What can cause high return temperatures in installations served by district heating?
Causes for high return temperatures in district heating networks can be very diverse. One can categorize causes for high return temperatures according to the following characteristics:
- Origin: planning default, defect in installation or repair, progressive degradation (fouling, sensor drift), sudden failure etc.
- Responsible entity, which may for instance be planner, installer or occupant. A distinction can be made between design faults, construction and assembly faults, and operational and maintenance faults .
- Time course: faults may be present from the start (installation), happen gradually or suddenly.
- Location in the system: in the substation or on the secondary side, respectively in the space heating or domestic hot water system. An overview of issues grouped by subsystem is provided in Figure 1.
- Affected components, if applicable: e.g. heat exchanger, valve, sensor. However, it may also be that the system rather than a single component is responsible for high return temperatures.
- Impact: issues causing high return temperature may also be responsible for thermal discomfort or increased energy consumption, or even prevent the system from functioning altogether. Faults resulting in severe comfort impairment are usually easily found .
Decision to perform an audit
Operation, maintenance and repair of district heating substation and secondary systems are usually the responsibility of the building owner and energy consumers, although many district heating suppliers offer service plans . Regular inspections of district heating substations are recommended ( p.43) but usually not required by any regulations, and thus often omitted. In any case, the physical accessibility of these installations is an important prerequisite for a successful audit and fault handling . Monitoring data may draw the attention of district heating suppliers to a substation or building. With increasing availability of high-resolution metering and monitoring data (with hourly or even sub-hourly time steps), it is expected that suboptimal behaviour can to a large extent be detected based on these data. Methods such as the overflow method may allow substations to be ranked by order of priority . The overflow is defined as the difference between measured annual flow and the flow that would have been necessary to carry the measured amount of energy with an expected temperature difference. The resulting quantity can be considered to be proportional to the additional costs associated with a return temperature higher than expected, so that it can be used to establish an order of priorities among substations . Complaints from building occupants are another possible trigger for audits.
Audit preparation includes the gathering and analysis of background information and the timing of the audit.
Background information should be collected regarding the substation, the secondary systems and the served building. System information includes diagrams and manufacturer data. Measurements should also be considered.
If possible, collect the following useful documents:
- Guidelines and technical requirements of local district heating provider (e.g. ) or national association, including maintenance checklist (e.g. )
- Manual or operating guide of the substation, if it is prefabricated (e.g. ).
- Manual of controller, if not included in substation manual (e.g. )
A basic step to auditing building installations served by district heating is to find about the requirements formulated by the local district heating provider and ensuring they are fulfilled. Examples of such requirements are summarized in Table 3. Note that these requirements may differ from district heating network to network, and that the specific requirements applicable to your network should be considered.
The audit itself includes different tasks. A basic inspection should be carried out, to determine the state of the various components. This inspection is primarily a visual inspection, for instance checking for corrosion, leaks or lacking insulation. Noise should also be considered. Checklists can be used for the inspection. An example is provided below, based on the Euroheat & Power guidelines for district heating substations  and the manuals of some prefabricated substations , .
- Check all connections, check for leaks
- Check all parameters to nominal / actual values or admissibility
- General visual inspection of all components, check valves, check for leaks, check for corrosion, check pipe insulation
- Check and clean strainers
- Perform a functional and usability check of all components
- Control of the heat exchanger, check for leaks inside and outside
- Perform a visual inspection of the heat meter and water meter
- Perform a visual inspection of the measuring devices
- Check supply and return temperatures
- Check pressure levels in the system
- Check heating curve
- Check time control (on/off times, reduced temperature times, summer/winter time)
- Check wiring of control valve
The next goal is fault detection, that is to identify whether the system functions in a suboptimal way or not. If suboptimal behaviour is identified, the reason for it is to be determined (diagnosis). Measurements at various points in the system and in different operation states are an important means of fault identification and diagnosis. Readings of installed sensors may be complemented by additional measurements with mobile sensors. Manual modification of control settings can be used to investigate presumable issues.
In cases where a diagnosis can be made, some adjustments may be carried out during the audit. Some issues may require a later intervention. Ways to remedy specific issues are indicated in the issue profiles.
Documentation of the audit and corrections to the system is important. It serves not only to justify payment, but also for knowledge gain (for the district heating supplier, for technicians and potentially for other interested parties), and for follow-up. Documentation may make use of checklists. Documentation should include:
- Items that have been checked (possibly in the form of checklists)
- Measurements and readings performed during the audit)
- Changes to the system that were carried out
 H. Lund et al., 4th Generation District Heating (4GDH): Integrating smart thermal grids into future sustainable energy systems, Energy, vol. 68, pp. 1â€“11, 2014.
 Wien Energie, Technische Richtlinie TR-HS - Hausstation sekundÃ¤r. 2009.
 H. Zinko et al.,
Improvement of operational temperature differences in district heating systems,
 Euroheat & Power,
Guidelines for District Heating Substations.
District Heating Substations - Operating Guide.
 J. C. Visier, R. A. Buswell, Y. Akashi, P. Andre, O. Bauman, and al,
Commissioning Tools for Improved Building Energy Performance - ECBCS Annex 40 Project Summary Report.
 A. Bres, C. Johansson, R. Geyer, P. Leoni, and J. SjÃ¶gren,
Coupled Building and System Simulations for Detection and Diagnosis of High District Heating Return Temperatures,
in Building Simulation 2019, 2019.
 R. Geyer,
Projekt heat_portfolio (FFG-Nr. 848849). Deliverable D6.1 & D6.2. Katalog zur Implementierung der nutzerseitigen MaÃŸnahmen. Szenarien unterschiedlicher Durchdringung der MaÃŸnahmen als Basis fÃ¼r die Simulation in AP7.
 AIT Austrian Institute of Technology; and AEE Intec,
Fehlerursachen Dokumentation - Internal document for FFG project T2LowEx.
 A. Dexter and J. Pakanen,
Demonstrating Automated Fault Detection and Diagnosis Methods in Real Buildings,
 Wien Energie,
Technische Richtlinie FernwÃ¤rme - Leitfaden allgemeine Bestimmungen.
 S. MÃ¥nsson,
Fault handling in district heating substations - Experiences from the industry,
in 4th International Conference on Smart Energy Systems and 4th Generation District Heating, 2018.
 H. Gadd and S. Werner,
Achieving low return temperatures from district heating substations,
Appl. Energy, vol. 136, pp. 59â€“67, 2014.
 Swedenergy (Energi FÃ¶retagen Sverige),
District Heating Substations - Design and Installation - Technical regulations,
District Heating Controller RVD120 RVD 140.
Siemens Switzerland Ltd., 2018.
 Wien Energie,
Technische Richtlinie TR-ZT - Zentrale TrinkwassererwÃ¤rmung.
 Wien Energie,
Technische Richtlinie TR-SZT - Schemen, Zeichnungen, Tabellen.
 Energie Graz,
Technische Anschlussbedingungen FernwÃ¤rme.
Energie Graz GmbH & Co KG, Graz, 2011.
Bedienungs- und Wartungsanleitung - FernwÃ¤rme-Kompaktstation fÃ¼r indirekten Anschluss an FernwÃ¤rmenetze.
 O. GuÃ°mundsson,
Detection of fouling in heat exchangers,
University of Iceland, 2008.
 K. Yliniemi,
Fault detection in district heating substations,
LuleÃ¥tekniska universitet, 2005.
 S. Frederiksen and S. Werner, District Heating and Cooling. Lund: Studentlitteratur AB, 2013.
Datasheet - Strap-on temperature sensor QAD2.
EN 14336. Heating systems in buildings â€“ Installation and commissioning of water based heating systems.
 M. Kaufmann, F. Jochum, and W. Schlader,
Endbericht Pilotprojekt â€˜Hydraulischer Abgleich in groÃŸen GebÃ¤uden,â€™
Hydraulics in building systems.
 aqotec GmbH,
Ratgeber zur Optimierung der SekundÃ¤ranlage beim FernwÃ¤rmeabnehmer.
aqotec GmbH, WeiÃŸenkirchen, 2011.
 P. Lauenburg and J. Wollerstrand,
Adaptive control of radiator systems for a lowest possible district heating return temperature,
Energy Build., vol. 72, pp. 132â€“140, 2014.
 Austrian Standards Institute,
Ã–NORM B 5019 - Hygienerelevante Planung, AusfÃ¼hrung, Betrieb, Ãœberwachung und Sanierung von zentralen Trinkwasser-ErwÃ¤rmungsanlagen.
Austrian Standards Institute, 2011.
 C. BÃ¤cker,
Planung, AusfÃ¼hrung und Dimensionierung von Trinkwasser-Installationen - Zirkulationssysteme und Hydraulik.
 M. Crane,
Individual apartment substation testing - Development of a test and initial results,
in The 15th International Symposium on District Heating and Cooling, 2016.
Instructions - Akva Lux VX - District heating substation for indirect heating and domestic hot water.
 M. Brand, A. Dalla Rosa, and S. Svendsen,
Energy-efficient and cost-effective in-house substations bypass for improving thermal and DHW (domestic hot water) comfort in bathrooms in low-energy buildings supplied by low-temperature district heating,
Energy, vol. 67, pp. 256â€“267, 2014.
Funded by the European Union's H2020 Programme under grant agreement 768936.
The sole responsibility for the content of this webpage lies with the authors. It does not necessarily reflect the opinion of the European Union. Neither the EASME nor the European Commission are responsible for any use that may be made of the information contained therein.
Senior Research Engineer
Center for Energy
AIT Austrian Institute of Technology GmbH
Giefinggasse 6 | 1210 Vienna | Austria
T +43 50550-6695 | M +43 664 2351901