In city areas with low-temperature district heating: RETURN TEMPERATURE OPTIMIZATION OF RADIATORS IN MULTI-FAMILY HOUSES

For the district heating (DH) network to become more energy efficient in the future, it is relevant to improve the DH technology further. By lowering the temperatures, the production of heat and surplus heat is available from an even greater amount of variable energy sources, and the renewable sources and network become even more efficient. When the DH temperatures are lowered, it is, however, essential to ensure that the thermal comfort in the buildings is maintained.

By Ida Bach Sørensen, Project manager, Damgaard Consulting Engineers

The main barrier to lower the temperature is a lack of knowledge on how to implement this in existing areas without compromising thermal comfort in the buildings and a lack of knowledge on the use of low-temperature DH in buildings other than single-family houses. Former projects have aimed at single-family houses, mainly with floor heating. Until now, no one has solved the challenge in most existing homes, i.e., multi-family houses with radiators. Low-temperature DH is challenging in the existing building stock with radiators which requires a higher supply temperature than floor heating.

Another massive problem in multi-family houses is a lack of hydronic balancing in the building, which leads to poor distribution of heat between all radiators on all floors in multistorey buildings. Today mechanical balancing valves are on the market that can be adjusted to control the proper flow, but they have the disadvantage that their (manual preset) settings are adjusted to cope with the coldest day of the year – which seldom occurs. This leads to non-optimal balancing in partial load situations most of the year.

This is due to the lack of technology to cope with this segment, and not due to the relevance: Far the biggest part of living space, and most of the energy spent for space heating is consumed in cities with older multi-family buildings equipped with radiators.

Field testing

This project has investigated how the control of a given heating system within an apartment building can be improved by implementing new innovative thermostats from Danfoss. The thermostats have been developed to ensure better radiator performance and obtain a lower return temperature into the DH grid, thereby a more energy-efficient system.

Compared to the existing electronic thermostat design available in the market, this new one is equipped with an additional temperature sensor connected to the return side of each radiator and an algorithm to secure more accurate control of the opening or closing of the valve based on a maximum return temperature setting. The purpose of this functionality is two-fold.

First, it should help ensure low return temperature from the space heating systems by keeping a high delta-T across the radiator (Delta-T). This will help obtain a low DH return temperature and provide convenience for the occupant since it will ensure proper heating system operation even when windows are opened, or holiday setbacks are applied.

Second, due to the automatic flow limiting effect of the return temperature control, the thermostat should help provide automatic hydronic balancing of the heating system. This new thermostat could provide one combined solution for some of the essential issues in space heating systems in buildings connected to future low-temperature DH systems.

The return temperature control functionality is becoming increasingly relevant for both the primary and secondary sides of larger heating systems, as there are implications related to systems’ optimal performance (e.g., boilers operating in condensation mode are using less energy), environmental factors (less energy used means less CO₂ impact), as well as comfort-related factors (e.g., experienced by tenants in residential multi-family buildings, as well as in light commercial or public buildings).

Lowering the return temperature will lead to higher efficiency in DH applications and save the building owner the ‘incentive costs. In boiler applications, the boiler’s efficiency will be increased, and the solution implementation will also have a larger positive effect on the hydronic balancing in buildings.

The concept will address both buildings with known problems with return cooling and balancing issues, as well as typical buildings where cooling and energy efficiency can be at least optimized, if not radically improved.

Testing the functionality

The prototypes of the thermostat were installed on all radiators in part of two buildings to test the functionality of the new thermostat. Measurements on the heating system operation before, during, and after the test were collected and analysed to document how the thermostats performed to limit the return temperature to the district heating substation and ensure hydraulic balancing in the heating systems.

The new electronic thermostats were tested in two multi-family buildings in Denmark, both connected to the local DH network. The results documented that this prototype thermostat can embed the functionalities of the traditional thermostatic radiator valve and the riser balancing valves in the same design. Integrating an extra return temperature sensor ensured better flow control through the radiators and the lowest possible return temperature. In addition, installing these thermostats can allow the automatic hydronic control of the heating system, making the risers balancing valves redundant. It was found in Building A that the DH cooling – defined as the difference between the supply and return temperature – was 4-12 °C higher in 2019 during the test compared to winter and spring 2020 when the prototypes were replaced with state-of-the-art thermostats.

The measurements also suggested how the optimal operation in large buildings is sensitive even though the buildings have a well-controlled heating system. Only two uncontrolled radiators out of 175 were able to contaminate the overall return temperature in Building B. In one case, the reason was a manufacturing problem – which was an acceptable issue in the development of a new product –whereas, in the second one, it was due to the end-user tampering with the thermostat. This last aspect can be potentially critical for any space heating system control. The results highlighted the importance of remotely connected devices. The measurements from the thermostats helped pinpoint the faults in the heating system, although the end-users were not experiencing any discomfort. By also correcting the pump setting and closing the string balancing valves in the two risers to limit the flow to the uncontrolled radiators, it was possible to reduce the overall return temperature to 35°C, neutralizing the negative effect of the malfunctioning thermostats. Hence, the digitalization of the demand side represents a critical element for the transition towards low-temperature operations and can support and improve the quality of the activities of the building service personnel, currently, labor and time-consuming because manually performed.

Transition towards low-temperature operations

Key strengths & differentiators are that this concept is a world 1^st type of solution that allows for dry installation, provides continuous valve commissioning, and solves both hydronic imbalances & return temperature optimization.

However, there is a mass of opportunities with the solution: its unmatched versatility – as it can be used as a stand-alone solution without connecting it to the internet. In public buildings, the solution can be bundled with accessories, such as tamper-proofing (protective cases) and main power supply modules. The concept endeavors to solve key pain points with hydronic imbalance removing costly installer calls and labor needed to fix claims that may arise. It provides preemptive maintenance, live access, and monitoring in a simple and easily integrated, digitally fit solution.

The concept is also ground setting for savings through scheduling in large-scale public buildings.

These learnings will lead to further improvements of the system and give a lot of learnings of the effect in district utility grids and larger gas-boiler-fired installations.

Furthermore, based on the massive amount of data gathered in the two test buildings, we can now see that the data can be used to diagnose various kinds of errors in how the heating systems are built and used. This could potentially lead to advanced diagnostic systems that can monitor vast amounts of radiators and buildings.

Having this in place could save a lot of time for janitors and engineers to supervise and troubleshoot installations, saving time and money in housing associations, municipalities, etc. 

The key finding for future product development is to secure the most robust design that could minimize the negative impact of the end-user’s misuse of local controllers in the heating elements. An improved design may integrate a new safety functionality that closes the valve or limit the flow to a minimum in case of damaged thermostats or wrong signal from the sensors. This will not be detrimental for the indoor comfort in the flats and avoid a few radiators compromising the low-temperature operation of an entire building. 

The assessment of implementing low-temperature district heating grids shows that the existing grid can lower the temperatures. It might be legally possible for the district heating companies to support the transition to low temperatures by investing in intelligent thermostats. Furthermore, the results showed that the digitalization of the demand side represents a critical element for the transition towards low-temperature operations and the overall green transition.

For further information please contact: Ida Bach Sørensen, ida-bach@hotmail.com