When the design of variable flow hydronic systems was initiated 30 to 40 years ago, little emphasis was placed on the selection of the modulating two-way coil control valves. The existing valve specifications that were used for constant flow were often applied to variable flow systems. In many instances the valves that were used were not constructed to meet the high differential pressures encountered in variable flow systems. As a result many variable flow chilled water systems experience Delta T’s that are below design and have chillers that operate inefficiently. This low Delta T is often caused by valves leaking due to either insufficient close off capabilities or erosion (often referred to as “wire drawing”) of the seat or plug.