Residential condensers have liquid and suction service valves: closed, they hold the factory charge in the outdoor unit while the gauge port still reaches the line set, so the line set and indoor coil can be evacuated before the valves are opened. Commercial compressor and receiver valves have three positions: back-seated closes the gauge port, mid-seated opens all three paths, and front-seated isolates the line. Both kinds allow pump-down for repairs.
Front-seat the liquid service valve and pump it down, then we can change the drier without losing the charge.
Related terms
Pump down
Moving a system's refrigerant into the condenser or receiver by shutting the liquid line and running the compressor.
Schrader valve
A spring-loaded valve core in a refrigerant service port, opened by the depressor in a gauge hose fitting.
Gauges
The manifold gauge set techs connect to service ports to read system pressures, saturation temperatures and charge.