Bryan teaches a class about diagnosing poor compression, which is a less common fault than shorted and grounded compressors.

In modern compressors, poor compression may happen due to stuck or active safeties. You’ll notice that the suction pressure may be high, and the head pressure will typically be low; there will be a smaller pressure differential between the suction and head pressure. However, those symptoms are also consistent with refrigerant bypass through a stuck refrigeration valve.

The reversing valve relies on a solenoid to shift the pilot valve, but pressure differential primarily help shifts the valve. There are cases when poor compression makes the valve unable to shift fully.

Good compression ratios for residential HVAC equipment are typically around 2.3-2.7. Higher compression ratios contribute to inefficiency due to a reduction in mass flow rate. However, a low compression ratio (below 2) indicates that the compressor isn’t pumping refrigerant. The compression ratio is the fraction of absolute discharge pressure over absolute suction pressure (at the compressor). MeasureQuick can calculate that ratio automatically.

Compressors that don’t pump properly also often have low current, usually less than 50% of the rated load amps (RLA). However, a bypassing reversing valve may result in a normal or higher current.

Potential causes of poor compression include broken valves or cylinder wear (on reciprocating compressors). In rotary or scroll compressors, you could be dealing with significant wear inside the compression chamber. However, the most common cause of poor compression is a tripped or stuck safety. You have to let a compressor power off and equalize the pressure to prevent poor compression caused by bypassing. Compressors might also run backward, meaning that it doesn’t pump properly.

Overall, it’s best to shut the compressor off, let it sit, and make sure everything is connected before letting the compressor start up again. Before replacing the compressor, make sure you can rule out every other possible cause of poor compression.

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