What is a flow restrictor and why should i use it reverse osmosis units work by creating a back pressure on the exiting water to force the pure water through membrane this back pressure is created by adding a flow restrictor on the waste pipe and unless this is fitted you will never achieve good quality water the new generation of d d ro units now have a flow restrictor with a tap on the top of it this tap allows the user to take the restrictor out of line allowing a greater water flow to pe