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The bane of district cooling

How long should we have to live with the migraine that the Low ΔT Syndrome has come to represent, ask Philips Tharakan and Mohammed Faiz, adding that the answer may lie in deploying Intelligent ΔT Management Systems

| | Jan 5, 2021 | 5:00 am
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Philips Tharakan

The HVAC industry has acknowledged the problem of the Low ΔT Syndrome for a long, long time. It has been a topic of widespread debate among district cooling industry stakeholders, with several having hypothesised solutions to the issue and then having found progress impeded by various concerns.

Typically, the concerns include the cost of implementation, the uncertainty of the return on investment (ROI) and the practical difficulties associated with making changes to existing installations. The fear is that the existing ΔT controllers will reduce the waterflow to a point where it is lower than the current waterflow, which gives rise to the associated phobia of the potential increase in room temperature.

While proper design, selection, operation and maintenance can help avoid Low ΔT in upcoming buildings, the causes of the problem in existing buildings cannot be practically eliminated if the airside equipment are not capable of handling the room load with the designed waterflow. Currently, we are in a phase that has found interesting ideas but no substantial solutions.  

Mohammed Faiz

While various groups have hypothesised ideas that could work, in theory – and some have tested the said hypotheses – in one way, all of them have failed, as they published the results limited to one component or part of the system. In short, no significant implementations have been made to date to combat the long-standing issue of Low ΔT in existing buildings. We must add the disclaimer that if there are, indeed, any solutions, the findings of these experiments are not publicly available, making it impossible for other professionals to adapt and improve.

For the uninitiated, Low ΔT is the result of underutilising the chilled water in the terminals of air-handling units (AHUs) or fan-coil units (FCUs). The unused chilled water is just recirculated back through the chiller or through intermediate heat exchangers. If the chilled water is produced by individual systems installed within the facility, then the Low ΔT reduces the chiller efficiency. This lowered efficiency leads to increased electricity cost, but there is also a problem of higher pumping volume – both of which increase the cost of electricity. However, this unfortunate phenomenon not only makes for an inefficient system and uneven indoor temperature distribution, but it also attracts penalties from district cooling providers. In other words, Low ΔT poses a major headache to professionals in the industry.

The problem is more than just a curiosity for us as authors, since we are keen on seeking out HVACR-related solutions that encompass technical as well as business aspects. Given the gravity of the problem, we were compelled to conduct an investigation. More specifically, we explored the concern of how FCUs affect ΔT in existing and upcoming buildings, because there are economically viable solutions available for larger equipment, like AHUs but not for smaller equipment, like FCUs.

Our investigation

Our investigation involved the examination of published articles by entities that claimed to have a solution to the problem. We found that each entity offered a different solution – chiller manufacturers approaching the issue with the perspective of chillers alone, valve manufacturers recommending the replacement of manual balancing valves to automatic balancing valves and flow measuring teams recommending continuous flow monitoring, to name three.

Most of the solutions were limited in some capacity, and as stated earlier, our focus is restricted to FCUs. This criterion eliminated the above ideas, and it led us to two possible solutions – the first, an external implement, known as a ‘ΔT Control System’, and the second. a native solution, known as an ‘Intelligent ΔT Management System’.

Solution 1: ΔT Control System (external)

There are various externally mounted ΔT Control Systems available in the market. These systems work by installing two temperature sensors at the inlet and outlet of a unit. The sensors help determine the temperature difference between the terminals and use the difference to perform a control logic, which controls the position of an actuator to maintain ΔT.  If the ΔT is within the design conditions, the temperature sensor in the room, or the Building Management System (BMS), takes control.  In short, once the ΔT is achieved, the actuator opens or closes, based on the signal from the BMS or from the room temperature sensor. In some cases, the system has been combined with pressure-independent control valves (PICVs) to give better control, while others have added flow measurement and an individual valve control along with coil performance logging to the system.

Belimo Energy Valve

Solution 2: Intelligent ΔT Management System (native)

The Intelligent ΔT Management System works similarly to the ΔT Control System, explained earlier. However, the former serves more as an intelligent FCU than as an auxiliary solution.  The system minimises the number of fittings by replacing flow measuring, balancing and PICVs with economical two-way actuators. The difference between the above solution and the Intelligent ΔT Management System is that the latter integrates the controller and the fan. The integration allows for flow rate control and fan speed modulation, thus making the FCU utilise all of the chilled water available while maintaining the room temperature. The Intelligent ΔT Management System, thus, improves the efficiency of an FCU. Furthermore, since the Intelligent ΔT Management System is part of the fan coil, it allows the system to assess the airflow, the room temperature and the water temperature in tandem to balance the waterflow and maintain ΔT.

Although both the solutions provide answers to the concern of Low ΔT, the latter provides enough information to eliminate the biggest phobia with these solutions – that of ΔT controllers disregarding room comfort.

Room comfort and a comparison of the two systems

We mentioned earlier that there are economically viable solutions for larger equipment but not for smaller units. One of the solutions is to use PICVs along with ΔT controllers, but they are not a justifiable investment for FCUs. PICVs create a direct relationship between valve position and water flow rate by eliminating the pressure variability in a flow system. However, flow rate and load are not linearly related, and thus the load of the space is not linearly related to valve position. This results in the system modulating the water flow, but if the coil does not use up all the delivered energy, Low ΔT will rear its menacing head in the system.

The economic constraints and the possibility of continuing Low ΔT eliminate PICVs as the solution for Low ΔT attributed to FCUs. However, using an external ΔT controller ensures constant ΔT but does risk under-delivering on the energy requirements of the room, resulting in undercooling. Here, the load is independent of the control of the valve, and the valve simply limits flow in order to maintain an arbitrary ΔT.

The Intelligent ΔT Management System seems to have a solution for the concern of undercooling. Since it approaches the Low ΔT issue from the load point of view, the system not only controls the valve for ΔT but also modulates the fan to ensure that all the energy is delivered to the coil that conditions the space. Should the load increase, the fan speed will also increase, which will cause the ΔT to increase. Consequently, the valve position opens to increase flow and deliver more energy to the coil. Thus, the system avoids undercooling by modulating the fan in the unit. The Intelligent ΔT Management System, therefore, adds intelligence to itself by connecting all relevant parameters of the ΔT problem: Water flow rate, ΔT and the load of the space.


Among the two hypotheses we explored, the Intelligent ΔT system offered a more comprehensive solution. If the Intelligent ΔT System is the answer to this long-standing issue, then it is our collective responsibility to investigate it extensively. To do so would require us to implement the said solution in a real, living building and determine the system’s efficacy. As authors, we urge facility managers, building owners and service providers of existing buildings, as well as consultants and building owners of upcoming buildings to consider this proposition and collaborate with us to implement this enticing prospect.

If we can implement the Intelligent ΔT System in a new or existing building and follow that up by monitoring it, troubleshooting, and by investigating its performance, we can publish our results to add another piece to the collaborative puzzle of improving HVAC technology. If we can help dissipate the cloud surrounding the problem and collaborate on implementing the solution in an actual project, we may finally be able to get relief from the debilitating migraine.

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