Carter tells us why compression ratio is important, what it means, why it changes so much on heat pump systems, and the effect it has on system operation. We also talk a bit more about heat pumps and their unique challenges beyond compression ratio.

Compression ratio is a comparison of discharge pressure to suction pressure. A ratio of 3:1 indicates that the discharge pressure is three times higher than the suction pressure. The higher the pressure difference, the less gas you move and the less efficient your system is. The compressor has a fixed volume, but the gas's actual mass varies based on density and pressure. So, lower suction pressure results in less gas being moved. Dirty filters, coils, and other means of clogging the system can drastically increase the compression ratio.

Heat pumps are especially sensitive to compression ratio changes because they move varying amounts of refrigerant depending on the operating mode. As such, charging heat pumps can be a challenge. Some heat pump manufacturers use a charge compensator to help make charging a slightly less difficult task. Heat pumps may also have coils with smaller surface areas, which can drive up the compression ratio.

Heat pumps have highly variable evaporator temperatures, and refrigeration systems have highly variable condensing temperatures. Both of these highly variable conditions may indicate systems with susceptibility to high compression ratios. In the case of refrigeration systems, the metering devices are critical components for reducing keeping the compression ratios at bay.

If you cannot find manufacturer literature or are working on an old heat pump, Carter recommends using airflow and temperature difference to determine how much heating the system is accomplishing.

Carter and Bryan also discuss:

• Rheem and Ruud heat pumps
• Centrifugal blowers
• Plenum placement
• New inverter-driven compressors

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