Abstract: A system of purifying water in a pressurised system using a venturi to contact a chemical with the water, the system having a main water line from inlet (5) to faucet (15) and including a bypass loop (13) incorporating the venturi (6) to add the chemical to the water, a pump (12) in the bypass loop (13) to pass water through bypass loop (13) at constant pressure whereby the venturi (6) delivers the chemical at a constant rate irrespective of the variation of water pressure in the main water line (5) due to opening and closing of the faucet.