Less Compressor, More Suction Pressure – Short #304

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

• Compression Ratio (video)
• Why Compression Ratio Matters (tech tip)
• HVAC School calculators
• Submit a podcast question
• Learn about the GRIT Foundation

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