The Copeland 9R and 9D compressors use an external hot gas by-pass system. When the solenoid is de-energized the by-pass valve directs the gas from one cylinder to the condenser. When energized the the gas is directed back into the suction of the compressor.
Compressors equipped with this type unloading must have it installed. There is no internal provision to send the gas from one cylinder to the discharge or suction, this is done with the external solenoid and piping. To operate these compressors without the unloader the compressor head must also be changed. Failure to do so could result in equipment damage and/or personal injury.