Abstract: A water purifier includes a canister containing a reverse osmosis filter unit and threadedly engaged with a control head which is adapted to be coupled to a source of tap water, to a purified water storage tank, to a faucet or other remote usage location and to a waste water drain. The head includes a normally-open inlet valve controlling the flow of tap water to the filter unit and a drain valve controlling flow of waste water to the drain. The inlet valve closes in response to back pressure in the storage tank when it is filled to shut off the apparatus. A normally-closed and normally-disabled bypass valve can be selectively enabled for opening in response to a predetermined lower back pressure in the storage tank when it is less than completely full for diverting purified water from the filter unit to the drain. The drain valve is continuously adjustable between fully open and fully closed conditions.