2. The SENTINEL prevents boiler scaling as well as premature piping failure by replacing that troublesome relic, the ball-float valve. The ball-float is used to  maintain the level of water in the return tank. Ball-float valves are known for leaking and/or failing open. This allows new city water to continuously leak into the return tank diluting boiler compound which results in scale formation. In addition city water contains lots of free oxygen which combines with carbon dioxide to create carbonic acid. Carbonic acid destroys pipe work and boiler internals.