A common method of regulating the heating or cooling output of hydronic air handlers and other types of HVAC terminal units is to vary the water flow rate through their coils. The most common flow control element is a two-way modulating valve. The valve's stem position is controlled using a pneumatic or electrical actuator that receives input from a building automation system. Feedback from a temperature sensor at the discharge of the air handler provides for closed-loop feedback control of flow.
The modulating valve is customarily selected so the pressure drop through its fully open body is at least 50% of the pressure drop across the crossover piping assembly of the distribution system serving the air handler, as shown in Figure 1. This gives the valve the necessary "authority" to provide stable output control under partial load conditions.