How ASHRAE 90.1-2016 Changed the Game for Chilled Water Coil Design

The HVAC industry is always evolving to improve energy efficiency, reduce costs, and promote sustainable practices. One of the most impactful changes came with the 2016 update of ASHRAE Standard 90.1, where a key revision was made to the required chilled water temperature difference (ΔT) across cooling coils.

This seemingly small change—from a ΔT of 10°F to 15°F—has massive implications for energy savings, system design, and long-term performance. In this post, we’ll break down what this update means, why it matters, and how it benefits HVAC engineers, building owners, and the environment.

The Old Standard: 10°F ΔT

Prior to the 2016 update, most chilled water systems were designed with a ΔT of 10°F (about 5.5°C). While this approach met cooling demands effectively, it required higher chilled water flow rates. More water had to be pumped through the system to remove the same amount of heat, leading to increased pump power consumption, larger piping, and higher installation and operational costs.

The 2016 Revision: 15°F ΔT Requirement

With ASHRAE 90.1-2016, Section 6.5.4.7 mandates that cooling coils be selected for a minimum ΔT of 15°F (8.3°C). This means the same cooling capacity can now be achieved with less water flow, which directly results in:

• Reduced energy consumption
• Smaller pump and piping sizes
• Improved chiller performance

This change pushes the industry toward designing systems that are leaner, more efficient, and better aligned with green building goals.

Why Was the Change Needed?

There are several performance drawbacks to designing systems with a lower ΔT:

1. Higher Pump Energy Usage

Lower ΔT means more gallons per minute (GPM) are needed to meet the load. This increases pump power demand, friction losses, and pressure drops throughout the system.

2. Inefficient Chiller Operation

Chillers are more efficient at higher ΔT values. With lower ΔT systems, chillers operate at poor part-load efficiency. Compressors run harder and longer, and the Coefficient of Performance (COP) drops significantly.

3. System-Wide Inefficiencies

Increased flow leads to additional strain on cooling towers and more heat rejection requirements. The result? A system that consumes more power across the board.

The Benefits of Increasing ΔT to 15°F

Switching to a higher ΔT brings multiple advantages that go beyond energy efficiency:

1. Lower Pumping Costs

Pump power is proportional to the cube of the flow rate. A 33% reduction in flow can cut pump energy consumption by more than 50%. That’s a significant operational cost saving.

2. Improved Chiller Performance

Higher ΔT enables chillers to transfer heat more effectively. For example, a chiller with a COP of 5.0 at 10°F ΔT can reach a COP of 6.5 at 15°F ΔT—translating to less power needed for the same cooling output.

3. Smaller Equipment Sizes

Reduced water flow means smaller pipe diameters, lower valve pressure drops, and potentially even smaller pumps. This brings down both first costs and ongoing maintenance.

Real-World Example: 100-Ton Cooling Coil

Let’s break it down using a 100-ton coil (1,200,000 BTU/hr):

• At 10°F ΔT:

       GPM = 1,200,000 / (500 × 10) = 240 GPM

• At 15°F ΔT (ASHRAE 2016 Standard):

       GPM = 1,200,000 / (500 × 15) = 160 GPM

This 33% reduction in flow drastically cuts down pumping power and improves overall system efficiency.

Final Thoughts

The ASHRAE 90.1-2016 update to increase the minimum chilled water coil ΔT from 10°F to 15°F might seem subtle, but its impact is anything but. From cutting pump energy in half to boosting chiller COP, the change represents a major step forward in HVAC system design.

By adopting higher ΔT strategies, engineers and building owners not only comply with code but also contribute to a more sustainable and cost-effective future.