Less Compressor, More Suction Pressure - Short #304 · HVAC School - For Techs, By Techs summary

Key takeaways

An increase in suction pressure during compressor unload is a normal response and typically not a sign of failure.
Compressor staging involves complex interactions between components, including the compressor, blower, metering device, and system controls, which all influence pressure readings.
To accurately diagnose, technicians should record system pressures at multiple stages of operation and understand the manufacturer’s intended staging sequence.
Changes in airflow, such as from blower adjustments, significantly impact evaporator load and system pressures, and should be considered during diagnostics.
Pressures immediately after a stage change are not representative of steady-state operation; wait for the system to stabilize before making adjustments or conclusions.
Higher suction pressure during low-stage operation does not necessarily mean a compressor problem; it can be a normal aspect of system operation.
Before condemning a compressor, verify the system’s sequence of operation and ensure pressures have stabilized to distinguish between normal unloading and faults.
Understanding the dynamics of the system and its controls is essential for proper diagnosis and avoiding unnecessary component replacements.

Summary

Introduction and Sponsor Mentions

The episode begins with Brian introducing the podcast's focus on HVAC topics like compressors and compression ratios. He then proceeds to thank the show's sponsors, starting with Refrigeration Technologies, highlighting the Viper Wet Rag Heat Shield available through distributors or at trutectools.com, with a discount code 'GetSchooled'. Next, he mentions Carrier, emphasizing their long-term sponsorship and wide range of HVAC products, encouraging listeners to visit Carrier.com for more details. The segment concludes with mentions of Navac, TrueTech Tools, and details about the upcoming 2027 AHR Expo, including the opportunity to catch live podcasts and register at AHRExpo.com.

Understanding Compressor Unloading and Its Effects

Brian explains the process of a compressor entering low stage or unloading, which may involve opening scroll set ports, slowing down via variable frequency drives, or switching compressors. The core question addressed is whether the rise in suction pressure during unloading indicates a problem or is a normal response. He discusses how unloading reduces the compressor's ability to pump vapor, which affects head and suction pressures, and emphasizes that this is a normal part of compressor operation. The conversation includes how the compressor acts like a heart, pulling in and pushing out refrigerant, with pressures shifting accordingly—higher suction during unloading, lower head pressure. Brian highlights that the ability of the evaporator and the metering device to respond also influences system pressures, and that the heat entering the evaporator remains constant at the moment of unload, causing a temporary rise in suction pressure.

Impact of System Components and Conditions on Pressure Readings

Brian discusses how changes in airflow—whether from the compressor unloading or other factors like blower speed—affect evaporator load and pressures. If airflow drops along with compressor unloading, the suction pressure can decrease because less heat enters the evaporator. Conversely, reducing compressor capacity without changing airflow can cause suction pressure to rise. He notes that the metering device, such as a TXV or EEV, needs time to respond and stabilize after changes, which influences pressure readings. Additionally, system controls that target specific suction pressures may modulate compressor unloading or speed, complicating diagnosis. Brian stresses the importance of understanding the sequencing of components and recording system pressures at different stages to accurately evaluate system performance.

Approach to Diagnostics and Testing Stages

To properly diagnose system behaviors, Brian recommends identifying the equipment and understanding the manufacturer's testing procedures for staging. It's advised to record pressures and responses at multiple points in the staging cycle to see how the system reacts, rather than relying on a single pressure reading during a transition. He emphasizes that system responses can vary depending on factors like metering device type, blower operation, and control strategies. The importance of differentiating between normal unloading responses and faults is stressed, especially to prevent misdiagnosis leading to unnecessary part replacements. Brian also advises to wait until the system stabilizes before making charge adjustments, as pressures immediately after stage changes are not representative of normal operating conditions.

Conclusion and Key Principles for Correct Diagnosis

The episode concludes with Brian reinforcing that higher suction pressures during low-stage operation are typically normal and not an automatic sign of a problem. He urges technicians to consider what has changed in the system and whether components are functioning as designed. The importance of understanding the sequence of operation and only condemning components when certain they are faulty is underscored. He encourages using proper diagnosis techniques, such as recording pressures at different stages and consulting manufacturer guidelines. Ultimately, professionals should approach pressure changes with contextual awareness, recognizing that many factors—including system controls, airflow, and metering device responses—affect what is observed during system unloads.

Show notes

In this short podcast episode, Bryan talks about compression ratio and compressors, particularly why you see more suction pressure when the compressor unloads and what that means for the HVAC system's operation.

When the compressor drops into low-stage or unloads, the suction pressure goes up. Regardless of the cause of reduced pumping, the head pressure will also drop as the suction pressure rises. If the conditions stay the same and the compressor stages down, the pumping capacity drops; the compressor moves less vapor while the evaporator still needs to absorb the same amount of heat.

However, if the indoor airflow also drops along with the compressor stage, the load on the evaporator also changes and drops the suction pressure. Dynamic metering devices (such as TXVs and EEVs) also need to equalize and factor into the compression ratio.

We should understand the sequence before condemning a compressor and then use that sequence in diagnosis. To troubleshoot and test staging, we want to identify the equipment and sequence (including how the manufacturer wants us to test staging) and then record everything at each stage or at least a few different points. We want to see how the equipment responds, such as if one part does not respond when the compressor drops to a lower stage.

This episode contains embedded sponsor messages from Refrigeration Technologies, Carrier, NAVAC, TruTech Tools, and the 2027 AHR Expo.

Resources

Purchase your tickets or learn more about the 8th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

Less Compressor, More Suction Pressure - Short #304