Because most hot water/chilled water systems are or should be utilizing variable speed technologies to reduce energy costs and improve comfort, control valves play a vital role in success or failure of an efficient hot water/chilled water system. In direct return systems, the valves located closest to the pumps experience the highest differential pressure (DP), those hydronically furthest the lowest. Typical valves are selected based upon the equation Q = C_v*DP, where Q is the flow rate, C_v is a flow coefficient equal to the amount of flow thru the valve at a pressure drop of 1 PSI, and DP is the differential pressure across the valve’s control surfaces. C_v is a constant in all but a very select group of valves that employ variable C_v technology.

This is all very important because the differential pressure across the valves’ control surfaces varies constantly as other system components modulate. The below graph shows a real time graph of changes in flow rates – for a constant load - for a typical (constant C_v) control valve and one that uses variable Cv technology as the differential pressure varies from 5-15 PSI.

On variable flow systems, control valves experience large changes in differential pressure across their control surfaces as the flow rates modulate, usually to maintain an established system differential pressure setpoint established as required to satisfy all coil flow requirements. This is shown graphically in the chart below.

All of this is important because of the energy scourge known as low system temperature differentials. The combination of central plant piping, coil selection criteria and typically used control valve capabilities all combine – among other things of course – to cause system inefficiencies. Remember, the most efficient chiller system you have is the one you never turn on!