Guide for auditing building installations to reduce high return temperatures

Contents

Introduction

The present practical guidelines have been developed within the TEMPO project, funded by the European Union's H2020 Programme under grant agreement 768936. They should guide technicians in performing audits on energy systems in building to identify, diagnose and correct components or systems behaving in a suboptimal way. The flowcharts offer a first to gradually narrow down issues, and thus refer the user to issue profiles corresponding to probable issues. Issue profiles provide more detailed information on specific types of issues, grouping together issues affecting given components and/or sharing similar mechanisms or symptoms. Links are also provided between issues and metrics which are related to them and might support diagnosis. General information on issues causing high return temperatures and references can be found in the last sections.

Flowcharts

The flowcharts show a proposed methodology for detection and diagnosis of issues leading to high return temperatures in building installations served by district heating. The flowcharts are intended to serve as a walkthrough. The findings for this procedure were on one side derived from an analysis of simulation results and on the other side from an extensive literature research. Each flowchart applies to a certain type of system. System types vary in the presence or absence of space heating, the presence or absence of domestic hot water preparation, the substation connection type (indirect or direct), and where applicable the type of space heating system (radiator or floor heating) and the type of hot water preparation (instantaneous or with accumulation).
Please click on the colored activity rectangles for more information on specific issues.


Flowchart for subsystem targetting


Flowchart for space heating subsystem


Flowchart for domestic hot water subsystem



Fault profiles



Insufficient heat exchanger capacity

Background
Heat exchangers are used in indirect substations to transfer heat between primary and secondary side. Lowered or insufficient heat transfer capacity leads to higher primary return temperatures.
Description
Insufficient heat transfer capacity may be due to undersizing during planning, fouling, or a wrong connection. Heat exchanger fouling is known to occur frequently [20]. The erroneous exchange of connection ports, resulting in parallel flow rather counter-flow heat exchange, can also lead to reduced heat capacity.
Diagnosis
Lower heat transfer coefficient causes greater difference between secondary return temperature and primary return temperature, and thus higher primary return temperatures than achievable. However, the differences are often small enough that diagnosis is not trivial, even where measurements on the secondary side are available. In the case of fouling, one may observe a gradual decrease of the heat transfer coefficient, but also sporadic increases due to scaling layers detaching from the heat exchanger surface [21].
Remediation
In the case of a fouled heat exchanger: clean heat exchanger. If necessary, flush system. For the cleaning of brazed plate heat exchangers, liquids with a given chemical composition are circulated through the plates (cleaning-in-place) [22].
  • In the case of a wrong connection: reconnect correctly.
  • In the case of an undersized heat exchanger: replace with correctly sized heat exchanger, if technically and economically viable.

More
Tags: heat exchanger, substation
Related metric: Flowrate-weighted difference between primary and secondary return temperature
More specific faults: Undersized heat exchanger, Incorrect connection of heat exchanger (parallel flow instead of counterflow), Fouling of heat exchanger, Incorrect heat exchanger position (horizontal instead of vertical)
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Heat exchanger control issues

Background
In substations with indirect connection, heat exchanger control is a fundamental aspect which can affect performance and return temperatures. Issues may be related to the actuator (see primary control valve issues) or to the control scheme itself. Control possibilities depend on the type of valve: mechanical valves are self-operated and typically provide proportional-type control (P-control), whereas mechanical valves usually implement PID control (proportional with integration and derivation) [22].
Description
Mechanical valves are typically restricted to P-control and do not enable weather-compensation. Wrongly-tuned PID parameters will typically result in suboptimal control, with impacts on return temperatures, secondary supply temperatures and equipment durability.
Diagnosis
Oscillations or static deviations from set point may point to control issues.
Remediation
Determine whether control issue is due to the actuator (primary control valve issues), sensors or with the control scheme as such. Remediation may involve adjusting control parameters, changing the control scheme and/or changing the valve and control (e.g. from mechanical to motorised control valves).
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Tags: controls, heat exchanger, substation
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Hydraulic imbalance

Background
Hydraulic balancing refers to the adjustment of flow resistances in such a way as to achieve the desired water flow rates in the entire hydronic system. The adjustable flow resistances can be manual balancing valves, which ought to be set appropriately (based on iterative methods, manual calculations or computer-aided methods). Hydraulic balancing may also be based on self-adjusting valves.
Description
Hydraulic imbalance is known to be a frequent issue [8], even though hydraulic balancing is required by installation and commissioning standard EN 14336 [24]. Hydraulic imbalance typically leads to over- and undersupply of different heat delivery components.
Diagnosis
Unwanted temperature differences in the building during the heating season are typical indices of hydraulic imbalance. Further possible signs of hydraulic imbalance are noise in radiator valves, indicating excessive differential pressure, and poor control behaviour of radiator valves with overshooting [25]. Higher supply temperatures set to compensate for local undersupply are also a frequent indirect consequence of hydraulic imbalance [25]. Measurements of mass flow rates may be useful in diagnosing hydraulic imbalance.
Remediation
Hydraulic balancing should be carried out by a professional. Indications on how to perform hydraulic balancing can be found in EN 14336 [24].
More
Tags: comfort, heating system
More specific faults: Insufficient hydraulic balancing
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Primary control valve issues

Background
Control valves are used to control the flow of water on the primary side of heat exchangers. Valve control is especially significant and challenging for domestic hot water and in small substations [22].
Description
Different reasons may lead to primary valves not controlling primary flow as desired, typically resulting in high primary flow rates and according higher primary return temperatures than required. Oversizing of the primary control valves is a frequent issue which typically results in the inability of the valve to maintain low flow rate conditions (also depending on valve rangeability). The valve may become oversized after a refurbishment lowering the energy demand. Wear - due to normal operation and potentially worsened by suboptimal control - may also affect valve performance and result valves not closing properly or in excessive hysteresis. The valve control range and the valve characteristic should be adequate for proper control. This depends on the expected minimum and maximum flow rates in the installation. For a large control range, pairs of two valves in parallel or split-range valves can be used [22].
Diagnosis
Primary flow rate measurements are instrumental in diagnosing primary control valve issues. Poor control may result from issues with the primary control valve. Residual flow rates when flow rate should be null point to a primary control valve issue. This is easy to detect when the control signal of the primary valve is logged. Otherwise, a look at the behaviour in the periods of low power may also point to issues of primary valves not closing tightly. Unstable control in particular at low flow rates may indicate an issue with the primary control valve.
Remediation
Check valve condition and sizing. Check that the valve control range is adequate for the expected minimum and maximum flow rates. If necessary, replace the valve.
More
Tags: controls, substation
Related metric: Correlation coefficient between primary valve position and primary mass flow rate
More specific faults: Oversized district heating connection, Oversized primary valve, Primary valve not controlled, Primary control valve not closing tightly
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Sensor faults

Background
Sensors are vital components of district heating systems. They are used for control and billing, as well as fault detection in some cases. All sensors have a limited accuracy, which means that measured values can never be said to coincide exactly with true physical values. However, some conditions may cause the difference between values measured by sensors and the values to be measured to exceed the normal accuracy range.
Description
Various kinds of sensors may be affected by various kinds of faults. Sensor faults include:
  • Total sensor failure or output stuck at a constant value
  • Sensor bias (offset constant over time)
  • Sensor placement should also be considered: for tap water temperature, the temperature sensor should not be located too far from the heat exchanger [22].
  • Sensor drift (deviation over time, linear or not)
  • Sensor scaling error (multiplicative fault)
  • Excessive noise
  • Excessive inertia (sluggish sensor)
These types of faults can also be combined, and/or happen intermittently.
Diagnosis
The most basic means of diagnosis of sensor faults is to read sensor values and decide if they seem reasonable. This should allow gross faults to be diagnosed, but not small biases or drift. Comparison measurements with additional (mobile and calibrated) sensors may allow sensor faults to be diagnosed. Logged measurements give additional possibilities for fault detection. Plausibility checks can be done for single sensors based on instantaneous values or trends (detecting stuck values), as well as for several sensors (looking at temperature difference between secondary and primary return or comparing energy balance calculated on primary and secondary side).
Remediation
Make sure the sensor is located at a correct place in the system. Sensor manufacturer documentations provide recommendations on sensor location (e.g. water should be well mixed) and information on sensor accuracy [23]. In the case of sensor failures due to wiring or electricity supply, ensure correct rewiring of the sensor. In the case of contact sensors gone out of place, the remediation measure is to fix them at the appropriate location. Some sensors can be calibrated. If calibration is not possible, faulty sensors have to be replaced.
More
Tags: sensors, controls
More specific faults: Faulty or misplaced outdoor temperature sensor, Faulty or misplaced water temperature sensor, Sluggish water temperature sensor, Inaccurate primary temperature sensor, Inaccurate flow meter
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Suboptimal heat curve

Background
Space heating system controls usually use weather compensation: the secondary supply temperature is modified according to the outdoor temperature in order to respond to different space heating loads. This is done with a heating curve, which specifies the secondary supply temperature setpoint as a (piecewise linear, decreasing) function of outdoor temperature. The selection of an optimal heating curve should depend on building and heating system characteristics as well as climate in different seasons, so that suboptimal heating curves are not uncommon.
Description
This issue may take on different forms:
  • In the simplest and worst case, there is no weather compensation and hence no heating curve.
  • The heating curve may be suboptimal. Excessively high or flat heating curves will result in high return temperatures (on secondary and primary sides).
  • Secondary supply temperature may be stuck at a certain value. If the value is low, comfort problems may appear soon. If the value is high (or if there is no heating curve, but the same design set point is used all the time), this may lead to relatively high return temperatures when outdoor temperatures are higher.
  • The outdoor temperature sensor may provide faulty values and make an otherwise good heating curve yield suboptimal outputs. If the sensor is affected by a bias or stuck at certain value, the effects may be similar to those of the two previous points.
  • A faulty timer may also result in similar issues regarding secondary supply temperature setpoint.
Diagnosis
Continuously high secondary supply temperatures can be a sign that a heating curve is not used in an optimal way. Check that a heating curve is used, and that it is parameterized correctly. Check outdoor temperature sensor values for plausibility.
Remediation
Adjust heating curve parameters if not appropriate. Correct sensor position if needed. If weather compensation is not implemented, consider implementing it.
More
Tags: controls, heating system
Related metrics: Correlation coefficient between secondary supply temperature and outdoor temperature, Mean secondary supply temperature
More specific faults: Heating curve too high, Lack of temporal temperature reset where it would be useful, Faulty heating controller timer
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Delivery component heat capacity

Background
The heat transfer performance of heat delivery components (typically radiators or floor heating) has an impact on secondary and primary return temperatures. Issues may derive from sizing, occupant behaviour or unsuitable installation. Thermal power output depends on (supply and return) water temperatures through the delivery component, room temperature and heat transfer area.
Description
Undersized delivery components require a higher volume flow and/or higher temperatures to deliver the same heat rate, leading to higher return temperatures. Undersizing of heat delivery components during planning is not frequent, but rightly sized heat delivery components may "become undersized" if supply temperatures are lowered. Aside from undersizing in planning, the heat output of radiators may be reduced by occupants placing furniture in front of radiators, by radiator niches or by radiator connection. The heat output of floor heating may be reduced for instance by carpets with excessive thermal resistance. Supply and return connections at the same height may result in heat output lower than the nominal heat output, which is determined with supply connection above and return connection below. On the other hand, oversizing of heat delivery components is frequent. Also, heat delivery components may become oversized with respect to a decreased heat demand after thermal renovation. Oversized heat delivery components in themselves are not the cause for high return temperatures. Rather, they give the possibility to use lower supply temperatures and obtain lower return temperatures [28]. Also, the oversizing of heat delivery components may be associated with the oversizing of other components (valves, pumps) which may lead to higher return temperatures.
Diagnosis
Insufficient delivery component capacity will result in secondary return temperatures and secondary flow rates higher than optimal. This kind of issue is presumably difficult or even impossible to diagnose for reasons of access to the affected components (see remarks).
Remediation
If possible, make sure the heat transfer performance of delivery components is not adversely affected by furniture or incorrect connections.
More
Tags: comfort, occupant behaviour, heating system
Related metric: Flowrate-weighted mean secondary temperature difference
More specific faults: Undersizing of heat delivery components, Oversizing of heat delivery components, Use of only few radiators by occupants, Radiators blocked by furniture, Radiator connection causing reduced heat output, Air in heating system , Fouling in space heating system
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Control equipment

Background
Control equipment refers to physical components used for control, as opposed to control software. This includes valves, pumps and actuators.
Description
Different issues may affect control equipment. Please refer to the individual pieces of equipment for more detailed information.
More
Tags: controls
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Heating system unsuitability

Background
Issues may be related to the heating system itself rather than to individual components.
Description
System-level issues include unsuitable types of hydronic circuits, hydraulic separators and unnecessary bypasses. Bypassing typically leads to high return temperatures and should be avoided in systems served by district heating. Hydraulic separators should also be avoided, as they lead to increased return temperatures when the flow rate is higher on the supply side than on the demand side, in which case they are comparable to a bypass. Diverting circuits and injection circuits with 3-way valves return uncooled water. As a consequence, they yield high return temperatures and should be avoided. Suitable circuits suitable for systems served by district heating are throttling circuits, injection circuits with 2-way valves and mixing circuits [26], [27].
Diagnosis
Unlike other issues, an unsuitable system may be identified just by looking at an installation schema.
Remediation
An unsuitable circuit can be remediated by changing the main pump to a speed-controlled pump and implementing an appropriate control for the pump [27].
More
Tags: bypass, heating system
Related metric: Flowrate-weighted mean secondary temperature difference
More specific faults: Unsuitable type of hydronic circuit, Non-adjusted one-pipe distribution, Hydraulic separator, Redundant bypass
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Excessive recirculation flow

Background
Hot water recirculation uses additional pipes that form a closed circuit in the tap water system and allow a minimum temperature to be maintained in the system at all times [29]. It is used for reasons of comfort (keeping hot water quickly available) and hygiene (keeping hot water temperature above the temperature range favourable for legionella bacteria. Large systems (typically with a pipe volume over 3 litres) are required to have recirculation [30].
Description
An excessive recirculation flow has an adverse impact on return temperatures. Some district heating providers specify a maximum limit to recirculation flow rate (e.g. 20 % of loading flow rate [17]). The recirculation flow rate should be chosen such that temperature of water at the end of the recirculation pipe (at the coldest) does not fall under a certain limit (typically 50 °C [4] or 5 K under the hot water temperature at the beginning of the recirculation pipe). Note that this temperature at the end of the recirculation pipe depends on the insulation of the recirculation pipe: better insulation means lower thermal losses and leads to lower flow requirements to maintain the same temperature limits.
Diagnosis
Water temperature at the end of the recirculation pipe significantly above the minimum limit is a sign of excessive recirculation flow rate.
Remediation
If water temperature at the end of the recirculation pipe is significantly above the limit, adjust the controls to reduce recirculation flow. If recirculation pipes are poorly insulated, insulating them may be desirable.
More
Tags: domestic hot water
More specific faults: Unnecessary hot water circulation, Excessively high flow rate of hot water circulation, Insufficient circulation pipe insulation
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Inadequate tank outlet or return lance

Background
Primary return water from buffer tanks should be taken at a level where temperature is as low as possible. Where this is not the case, high return temperatures from the tank may ensue.
Description
Primary return water from buffer tanks should be taken at a level where temperature is as low as possible, typically at the bottom of the tank. Where this is not the case, high return temperatures from the tank may ensue. If sensed bottom tank temperature plays a role in control and the level of the return lance is higher, it may happen that the bottom of the tank remains cold and the tank keeps charging a high proportion of time.
Diagnosis
A comparison of lower buffer temperature with return temperature should allow this type of fault to be identified. Both temperatures should have similar levels, at least when water is flowing from the tank.
Remediation
Correct the position of the outlet lance or, if the lance is too short, replace it with a lance of adequate length.
More
Tags: storage
Related metric: Mean difference between primary return and bottom tank temperature
More specific faults: Inadequate return lance
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Suboptimal pump control

Background
Pumps are essential components of modern heating systems. Control possibilities depend on the pump component (and its motor): constant-speed pumps may only be subject to on/off control, whereas the speed of other pumps may be varied at discrete levels or continuously.
Description
Pump control may be suboptimal, faulty, or in the worst case even lacking. The control parameters of circulation pumps or pump groups may not be set correctly.
Diagnosis
A first step towards diagnosis of suboptimal pump control is to gather data on the installed pumps. Secondary flow rate measurements would allow pump control faults to be easily detected, but they are expensive and rarely available. Pump operation can be assessed more quantitatively based on sound and pump housing temperature.
Remediation
If necessary and possible, adjust pump control. In the case of constant-speed pumps, replacement may be the only remediation possibility.
More
Tags: controls
More specific faults: Lacking pump control, Faulty pump control
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Malfunctioning thermostatic radiator valves

Background
The role of a thermostatic radiator valve (TRV) is to measure room temperature with an in-built sensor and keep it at a constant level by adjusting water flow through the radiator. A TRV consists of a valve and a thermostat combined together. The thermostat values in some TRVs may be limited to a certain range narrower than the full range (e.g. 7 to 26°C ).
Description
Several problems may affect TRVs:
  • Especially in old systems, thermostatic valves might not be present at all.
  • Radiator valves may be stuck in one position.
  • Radiator valves may leak.
  • Radiator valves can be operated by occupants (by turning the thermostat handle), and occupant settings may not be optimal. Closed valve positions are rather unfavourable.
Diagnosis
If possible, check for the presence and good function of thermostatic valves.
Remediation
If thermostatic valves are not present, consider installing some. To fix a stuck thermostatic valve, the thermostat head should be removed. If occupant behaviour appears to be unfavourable, consider ways of raising awareness of the issue.
More
Tags: sensors, controls, heating system
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Tap water bypass

Background
Standby or idle heat functions use a bypass flow at times without heat demand to ensure a short time delay when hot water is required again. This results in an increase of return temperature, which can become significant if the corresponding mass flow is high. Excessive bypass flow rates in standby operation have been identified as a significant cause of high return temperatures in some compact substations [31].
Description
Bypass flow can be continuous or intermittent [31]. It can be controlled by a bypass thermostat, giving the possibility to set different temperatures [32]. A defective or wrongly adjusted bypass thermostat can result in high return temperatures [32].
Diagnosis
The measurement of flow rate and energy consumption in standby operation may allow this issue to be diagnosed. A high return temperature in standby mode combined with a cold heat exchanger would indicate a fault in the bypass thermostat (whereas a hot heat exchanger would indicate an issue with the controller) [32].
Remediation
If standby operation uses a bypass thermostat, make sure the corresponding temperature setting is adequate, and ensure the bypass thermostat works properly. If not, fix accordingly. Standby operation is sometimes not adjustable in compact substations [31]. It has been suggested to redirect bypass flow into bathroom floor heating to reduce return temperatures while improving thermal comfort [33].
More
Tags: substation, domestic hot water, bypass
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Storage tank temperature mixing

Background
Storage tanks can be used for hot water preparation, which allows heat exchanger and service pipes to individual substations to be sized smaller [22].
Description
The use of hot water accumulation generally tends to yield higher return temperatures than in systems without accumulation [4]. However, with appropriate tank stratification, the increase in return temperatures should be very limited. On the other hand, temperature mixing in the tank leads to unnecessarily high return temperatures. Multiple factors can potentially interfere with tank stratification:
  • High mass flow (see excessive recirculation flow) and accordingly high velocities can lead to tank temperature mixing.
  • Tank size plays a role, as do shape and position. The storage tank should be in vertical position for optimal stratification.
  • The design of internal heat exchangers and obstacles in storage tanks also has an impact on stratification.
  • The position of internal heat exchangers and/or inlets has an impact on tank temperature stratification. An inlet position for recirculation flow placed too low may for instance impair stratification.
  • Lacking insulation can lead to more frequent reheating and suboptimal stratification.
Diagnosis
Water temperature measurements at different points in the storage tank is the most obvious way to investigate stratification. Measuring the temperature of water coming from the tank may also help stratification to be checked.
Remediation
Make sure the tank is vertical and well-insulated, lower inlet mass flow if needed.
More
Tags: storage, domestic hot water
Related metric: Mean difference between top and bottom tank temperature
More specific faults: High mixing rate in storage tank, Excessive heat losses in storage tank, Faulty tank loading pump control, Constant flow tank loading pump
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Inadequate tank control

Background
Tank temperature stratification depends on loading and unloading times and powers.
Description
Inadequate tank control can affect buffer tanks for heating or accumulation tanks for domestic hot water. It can for instance take the form of excessive loading, where the tank continues being loaded although it is already full of hot water. This results in high bottom tank temperatures and consequently high return temperatures from the tank.
Diagnosis
This type of issue may be diagnosed by looking at tank bottom temperatures or directly the control parameters impacting tank loading (typically a switch-off temperature).
Remediation
Adjust the control parameters impacting tank loading.
More
Tags: storage, controls
Related metrics: Mean difference between top and bottom tank temperature, Mean tank bottom temperature
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Specific faults

Undersizing of heat delivery components
Problem description: Undersized delivery components require a higher volume flow and/or higher temperatures to deliver the same heat rate, leading to higher return temperatures. Undersizing of heat delivery components during planning is not frequent, but rightly sized heat delivery components may "become undersized" if supply temperatures are lowered.
Detection: Visual inspection of plant or plans and sizing calculation.
Measure: Replace or add delivery components (expensive), or increase supply temperature as much as required.

Related fault profile: Delivery component heat capacity
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Oversizing of heat delivery components
Problem description: Oversizing of heat delivery components is frequent. Also, heat delivery components may become oversized with respect to a decreased heat demand after thermal renovation. Oversized heat delivery components in themselves are not the cause for high return temperatures. Rather, they give the possibility to use lower supply temperatures and obtain lower return temperatures. On the other hand, the oversizing of heat delivery components may be associated with the oversizing of other components (valves, pumps) which may lead to higher return temperatures.
Detection: Visual inspection of plant or plans and sizing calculation.
Measure: Check possibility of decreasing supply temperature.

Related fault profile: Delivery component heat capacity
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Use of only few radiators by occupants
Problem description: Closing several thermostatic valves does not necessarily result in lower energy consumption, as believed by some occupants. If a reduced number of radiators have to compensate for closed radiators, higher supply temperatures may be required. Thermal discomfort may also occur.
Detection: On-site inspections or surveys on occupant heating behavior.
Measure: Occupant information.

Related fault profile: Delivery component heat capacity
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Insufficient hydraulic balancing
Problem description: Hydraulic balancing is not always (correctly) conducted. This may result in overheating in some rooms and underheating in others. The interdependencies between system and occupant behaviour may be complex. Put simply, following occupant complaints, system modifications compensating for underheating in the coldest rooms (e.g. increase in supply temperature) may lead to an increase in energy consumption.
Detection: Temperature measurements in several rooms. Occupant complaints for over- and underheating.
Measure: Conduct hydraulic balancing.

Related fault profile: Hydraulic imbalance
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Unsuitable type of hydronic circuit
Problem description: Diverting circuits and injection circuits with 3-way valves return uncooled water. Thus, they yield high return temperatures and should be avoided.
Detection: Visual inspection of plant or plans.
Measure: Replan and change hydronic circuit.

Related fault profile: Heating system unsuitability
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Non-adjusted one-pipe distribution
Problem description: One-pipe heat distribution is difficult to adjust, as more distant delivery components receive colder water and should accordingly be sized larger. The impact of non-adjusted one-pipe distributions is comparable to that of hydraulic imbalances.
Detection: Visual inspection of plant or plans.
Measure: Adjust or (expensive) install two-pipe distribution instead.

Related fault profile: Heating system unsuitability
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Hydraulic separator
Problem description: Hydraulic separators should be avoided in systems served by district heating, as they lead to increased return temperatures when the flow rate is higher on the supply side than on the demand side, in which case they are comparable to a bypass.
Detection: Visual inspection of plant or plans.
Measure: Replace hydraulic separator.

Related fault profile: Heating system unsuitability
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Redundant bypass
Problem description: Bypassing typically leads to high return temperatures. See also unsuitable type of hydronic circuit.
Detection: Visual inspection of plant or plans.
Measure: Remove bypass.

Related fault profile: Heating system unsuitability
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Incorrect ventilation behavior (e.g. continuously tilted windows)
Problem description: Intermittent full ventilation is generally more efficient than continuous tilt ventilation. The impact of the latter on return temperatures is not straightforward, as it depends on how room thermostats are affected by air movement.
Detection: On-site inspections or surveys on occupant heating behavior.
Measure: Inform building occupants.

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Radiators blocked by furniture
Problem description: Furniture blocking radiators results in lower heat transfer capacity. As in the case of undersized radiators, this means higher flow rates or temperatures are required to keep the rooms at the same temperature. Installation in radiator niches also results in reduced heat output, which should be taken into account during planning.
Detection: On-site inspections or surveys on occupant heating behavior.
Measure: Inform building occupants.

Related fault profile: Delivery component heat capacity
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Radiator connection causing reduced heat output
Problem description: The connection type has an influence on heat output. Supply and return connections at the same height may result in heat output lower than the nominal heat output, which is determined with supply connection above and return connection below.
Detection: On-site inspection in heated spaces.
Measure: Reconnect radiators.

Related fault profile: Delivery component heat capacity
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Incorrect control options (manual instead of automatic operation)
Problem description: Control stuck to manual position may result in unnecessary flow and high return temperatures. May affect pumps, changeover summer/winter etc.
Detection: Visual inspection of control system.
Measure: Check control parameters and set correctly.

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Heating curve too high
Problem description: A secondary heat curve set too high yields high return temperatures. This is especially the case if it is combined with other control issues leading to unnecessary secondary flow. A suboptimal heat curve may also be a consequence and symptom of other issues (e. g. reduced heat delivery capacity).
Detection: Visual inspection of control system.
Measure: Reset heating curve as low as possible while allowing heat demand to be covered.

Related fault profile: Suboptimal heat curve
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Lack of temporal temperature reset where it would be useful
Problem description: See "heat curve too high", as not resetting temperature at times of lower demand may result in temporarily too high heat curve. Temperature setback can be assumed to be useful when a building is not used for more than one day (e.g. office buildings on weekends). On the other hand, the usefulness of night-time temperature setback may depend on the building and system in which its is implemented and is often subject to debates.
Detection: Visual inspection of control system.
Measure: Implement temperature reset if deemed useful.

Related fault profile: Suboptimal heat curve
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Faulty heating controller timer
Problem description: A faulty timer can lead to temperature reset at the wrong times, resulting in comfort issues in some cases, and high return temperatures (issue "Lack of useful temporal temperature reset") in other cases.
Detection: Visual inspection of control system.
Measure: Reset controller timer.

Related fault profile: Suboptimal heat curve
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Control of substation and heating system not harmonised
Problem description: If substation control and heating control are not harmonised with one another, it may happen that primary flow is active while the secondary system does not demand heat.
Detection: Visual inspection of control system.
Measure: Harmonise control of substation and heating system

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Lacking pump control
Problem description: The pump components by themselves may not be controllable.
Detection: Visual inspection of plant or plans.
Measure: Exchange pump with variable speed pump

Related fault profile: Suboptimal pump control
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Faulty pump control
Problem description: The control parameters of circulation pumps or pump groups may not be set correctly. In particular, in pumps with multiple (e. g. 2 or 3) stages, a suboptimal stage can often be set manually. A fixed high stage may lead to excessive secondary mass flow and high secondary return temperatures. A fixed low stage may lead to insufficient heat delivery and comfort issues at times of high heat demand, while causing low secondary return temperatures.
Measure: Set pump control parameters correctly

Related fault profile: Suboptimal pump control
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Air in heating system
Problem description: Air in the secondary heating system may cause a reduced heat output.
Detection: Air in the system may also cause noise.
Measure: Vent the system.

Related fault profile: Delivery component heat capacity
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Fouling in space heating system
Problem description: Dirt in the secondary heating system may lead to reduced heat transfer, and may be linked to fouling in heat exchanger.
Detection: Full strainer or filter.
Measure: Flush system and clean affected components.

Related fault profile: Delivery component heat capacity
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Rapid heat-up
Problem description: In buildings with intermittent use or night temperature setback, rapid heat-up may result in short power peaks and, especially in combination with suboptimal control, high return temperatures.
Detection: Power peaks.
Measure: Adapt control strategy and/or inform users about the consequences of rapid heat-up.

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Heating water flowing in heating coil despite ventilation off
Problem description: Heating water flows uncooled in return, either because of faulty control, or because of the lack of control mechanism.
Detection: Checking hot water flow through heating coil.
Measure: Implement correct control setting off water flow when ventilation is off.

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Faulty backflow preventer
Problem description: Faulty or mispositioned backflow preventers may lead to unwanted water circulation.
Detection: Measurements.
Measure: Ensure backflow preventers are located where needed to prevent unwanted water circulation.

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Excessive bypass flow rate in standby operation
Problem description: Standby functions use a bypass flow at times without heat demand to ensure a short time delay when hot water is required again. This results in an increase of return temperature, which can become significant if the corresponding mass flow is high.
Detection: Measurement of flow rate in standby operation.
Measure: Implement more efficient standby operation.

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High mixing rate in storage tank
Problem description: A lack of stratification in the tap water storage tank may be caused by a horizontal positioning of the tank or internal heat exchanger. It leads to a more frequent heating of the tank, higher temperatures at the bottom of the tank and higher return temperatures.
Detection: Measurement of water temperature at the bottom of the tank.
Measure: Determine cause for excessive mixing (flow rates, heat exchanger) and eliminate it.

Related fault profile: Storage tank temperature mixing
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Excessive heat losses in storage tank
Problem description: Excessive heat losses in storage tank, caused by insufficient insulation, may lead to frequent reheating and consequently to increased return temperatures.
Detection: Visual inspection of tank.
Measure: Insulate storage tank.

Related fault profile: Storage tank temperature mixing
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Unnecessary hot water circulation
Problem description: Hot water recirculation may not be required if the system is small enough, in which case its installation would cause unnecessary energy consumption and increase of return temperatures.
Detection: Visual inspection of plans and calculation.
Measure: Reduce circulation flow rate to minimum required.

Related fault profile: Excessive recirculation flow
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Excessively high flow rate of hot water circulation
Problem description: A circulation flow rate higher than needed leads to high return temperatures.
Detection: Measurement of water temperature at the end of the circulation pipe.
Measure: Reduce circulation flow rate to minimum required.

Related fault profile: Excessive recirculation flow
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Insufficient circulation pipe insulation
Problem description: Insufficient pipe insulation may lead to increased heat losses, but also to unnecessary start due to water cooling in the absence of heat demand.
Detection: Visual inspection of pipes.
Measure: Insulate circulation pipe.

Related fault profile: Excessive recirculation flow
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Faulty tank loading pump control
Problem description: Loading pump working more often than needed or with higher flow rates than needed impairs tank stratification and leads to high return temperatures.
Measure: Configure pump control correctly

Related fault profile: Storage tank temperature mixing
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Constant flow tank loading pump
Problem description: Constant flow in tank loading pump impairs tank stratification and leads to high return temperatures and oscillating behavior.
Measure: Implement variable speed control

Related fault profile: Storage tank temperature mixing
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Wrong return temperature limiter setting
Problem description: Thermostatic return temperature limiters can be installed to ensure return temperature is cooled at least to a certain limit. If this is set too low, may lead to excessive oscillations or issues in comfort or hygiene. If set too high, may be useless.
Detection: Check temperature limiter setting.
Measure: Set temperature limiter correctly.

Back to overview

Unstable tap water temperature control
Problem description: Heat exchange control for instantaneous domestic water heaters is particularly challenging: temperature sensors placed too far away from the heat exchanger or stagnation because of unfavorable flow conditions at the sensor location may cause a time lag and in the worst case unstable control (Frederiksen and Werner 2013).
Detection: Unstable control
Measure: Place temperature sensor for supply domestic hot water in favorable location close to heat exchanger.

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Tap water set point temperature set too high
Problem description: A tap water set point temperature set higher than necessary will lead to higher return temperatures.
Detection: High tap water temperature, high return temperature during tapping.
Measure: Set tap water set point temperature to lower value.

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Undersized heat exchanger
Problem description: Undersized heat exchanger leads to higher primary return temperatures. Same effect as fouled heat exchanger (except heat transfer capacity is limited from the start) or incorrect connection of heat exchanger.
Measure: Replace heat exchanger.

Related fault profile: Insufficient heat exchanger capacity
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Incorrect connection of heat exchanger (parallel flow instead of counterflow)
Problem description: Connecting a heat exchanger in parallel flow instead of counterflow leads to a reduction of heat transfer.
Detection: Visual inspection.
Measure: Reconnect heat exchanger

Related fault profile: Insufficient heat exchanger capacity
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Fouling of heat exchanger
Problem description: Fouling in heat exchangers causes a decrease in heat transfer and higher return temperatures.
Detection: Accurate measurements of temperature differences, or measurements of pressure drop, or visual inspection of heat exchanger.
Measure: Clean heat exchanger.

Related fault profile: Insufficient heat exchanger capacity
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Leaking heat exchanger
Problem description: A leaking heat exchanger may result in primary water flowing directly into the secondary system. Commonly used plate heat exchangers are found to be very reliable, but cracks may develop in heat exchangers for DHW preparation because of frequent thermal cycling (Frederiksen and Werner 2013).
Measure: Repair or replace heat exchanger.

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Incorrect heat exchanger position (horizontal instead of vertical)
Problem description: Incorrect heat exchanger position may lead to dirt deposition.
Detection: Visual inspection.
Measure: Position heat exchanger correctly.

Related fault profile: Insufficient heat exchanger capacity
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Oversized district heating connection
Problem description: Generally oversized district heating connection with regard to actual demand can lead to high return temperatures because of poor controllability (see "Oversized primary valve") and excessively high flow rates.
Detection: Load calculation.
Measure: Implement primary flow reduction, replace primary control valve…

Related fault profile: Primary control valve issues
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Oversized primary valve
Problem description: Oversized valves tend to lead to poor control. The controllability of the valve also depends on the rangeability of the valve (ratio of maximum to minimum controllable flow): a valve with insufficient rangeability may lead to unstable control for low flow rates.
Detection: Oscillations in secondary water temperature. Load calculation.
Measure: Replace valve.

Related fault profile: Primary control valve issues
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Primary valve not controlled
Problem description: Faulty actuator or lack of control of primary valve leads to permanent flow through heat exchanger.
Measure: Repair actuator or implement control.

Related fault profile: Primary control valve issues
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Poor control scheme (e.g. P-controllers)
Problem description: Control can be problematic in different ways: unstable, or too sluggish etc. Jumpy behavior.
Detection: Measurement of oscillations in secondary supply temperature or primary mass flow.
Measure: Implement better control.

Back to overview

Primary control valve not closing tightly
Problem description: A primary valve not closing tightly, for instance because of a faulty actuator or low rangeability, will cause a flow on the primary water even when it is not necessary, leading to high return temperatures in these cases. This issue has the highest chances of being detected at times of low or zero demand, where its manifests itself as a "residual flow".
Measure: Repair or replace valve or actuator.

Related fault profile: Primary control valve issues
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Faulty or misplaced outdoor temperature sensor
Problem description: For instance, an outdoor temperature sensor exposed to direct solar radiation will not yield reliable values.
Detection: Excessive daily variations in recorded outdoor temperature.

Related fault profile: Sensor faults
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Faulty or misplaced water temperature sensor
Problem description: Water temperature sensors at several points can be affected by various faults, depending on the type of sensor. Contact sensors can for instance get out of place.
Detection: In extreme cases, unplausible sensed values. Test with other sensor.

Related fault profile: Sensor faults
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Incorrectly used control functions (e.g. night or holiday setback)
Problem description: Incorrect temperature setback may lead either to comfort issues or higher return temperatures

Back to overview

Sluggish water temperature sensor
Problem description: A sluggish temperature sensor decreases controllability. For tap water, it is recommended to use temperature sensors with a time constant under 2 seconds. Time lag in sensing may also be caused by a position of the sensor too far away from the heat exchanger.
Detection: Poor controllability may be a symptom.

Related fault profile: Sensor faults
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Inaccurate primary temperature sensor
Problem description: For common systems, inaccuracies in primary temperature measurements should not have any impact on real return temperatures, but rather on billing and data analysis.

Related fault profile: Sensor faults
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Inaccurate flow meter
Problem description: Inaccuracies in flow meters are frequent. They should not have any impact on return temperatures, but rather on billing and data analysis.

Related fault profile: Sensor faults
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Inadequate return lance
Problem description: Primary return water from buffer tanks should be taken at a level where temperature is as low as possible, typically at the bottom of the tank. Where this is not the case, high return temperatures from the tank may ensue. If sensed bottom tank temperature plays a role in control and the level of the return lance is higher, it may happen that the bottom of the tank remains cold and the tank keeps charging a high proportion of time.

Related fault profile: Inadequate tank outlet or return lance
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Issue categories


Sensors

Heat exchanger

Domestic hot water

Heating system

Bypass

Controls

Substation

Occupant behaviour

Storage

Comfort

Metrics

Mean difference between top and bottom tank temperature

Difference between average value of top tank temperature and bottom tank temperature. A low value points to a lack of temperature stratification.
Related faults: Storage tank temperature mixing, Inadequate tank control

Mean tank bottom temperature

Average of bottom tank temperature
Related faults: Inadequate tank control

Flowrate-weighted mean secondary temperature difference

A low value of average secondary temperature difference points to an issue in the secondary heating system, such as a lack delivery component capacity or a secondary bypass.
Related faults: Delivery component heat capacity, Heating system unsuitability

Correlation coefficient between secondary supply temperature and outdoor temperature

Pearson correlation coefficient between secondary supply temperature and outdoor temperature. A typical heating curve would yield a negative value like -0.5. A value close to zero would point to a lack of temperature reset.
Related faults: Suboptimal heat curve

Mean secondary supply temperature

Mean secondary supply temperature
Related faults: Suboptimal heat curve

Flowrate-weighted difference between primary and secondary return temperature

The difference between primary and secondary return temperature (in German Graedigkeit) is a common indicator of heat exchanger performance. High values of this indicator should be avoided. A typical limit value is 5 K.
Related faults: Insufficient heat exchanger capacity

Correlation coefficient between primary valve position and primary mass flow rate

Pearson correlation coefficient between primary valve position and primary mass flow rate. A value of 1 or close to it is expected for normal operation. A low value (e. g. below 0.8) may point to an issue with the valve.
Related faults: Primary control valve issues

Mean difference between primary return and bottom tank temperature

Mean difference between primary return and bottom tank temperature. Typically, primary return water should be extracted from the bottom of the tank, so that this difference indicator should be close to 0. Large positive values typically point to an issue with the tank primary outlet/return lance.
Related faults: Inadequate tank outlet or return lance

Supplied energy

Supplied energy especially depends on heat demand for space heating and domestic hot water, but also on system efficiency. Particularly high or low values of supplied energy may result from some issues, but interpreting these requires consideration of a variety of boundary conditions which may change across buildings and across time.

Mean primary return temperature

The average value of primary return temperature is a straightforward indicator when addressing the issue of return temperature in district heating networks. However, it may not be as informative as the flowrate-weighted average of primary return temperature, as it gives equal weight to all measurements, whether the substation is active or not.

Flowrate-weighted mean primary return temperature

The flowrate-weighted average value of primary return temperature is a key indicator for return temperature in district heating networks. Weighting by flowrate values allows the impact on the network to be better taken into account, by ignoring times where the substation is not active and giving more weight to times of high flowrate.

General information

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:

Audit steps

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 [11]. Regular inspections of district heating substations are recommended ([4] 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 [12]. 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 [13]. 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 [13]. Complaints from building occupants are another possible trigger for audits.

Audit preparation

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: 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.

Audit execution

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 [4] and the manuals of some prefabricated substations [5], [19].
Example checklist:
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.

Audit documentation

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:

References

[1] H. Lund et al., 4th Generation District Heating (4GDH): Integrating smart thermal grids into future sustainable energy systems, Energy, vol. 68, pp. 1–11, 2014.
[2] Wien Energie, Technische Richtlinie TR-HS - Hausstation sekundär. 2009.
[3] H. Zinko et al., Improvement of operational temperature differences in district heating systems, 2005.
[4] Euroheat & Power, Guidelines for District Heating Substations. 2008.
[5] Danfoss, District Heating Substations - Operating Guide. 2017.
[6] 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. 2010.
[7] A. Bres, C. Johansson, R. Geyer, P. Leoni, and J. Sjögren, Coupled Building and System Simulations for Detection and Diagnosis of High District Heating Return Temperatures, in Building Simulation 2019, 2019.
[8] R. Geyer, Projekt heat_portfolio (FFG-Nr. 848849). Deliverable D6.1 & D6.2. Katalog zur Implementierung der nutzerseitigen Maßnahmen. Szenarien unterschiedlicher Durchdringung der Maßnahmen als Basis für die Simulation in AP7. 2018.
[9] AIT Austrian Institute of Technology; and AEE Intec, Fehlerursachen Dokumentation - Internal document for FFG project T2LowEx. 2018.
[10] A. Dexter and J. Pakanen, Demonstrating Automated Fault Detection and Diagnosis Methods in Real Buildings, 2001.
[11] Wien Energie, Technische Richtlinie Fernwärme - Leitfaden allgemeine Bestimmungen. 2009.
[12] 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.
[13] H. Gadd and S. Werner, Achieving low return temperatures from district heating substations, Appl. Energy, vol. 136, pp. 59–67, 2014.
[14] Swedenergy (Energi Företagen Sverige), District Heating Substations - Design and Installation - Technical regulations, 2016.
[15] Siemens, District Heating Controller RVD120 RVD 140. Siemens Switzerland Ltd., 2018.
[16] Wien Energie, Technische Richtlinie TR-ZT - Zentrale Trinkwassererwärmung. Wien, 2009.
[17] Wien Energie, Technische Richtlinie TR-SZT - Schemen, Zeichnungen, Tabellen. 2009.
[18] Energie Graz, Technische Anschlussbedingungen Fernwärme. Energie Graz GmbH & Co KG, Graz, 2011.
[19] Aqotec, Bedienungs- und Wartungsanleitung - Fernwärme-Kompaktstation für indirekten Anschluss an Fernwärmenetze. 2014.
[20] O. Guðmundsson, Detection of fouling in heat exchangers, University of Iceland, 2008.
[21] K. Yliniemi, Fault detection in district heating substations, Luleåtekniska universitet, 2005.
[22] S. Frederiksen and S. Werner, District Heating and Cooling. Lund: Studentlitteratur AB, 2013.
[23] Siemens, Datasheet - Strap-on temperature sensor QAD2. 2017.
[24] CEN, EN 14336. Heating systems in buildings – Installation and commissioning of water based heating systems. 2004.
[25] M. Kaufmann, F. Jochum, and W. Schlader, Endbericht Pilotprojekt ‘Hydraulischer Abgleich in großen Gebäuden,’ Dornbirn, 2017.
[26] Siemens, Hydraulics in building systems. 2017.
[27] aqotec GmbH, Ratgeber zur Optimierung der Sekundäranlage beim Fernwärmeabnehmer. aqotec GmbH, Weißenkirchen, 2011.
[28] P. Lauenburg and J. Wollerstrand, Adaptive control of radiator systems for a lowest possible district heating return temperature, Energy Build., vol. 72, pp. 132–140, 2014.
[29] Austrian Standards Institute, ÖNORM B 5019 - Hygienerelevante Planung, Ausführung, Betrieb, Überwachung und Sanierung von zentralen Trinkwasser-Erwärmungsanlagen. Austrian Standards Institute, 2011.
[30] C. Bäcker, Planung, Ausführung und Dimensionierung von Trinkwasser-Installationen - Zirkulationssysteme und Hydraulik. Graz, 2019.
[31] M. Crane, Individual apartment substation testing - Development of a test and initial results, in The 15th International Symposium on District Heating and Cooling, 2016.
[32] Danfoss, Instructions - Akva Lux VX - District heating substation for indirect heating and domestic hot water. Danfoss, 2005.
[33] 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. 256–267, 2014.

Acknowledgement

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.


Contact

Contact person:
RALF-ROMAN SCHMIDT
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
ralf-roman.schmidt@ait.ac.at | https://www.ait.ac.at/