Three-port control valve
Selection of Control Valves in Variable Flow Systems (BG 51/2014) written by Chris Parsloe and published by BSRIA in June 2014, defines a Three-port control valve (3PV) as: ‘An actuated control valve with three ports (usually two inlet and one outlet) which varies the flow through a terminal device by transferring a proportion of the flow through a separate piped bypass…. The majority of three-port valves used in HVAC are mixing valves and as such have two input ports and one output port. A few can be used as diverting valves with one input port and two output ports, but this is rare (except for rotary shoe type valves). As such, there is no ‘diverting’ port when used in most HVAC applications. Instead the mixing valve is installed on the return from a terminal device and as such is described as a ‘mixing valve used in a diverting application.’
It suggests that typical applications include; modulating control of heating or cooling outputs for air handling units, large fan coil units, and plate heat exchangers in constant flow branches.
--BSRIA
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