offering tariff incentives and using local temperature boosting for critical users.

The article also compares Denmark’s progress—where average supply/return temperatures are 70/40°C—with Germany’s slower transition, identifying smart metering, consumer engagement, and network optimization as essential for Germany’s success.

Finally, it stresses that new district heating markets have an opportunity to build next-generation low-temperature systems from the outset, benefiting from Denmark’s extensive experience.

For further information, please contact Pernille M. Overbye at pmo@ramboll.com