Abstract: An industrial central vacuum system has a filter chamber having an intake port connectable to a plurality of intake nozzles and an output port, a multispeed blower having an intake side and an output side and energizable for drawing in air at its intake side and expelling it at its output side, a conduit connected between the intake side of the blower and the output port of the filter chamber, and a valve having an outer side opening to ambient pressure and an inner side opening into the conduit. Thus when the valve is open ambient air can be admitted directly into the conduit. A controller is between the blower and the valve for opening the valve prior to energizing and de-energizing the blower and for closing the valve a predetermined interval after energizing the blower. A pressure sensor is provided in the conduit and the blower is operable at a high speed corresponding to a relatively large blower throughput and at a low speed corresponding to a relatively small blower throughput.