SL1529-NH the fresh air heat pump controller is divided into internal and external machine control, and can be expanded up to 8 single-system external machine control boards. Each external machine board supports up to 3 compressors, or can be expanded up to 4 dual-system external machine control boards. the board supports up to 4 compressors. Support electronic expansion valve PID adjustment, support high and low pressure sensors, and can detect compressor and fan currents through the extended current module.