A Duplex Water Softener consists of two vessels containing ion exchange resin, once the first column of resin has exhausted and goes into regeneration, the second column goes into service.

This maintains soft water to service at all times. The flow of water to service is metered and when the first column reaches exhaustion the control valve automatically switches the service flow to the second column and then regenerates the first column. It is then put on standby until required again for service.