The modulating float valves are not tight-closing, in fact, our valves are designed to pass some flow when the ball is in the full up or “closed” position, so as not to stop main drain flow entirely. The optimum operating condition for the float controlled valve is between 60% and 70% open. The resistance to flow between these two opening positions establishes the minimum depth required for the center of the main drain line so that the desired flow can be maintained. If the depth and size of the line are incorrect the proper flow rate will not be attainable.