3-Way Zone Valves

A 3-way zone valve is a bypass version of a 2-way zone valve with 3 ports, commonly used with fan coils, heating units or other central heating equipment.
On Taco 560 series valves, bottom port is always a bypass inlet, which remains closed while equipment is in use. Supply inlet port is used for return line from a fan coil. When it is required to bypass the equipment, the supply port of the valve is closed (automatically or manually) and the water flows though the bypass port.
On Honeywell V8044/V4044 valves, bottom port is always a supply inlet. One of the side ports is a bypass, depending on the model.