Delta T Talk w/ JD Kelly

Key takeaways

Delta T is the temperature difference between two points of the same medium and should be correctly measured at the equipment for accurate results.
Humidity significantly affects Delta T readings; in humid environments, lower Delta T is expected due to increased energy used for latent heat removal.
The '20-degree Delta T' is not a one-size-fits-all rule and should be contextualized based on regional and environmental conditions.
High Delta T readings can be misleading and indicative of potential issues, not necessarily good performance.
Technicians should consider Delta T alongside other diagnostic tests for a holistic view of HVAC system performance.

Summary

Understanding Delta T

In this episode, Bryan and JD Kelly discuss the fundamental concept of Delta T, which is the temperature difference between two points of the same medium, typically the return and supply air in an HVAC system. The key misunderstanding revolves around how Delta T is often measured incorrectly, with technicians placing probes at registers instead of directly at the equipment. This can lead to inaccuracies in readings due to external factors such as duct leakage or heat gains from an attic.

The Impact of Humidity on Delta T Measurements

Humidity plays a crucial role in Delta T readings, especially in climates with high moisture levels like Florida. As JD explains, when humidity levels are high, more energy is expended towards removing moisture from the air (latent heat) rather than simply cooling it (sensible heat). This results in lower Delta T readings, which can confuse technicians measuring system performance immediately after repairs. Over time, as indoor humidity levels decrease, Delta T readings tend to improve.

Common Misconceptions and Measurement Techniques

Bryan and JD highlight common misconceptions surrounding Delta T, particularly the belief in the '20-degree Delta T' rule as a universal standard. They emphasize the importance of context when evaluating Delta T values, illustrating how factors such as airflow (CFM per ton) and regional differences can affect expected results. Proper probe placement and understanding the system's unique conditions are critical for accurate assessment.

Challenges with High and Low Delta T Readings

The discussion also covers the challenges technicians face with high and low Delta T readings. A high Delta T isn't always indicative of effective equipment performance, particularly if it correlates with high humidity conditions that can lead to equipment issues down the line. Conversely, a low Delta T could signal problems, but may also just reflect seasonal or operational variations that aren't worth immediate adjustment.

Importance of a Holistic Approach to Diagnostics

Ultimately, the episode underscores the need for technicians to use Delta T readings as one piece of a larger diagnostic puzzle rather than the sole indicator of system health. By combining Delta T measures with other diagnostic tests, technicians can achieve a more comprehensive understanding of HVAC system performance and avoid making unnecessary adjustments that could worsen issues.

Show notes

Bryan from HVAC School interviews JD Kelly, known as "student of HVAC" on Instagram, about Delta T measurements in HVAC systems. This episode dives into the fundamentals of Delta T, common misconceptions, and proper measurement techniques.

JD explains that Delta T is simply a temperature difference between two points of the same medium, which is often misunderstood in the industry. Many technicians are taught to measure between return grills and supply registers, but this approach can lead to inaccurate readings due to external factors like duct leakage or heat gain from attics. The proper measurement should be taken directly at the equipment to evaluate how the evaporator coil is performing.

Humidity plays a significant role in Delta T measurements, which JD discovered when researching the fundamentals. Bryan elaborates on this point, explaining that in high-humidity environments (like Florida), more energy goes toward removing moisture (latent heat) rather than cooling the air (sensible heat), resulting in lower Delta T readings. This is why a newly repaired unit in a humid house might initially show lower readings but improve as the home dries out. The discussion highlights why the common "20-degree Delta T" rule of thumb isn't universally applicable and requires context.

Key Topics Covered:

Definition of Delta T: Temperature difference between two points of the same medium
Difference between Delta T and TD (temperature difference)
Proper probe placement for accurate Delta T readings
Impact of humidity on Delta T measurements
How regional differences affect expected Delta T values
Relationship between CFM (airflow) per ton and Delta T readings
Common mistakes: Adjusting equipment based solely on Delta T readings
Challenges of applying Delta T in heat pump heating mode
How temperature dead spots can affect readings
Why Delta T readings must be considered alongside other diagnostic tests
The danger of making unnecessary adjustments to HVAC systems
Why high Delta T isn't always good and low Delta T isn't always bad

JD's Delta T tech tip: https://hvacrschool.com/solving-delta-t/

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