Background Story
In chilled water systems, the temperature difference between the supply and return water — known as Delta-T — is a key indicator of system performance. A low Delta-T means chilled water is returning warmer than expected, forcing chillers to work harder, pumping more water to achieve the same cooling effect. This leads to increased energy use, reduced system capacity, and higher operating costs. While Delta-T degradation often creeps in unnoticed, its impact is substantial — especially in large commercial and institutional buildings. The causes are rarely singular. Instead, it’s a combination of issues spanning HVAC design, maintenance practices, control strategies, and tenant behavior.
Problems
Low Delta-T is typically the result of systemic issues across multiple disciplines:
- Poor HVAC design decisions (e.g., using 3-way valves or improperly sized coils)
- Maintenance shortcomings (e.g., fouled coils, leaking valves, dirty filters)
- Control and sensor calibration issues
- Lack of real-time monitoring or alarms
- Improper operation or lack of awareness by facility staff or tenants
These problems not only reduce efficiency, but also limit chilled water plant performance and compromise occupant comfort.
Main Objectives
This insight aims to help facility managers, HVAC consultants, and building owners understand:
- The diverse causes behind low Delta-T
- Why an interdisciplinary approach is needed to resolve it
- The consequences of ignoring this “invisible” issue
- Steps to diagnose and fix Delta-T degradation
Approach
To visualize the complexity of low Delta-T causes, we’ve grouped them into four contributing domains based on real-world experience:
- Maintenance
- HVAC Design
- Tenant Spaces
Operation
Each domain introduces unique risk factors, including faulty actuators, poor BMS commissioning, incorrect valve selection, and lack of preventive maintenance. For example:
- Use of 3-way valves eliminates temperature drop across terminal units
- HEX fouling or chemical imbalance in CHW loops causes inefficient heat transfer
- Thermostats and sensors, if miscalibrated, introduce false demand signals
- Dirty FCU filters reduce airflow, leading to poor coil performance
Results
Buildings with persistently low Delta-T typically see 10–30% higher chilled water pumping energy and reduced chiller efficiency.
In extreme cases, systems must oversize pumps or run additional chillers to compensate for the false load.
Once root causes are addressed (e.g., replacing 3-way valves, cleaning coils, recalibrating BMS), Delta-T improves — along with system capacity and energy savings.
This underscores a vital point: Low Delta-T isn’t just a symptom — it’s a warning sign that your HVAC system needs a tune-up, both technically and operationally.