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Inverter Driven: No Gauges, Part 2

By Dan Applegate

The old-school method for checking these compressors was pretty simple. Sometimes a slug of liquid oil or refrigerant would make its way back to the compressor, and if there were no suction accumulator it would be drawn into the compressor.

Dan Applegate
Dan Applegate

Compressors are pneumatic, not hydraulic; when the compressor piston would reach top dead center the compressor piston-to-valve clearance would be at its minimum, and since liquids can’t be compressed, the liquid oil or refrigerant would have no place to go. If the slug of liquid were large enough, sometimes the compressor would lock up and draw “Locked Rotor Amperage”, in some instances the connecting rod would break, but in most cases the discharge valve would be “tweaked”, slightly bent, as the liquid was forced past the valve and into the compressor discharge.

The conditions would be revealed this way: the compressor is running, but the amperage is way below name plate, there is little or no performance at the evaporator, and the differential between high and low side is nominal. Now, the purpose of a compressor is not to pump the refrigerant, but to create a pressure differential such that the refrigerant migrates through the system. Like a boiler circulator, it’s a pressure differential device, not a pump. So naturally, when the differential between the high and low side is nominal, and that evidence is supported by low running amperage (no load on the compressor because of the bent valve), the compressor is diagnosed “running, not pumping, replacement needed.”

If one were to use those very same diagnostics on a current single phase VRF system, compressors would be replaced every day! Using gauges is not only not necessary (helpful in pressure testing during installation, but otherwise not used) but can actually be harmful when servicing these systems. Checking pressures can’t really be done, and when it is attempted, it gives false information and alters the charge in the system. These residential VRF/DFS systems are different from what most are used to. The gauges, once the most ubiquitous tool in the service man’s truck remain that. In the truck.

Dan Applegate is a DFS/VRF National Trainer and a Senior Product Specialist.